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Effect of host plant on bionomic and life history parameters of Anagyrus pseudococci (Hymenoptera: Encyrtidae), a parasitoid of the mango mealybug Rastrococcus iceryoides (Homoptera: Pseudococcidae)
Biological Control 65 (2013) 43–52
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Effect of host plant on bionomic and life history parameters of Anagyrus pseudococci (Hymenoptera: Encyrtidae), a parasitoid of the mango mealybug Rastrococcus iceryoides (Homoptera: Pseudococcidae) Chrysantus M. Tanga a,b, Samira A. Mohamed a,⇑, P. Govender b, Sunday Ekesi a a b
International Centre of Insect Physiology and Ecology (ICIPE), P.O. Box 30772-00100, Nairobi, Kenya Department of Zoology and Entomology, University of Pretoria, Pretoria, South Africa
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
" Mealybug acceptability by Anagyrus
a r t i c l e
i n f o
Article history: Received 17 December 2011 Accepted 19 November 2012 Available online 28 November 2012 Keywords: Host plants Anagyrus pseudococci Host acceptability Host suitability Life table parameters
Wasp’s larva.
Wasp’s eggs.
Mummified mealybug with exit hole.
Emerge adult wasp Time = 24 h: Wasps are reproductively active, ready to oviposit.
Parasitized mealybug.
Survival from egg to adult.
Time = 72 h: Wasps are released and females oviposit on mealybug-infested plants.
Cord
100
80
Handle
a ab
ab b c
Sleeve
% parasitism
pseudococci mirrored that of the host suitability. " Performance of A. pseudococci varied significantly with the host plant species. " Life table parameters were superior in parasitoids reared on mealybugs maintained on butternut squash. " These parameters were inferior in parasitoids reared on mealybugs maintained on weeping fig.
60
40
20 Cage
Infested plants (five host plant species were used, see graph on the right) in the screen house. Time = 24 h: Parasitoids removed monitored for parasitism.
0 C. moschata P. aculeata
M. indica Host plant
C. cajan
F. benjamina
Percentage parasitism on different host plants.
a b s t r a c t Anagyrus pseudococci Girault is a solitary koinobiont endoparasitoid of several mealybug species. The effect of five host plants (Mangifera indica L., Cucurbita moschata Duchesne, Parkinsonia aculeata L., Cajanus cajan L. and Ficus benjamina Roxb.) on host acceptability for oviposition and suitability for immature development of this parasitoid in the invasive mango mealybug Rastrococcus iceryoides Green were investigated. Effect of host plant on fitness traits (parasitoid size, egg load and longevity) and life table parameters were also assessed. Although A. pseudococci accepted the mealybug regardless of the host plant, the level of acceptability varied significantly. Percentage of parasitized nymphs was higher on C. moschata, followed by P. aculeata and M. indica, while it was lowest on F. benjamina. Host suitability was also strongly affected by the host plant and largely mirrored host acceptability for all the parameters evaluated. Female wasps reared on mealybugs maintained on C. moschata and P. aculeata were bigger and more fecund, while those reared from mealybugs maintained on F. benjamina were of inferior quality with regard to all fitness parameters evaluated. A. pseudococci achieved a greater intrinsic rate of natural increase (rm), net reproductive rate (Ro) and finite rate of increase (k) on mealybugs maintained on C. moschata and P. aculeata. In addition, the wasp had a shorter mean generation time (G) and population doubling time (Td) on mealybugs maintained on C. moschata. The reverse was true for those maintained on F. benjamina. The findings are discussed in view of improvement of laboratory mass rearing, as well as field enhancement of the parasitoid performance. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction ⇑ Corresponding author. Fax: +254 20 8632001. E-mail address: [email protected] (S.A. Mohamed). 1049-9644/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.biocontrol.2012.11.003
The mango mealybug, Rastrococcus iceryoides Green (Hemiptera: Pseudococcidae), an important pest of mango, Mangifera
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C.M. Tanga et al. / Biological Control 65 (2013) 43–52
indica L. (Anacardiaceae), is native to southern Asia. The pest was accidentally introduced into Africa (mainly Tanzania, Kenya and northern Malawi) where it has become a serious pest of mango, several other food crops, ornamental plants and forest trees (Luhanga and Gwinner, 1993; Williams, 1989). In Tanzania, Kenya and Malawi, mango losses can range from 30% to complete crop failure in unmanaged orchards (CABI, 2000; C. Tanga, unpub.). Both adults and nymphs cause direct damage by sucking sap from fruits, tender leaves, petioles, inflorescences and young shoots. Indirect damage is caused by excretion of honeydew on which sooty mould develops reducing the photosynthetic activity of the infested plant. Heavily infested leaves turn yellow and gradually dry out while infestation on inflorescences results in their excessive shedding, resulting in reduced fruit set. Severely infested fruits drop prematurely, and the few that reach maturity will be contaminated with honeydew and sooty mould, which greatly reduces their market value. Additionally, honeydew excreted by mealybugs attracts ants, which may interfere with oviposition by the fruit fly parasitoids Fopius arisanus (Sonan) and Diachasmimorpha longicaudata (Ashmead) that were introduced into Africa for biological control of fruit flies and other major pests of mango. In Tanzania where R. iceryoides is widely distributed across several agroecological zones, this pest has become a major target for insecticidal sprays on mango, in addition to pruning and burning of infested plant parts (C. Tanga, unpub.; Willink and Moore, 1988). Apart from health and environmental hazards, chemical pesticide applications do not provide adequate control of mealybugs and the high cost or unavailability of pesticides have led some African mango growers to cut down mango trees, or completely abandon their cultivation. Mealybug populations may increase rapidly, causing severe damage when introduced to new areas where natural enemies are scarce. For example, the mango mealybug, Rastrococcus invadens Williams (Homoptera: Pseudococcidae), a species closely related to R. iceryoides, caused a marked decrease in mango production across West and Central Africa. However, it was brought under control by release of two encyrtid wasps, Gyranusoidea tebygi Noyes and Anagyrus mangicola Noyes, from India (Bokonon-Ganta et al., 2002; Neuenschwander et al., 1994). Another example is the South American cassava mealybug, Phenacoccus manihoti Matile-Ferrero (Homoptera: Pseudococcidae) that threatened the livelihoods of millions of Africans relying on cassava as one of their staple foods (Herren and Neuenschwander, 1991; Zeddies et al., 2000). This pest has been suppressed and yields increased by the introduction and release of coevolved parasitoids (e.g., Apoanagyrus lopezi De Santis (Hymenoptera: Encyrtidae)) from South America (Herren et al., 1987; Neuenschwander, 2001). Yet another example is Planococcus kenyae Le Pelley that was a major pest of arabica coffee in Kenya during the 1920s and 1930s. The introduction of several parasitoid species (especially Anagyrus sp. near kivuensis Compere) from Uganda in 1938 reduced the status of this mealybug to a minor pest (Le Pelley, 1943), and has remained so ever since. For R. iceryoides, classical biological control is likely the most suitable management option in Africa, considering that the pest is of no economic significance to mango production in its native home range, where several natural enemies cause up to 40% parasitism (Narasimham and Chacko, 1988; Tandon and Srivastava, 1980). However, the composition and efficacy of indigenous natural enemies that may have formed new associations with this pest in Kenya and Tanzania, need to be established before considering any such introduction. A survey carried out between 2008–2009 in Kenya and Tanzania (C. Tanga, unpub.) reported six Encyrtid parasitoid species (Anagyrus aegyptiacus Moursi, Leptomastix dactylopii Howard, L. tecta Prinsloo, Agarwalencyrtus citri Agarwal, Aenasius longiscapus Compere and Anagyrus pseudococci Girault) attacking R. iceryoides on different host plants, with A. pseudococci
accounting for 95% parasitism. However, information regarding the effects of host plant on the preference and performance of A. pseudococci is lacking. The objective of the present study was therefore to: (1) evaluate the effect of five host plants on acceptability for oviposition and suitability of R. iceryoides for the immature development of A. pseudococci, and (2) assess the effect of the host plant on the fitness traits, and the life table parameters of the parasitoid. 2. Materials and methods 2.1. Host plants Seedlings of four host plants, namely mango (M. indica L.) (Anacardiaceae), Jerusalem thorn (Parkinsonia aculeata L.) (Fabaceae), weeping fig (Ficus benjamina Roxb.) (Moraceae), and pigeon pea (Cajanus cajan (L.) Millsp. (Fabaceae), in addition to the rearing host butternut squash fruit (Cucurbita moschata Duchesne) (Cucurbitaceae) were used in this study. These plants were selected because of their known association with the invasive pest in the field (C. Tanga, unpub.). The plant seedlings were purchased from a commercial nursery and maintained in screen house cage (287 cm height 256 cm length 252 cm width) free from other pests and pesticides at the International Centre of Insect Physiology and Ecology (icipe), Nairobi, Kenya. In the screen house the plants were maintained in plastic containers (35 cm height 29 cm top diameter 20 cm bottom diameter or 19 cm height 21 cm top diameter 12.8 cm bottom diameter) and were fertilized with farmyard manure and watered every two days. 2.2. Host insect The colony was initiated from a cohort of 300 adult R. iceryoides that were collected from mango orchards in coastal Kenya in February 2008. In the laboratory, the insects were reared on mature fruit of butternut squash for approximately 20 generations before the start of the experiment. For colony maintenance, weekly or biweekly infestation of 10–20 butternut squash fruits was carried out, and after every 6 months, wild mealybugs from the field were introduced to maintain genetic diversity. Prior to the experiments, 30 adult female mealybugs with wellformed ovisacs obtained from the stock colony were placed on each host plant maintained in large cages (30 cm length 30 cm width 60 cm height) with screened sides and glass tops. Cultures were maintained under screen house conditions at 22.3 ± 1.07 °C, 40–75% relative humidity (RH), and a photoperiod of 12L: 12D for at least 3 generations to allow them to adapt to their new host. 2.3. Parasitoids The parasitoid colony was initiated from a cohort of 93 A. pseudococci wasps (72 $ and 21 #), which were collected from the same host plant and location as that of the host insect, and brought to the laboratory at icipe. They were reared on third instar nymphs and adults of R. iceryoides maintained on butternut squash fruits in Perspex cages (30 cm length 30 cm width 30 cm height). The parasitoids were provided with streaks of pure honey as food and the colony was maintained at ambient condition (26– 28 °C, 45–60% RH, and photoperiod of 12L:12D). Preceding the experiment, wasps were collected from the stock colony and maintained under screen house conditions using the same procedure as described for R. iceryoides above. Every two weeks, 20 A. pseudococci adult females were released into Perspex clip cages (12 cm length 12 cm width 12 cm height) contain-
C.M. Tanga et al. / Biological Control 65 (2013) 43–52
ing a host plant infested with an average of 250 third nymphal instar R. iceryoides. Two small holes (1 cm in diameter) were made on the top and bottom of the cages to accommodate the plant stem. Prior to the experiment, parasitoids were maintained on a host plant species for at least three generations (from egg to adult) to allow them to adapt and to remove maternal effects (Lacey, 1998), thus minimizing the effects of associative learning. At the start of the experiment, 12 days after the release of the parasitoids, mummies were collected from the host plants and individually placed in gelatin capsules. Mummies (<24 h) were observed twice daily and newly-emerged parasitoids were used for the experiments. 2.4. Bioassays 2.4.1. Effect of host plant on R. iceryoides acceptability for oviposition by A. pseudococci Prior to the introduction of the parasitoid wasps, seedlings of test host plants and butternut squash fruits infested with mealybugs were obtained from the stock culture and placed in test cages (30 cm length 30 cm width 30 cm height). Only 100 R. iceryoides nymphs of the suitable host stage (third nymphal instar) were retained on each host plant. Thereafter, 10 mated, naive and fed 3day-old wasps (5$ and 5#) were introduced into the cages. The parasitoids were left to forage and oviposit for 24 h at the ambient conditions described above, after which, the parasitoids were removed and the exposed mealybugs allowed to continue feeding on their respective host plant for two days. All surviving R. iceryoides were later dissected in phosphate buffer solution under a stereomicroscope and the number of nymphs containing at least one parasitoid egg recorded. The number of superparasitized nymphs and encapsulated eggs was also noted and the percentage of parasitized nymphs determined. This experiment was replicated eight times for each plant species. 2.4.2. Effect of host plant on R. iceryoides suitability for development of immature A. pseudococci To further test for the effect of host plant on R. iceryoides suitability for development of immature A. pseudococci, additional sets of 100 third nymphal instars of R. iceryoides were exposed to the parasitoids. Number and status of the parasitoid used, and the duration and conditions of the exposure were the same as those described above for host acceptability, except in this experiment the hosts were allowed to develop till mummification and parasitoid eclosion. Mummies from each host plant were collected, counted and stored separately in transparent plastic cups (4 cm height, 5.5 cm base diameter and 7.5 cm top diameter) until parasitoid emergence. The number and sex of emerging wasps were recorded and the length of their left hind tibia measured. Percentage of mummified nymphs was computed based on the initial number of exposed hosts (100 nymphs) while the percent parasitoid emergence was computed based on the number of the mummified nymphs for each host plant. The female offspring were computed as a percentage of the emerging females over the total number of emerging wasps. The experiment was replicated eight times for each host plant. 2.5. Effect of host plant on some fitness traits of A. pseudococci 2.5.1. Female body size and egg load Mated A. pseudococci females, newly emerged from R. iceryoides on different host plants, were held separately with access to pure honey and water in Perspex cages (15 cm length 12 cm width 12 cm height). Females were dissected at ages 0, 1, 3 and 9 days, and egg loads were recorded. Each female wasp was placed in a drop of phosphate buffer solution on a glass slide.
45
The specimen was dissected and observed at 35 with a Leica EZ4D stereomicroscope with an integral digital camera [Leica Microsystems (Switzerland) Limited]. Only mature oocytes with a well-defined neck connecting the two bulbs were counted. After the number of mature eggs was recorded, the wasps’ left hind tibia (LHT) was measured to the nearest 0.0025 mm under the same magnification.
2.5.2. Effect of host plant on parasitoid adult longevity of nonovipositing wasps Four groups of 20 parasitoid wasps (10$, 10#) that had emerged from mealybug-infested host plants on the same day (08:00– 11:00 h) and fed on pure honey were aspirated and maintained in a cage (20 20 20 cm) for each host plant separately. The cages were kept under the same ambient conditions as above and the parasitoids monitored daily. Dead wasps were removed and their longevity recorded. Thereafter, their left hind tibia lengths were measured as described above. Individuals that drowned in excess solution were excluded.
2.6. Life table experiment and calculation of demographic parameters One pair of newly emerged and fed A. pseudococci adults originating from each host plant culture was introduced into a test Perspex clip cage described above, containing 20 nymphs of the suitable host stage on their respective host plants. Parasitoids were left to forage and oviposit for 24 h. Adult female parasitoids were removed and transferred to another infested host plant with 20 new mealybugs each day for the entire life span of the parasitoid female. These observations were replicated using 10 females for each host plant. Upon dying, the left hind tibia of each female was measured as described above. For each exposure, mummified mealybugs were carefully removed and transferred into transparent plastic cups (4 cm height, 5.5 cm base diameter and 7.5 cm top diameter). Mummies were checked twice a day for parasitoid emergence. Parameters recorded during this experiment were days to mummification, developmental time, the number of offspring emerging, total number of offspring for each parasitoid female during its lifetime, and female longevity. Data obtained from this experiment were used to generate demographic growth parameters: the intrinsic rate of natural increase (rm), net reproductive rate (Ro), mean generation time (G), population doubling time (DT) and finite rate of increase (k).
2.7. Statistical analysis The percentages of parasitized nymphs, mummified mealybugs and emerged adult wasps, female offspring and lifetime fecundity were analyzed using one-way analysis of variance using a general linear model (SAS Institute, 2010). Developmental time, adult wasp size and longevity of host-deprived wasps were analyzed using two-way ANOVA, with host plant and sex as factors. Also egg load and longevity of ovipositing females were analyzed using two-way ANOVA with host and female age as factors. Count data were log10 transformed, while the percentages were arcsine transformed before statistical analysis (Sokal and Rohlf, 1981). When the ANOVA was significant, means were separated using the Student–Newman–Keuls (SNK) test. Demographic parameters—intrinsic rate of increase (rm), net reproductive rate (Ro), mean generation time (GT), doubling time (Td) and finite rate of increase (k)—were calculated using the Jackknife procedure described by Hulting et al. (1990).
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3. Results 3.1. Effect of host plant on R. iceryoides acceptability for oviposition and suitability of the immature development of A. pseudococci An acceptable host was defined as a host containing at least one parasitoid egg. A. pseudococci females accepted R. iceryoides regardless of the host plant on which it was cultured. However, acceptability varied significantly among host plants, with the highest percentage of parasitized nymphs occurring on mealybugs cultured on butternut squash, Jerusalem thorn and mango (Table 1). Superparasitism was recorded only on hosts maintained on butternut squash, Jerusalem thorn and mango, with an average percentage superparasitism of 2.4 ± 1.02, 1.7 ± 0.77 and 1.1 ± 0.73, respectively. No encapsulation of eggs or larvae by R. iceryoides was recorded on any of the tested host plants. Host suitability (as measured by number of days to mummification, percentage of mummified hosts, percentage eclosion of adult parasitoids, percentage female offspring and preimaginal developmental time) were strongly influenced by host plant, and the trends observed largely mirrored host acceptability (Table 1). However, sex ratio of the parasitoid offspring was female-biased, irrespective of the host plant (Table 1). A. pseudococci completed development on R. iceryoides maintained on all of the host plant species. However, the duration of preimaginal development time varied considerably across host plants (Table 2). Both sexes took significantly less time to emerge from butternut squash-reared mealybugs, and longer from weeping fig-reared mealybugs (Table 2), and the same trend was observed for the overall mean developmental time of the parasitoid (Table 2). Females took significantly longer to develop than males
for all host plants tested except on butternut squash (Table 2). Males emerged approximately one day earlier than females for all host plants except Jerusalem thorn (Fig. 1). 3.2. Effect of the host plant on parasitoid fitness parameters 3.2.1. Parasitoid adult size Body size of both male and female wasps, as measured by left hind tibia length, was strongly influenced by the host plant (Table 1). Both sexes attained a significantly larger body size when the hosts were maintained on butternut squash and Jerusalem thorn, while those that emerged from hosts maintained on weeping fig were significantly smaller (Table 1). The data also revealed that female wasps emerging from R. iceryoides were significantly larger than males for all host plants tested (Table 1). 3.3. Parasitoid adult longevity 3.3.1. Longevity of ovipositing female (host provided), and lifetime fecundity The survival of female wasps reared on, and offered, hosts maintained on any of the host plants followed a type I survivorship curve (Fig. 2). However, the overall mean life span of A. pseudococci females was significantly longer when the parasitoid was reared on, and offered, hosts maintained on weeping fig (16.8 ± 0.66 day) and pigeon pea (15.2 ± 0.36 day) and it was shorter when the parasitoid was reared on, and offered, hosts maintained on butternut squash (11.4 ± 0.72 days). However, female parasitoids reared on and offered hosts maintained on mango and Jerusalem thorn had a similar life expectancy.
Table 1 Effect of five host plants on biological parameters of Anagyrus pseudococci produced from 3rd nymphal instar Rastrococcus iceryoides (values represent mean ± SE). Host plant
M. indica C. moschata P. aculeata C. cajan F. benjamina F df P
Parasitized nymph (%)
Time to mummification (days)
Host mummified (%)
Adult eclosion Female (%) offspring (%)
Tibia length [mm] Female
Male
F
df
73.13 ± 2.48ab 79.13 ± 1.19a 74.25 ± 1.52ab 70.75 ± 1.52b 58.75 ± 2.17c 16.36 4, 35 <0.0001
10.47 ± 0.48bc 9.40 ± 0.38c 9.93 ± 0.36bc 11.20 ± 0.38b 12.73 ± 0.34a 11.03 4, 70 <0.0001
81.5 ± 3.02ab 89.75 ± 1.46a 83.75 ± 3.73ab 77.38 ± 3.91b 48.38 ± 2.67c 23.06 4, 35 <0.0001
68.25 ± 2.17ab 83.0 ± 1.72a 74.50 ± 4.47ab 67.25 ± 5.04b 38.63 ± 2.60c 26.72 4, 35 <0.0001
0.443 ± 0.002bA 0.452 ± 0.002aA 0.450 ± 0.003abA 0.443 ± 0.003bA 0.400 ± 0.002cA 76.47 4, 118 <0.0001
0.371 ± 0.002cB 0.415 ± 0.006aB 0.385 ± 0.004bB 0.360 ± 0.003cB 0.304 ± 0.003 dB 100.64 4, 46 <0.0001
393.74 56.0 172.94 263.46 523.61
1, 1, 1, 1, 1,
59.64 ± 1.01ab 65.79 ± 1.45a 62.25 ± 2.75ab 59.53 ± 1.70b 57.07 ± 2.33c 2.9 4, 35 0.0359
Statistics ($ and #) P 31 37 30 33 33
<0.0001 <0.0001 <0.0001 <0.0001 <0.0001
Means within the same column followed by the same lower letters and within the same row followed by the same upper case letters are not significantly different (Student– Newman–Keuls test, a = 0.05).
Table 2 Egg-adult developmental time and lifetime fecundity (mean ± SE) of female A. pseudococci on third instar R. iceryoides reared on five different host plant species. Host plant
M. indica C. moschata P. aculeata C. cajan F. benjamina F df P
Mean developmental time (in days ± s.e.)
Statistics ($ and #)
#
$
F
df
18.81 ± 0.19bB 15.52 ± 0.25dA 17.22 ± 0.33cB 19.41 ± 0.17bB 21.50 ± 0.38aB 67.77 4, 79 < 0.0001
19.83 ± 0.12cA 15.76 ± 0.14eA 19.08 ± 0.14dA 20.68 ± 0.21bA 23.73 ± 0.17aA 329.14 4, 166 < 0.0001
21.86 0.80 37.04 15.90 38.59
1, 1, 1, 1, 1,
Combined developmental time (#$)
Number of life time progeny production
19.51 ± 0.12c 15.68 ± 0.13e 18.47 ± 0.19d 20.23 ± 0.17b 23.03 ± 0.24a 253.63 4, 250 < 0.0001
27.6 ± 1.00bc 34.6 ± 1.10a 30.1 ± 1.66ab 25.2 ± 1.50bc 22.2 ± 3.17c 6.50 4, 45 0.0003
P 49 61 53 46 36
< 0.0001 0.3759 < 0.0001 0.0002 < 0.0001
Means within the same column followed by the same lower letters and within the same row followed by the same upper case letters do not differ significantly by StudentNewman-Keuls test (P < 0.05).
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C.M. Tanga et al. / Biological Control 65 (2013) 43–52
Male
M. indica
14
Female
Mean number of emerged wasps
10
Female
8 6 4 2
26
27
12 10 8 6 4 2
0
0 19
20
21
22
23
24
25
26
27
20
21
22
Days from oviposition
23
24
25
Days from oviposition
C. moschata
9
Male
F. benjamina
Male
Female
Female
8 Mean number of emerged wasps
18 16 Mean number of emerged wasps
Male
14
12 Mean number of emerged wasps
C. cajan
16
14 12 10 8 6 4
7 6 5 4 3 2 1
2
0
0 15
16
17
18
19
20
21
22
23
22
24
23
24
25
26
27
28
29
Days from oviposition
Days from oviposition
P. aculeata
Male
Female
16
mean number of emerged wasps
14 12 10 8 6 4 2 0 17
18
19
20
21
22
23
24
25
26
Days from oviposition
Fig. 1. Anagyrus pseudococci emergence pattern on various host plants.
3.3.2. Longevity of non-ovipositing A. pseudococci female (hostdeprived) Host plant and parasitoid sex had a strong influence on the life expectancy of wasps (Table 3). Females lived significantly longer when maintained on hosts reared on butternut squash and Jerusalem thorn (Table 3). Longevity of males was comparable across all the host plants, except for weeping fig on which it was shorter. When males and females were reared on the same host plant, females lived significantly longer than males when reared on hosts maintained on butternut squash, Jerusalem thorn and weeping fig. Longevity was comparable for both sexes when the wasps were reared on mango and pigeon pea (Table 3).
3.3.3. Effect of host plant and female age on egg load Host plant of R. iceryoides and the parasitoid female age had a significant effect on parasitoid egg load, as did their interaction (F = 6.86; df = 8, 322; P < 0.0001) (Fig. 3). Females reared on hosts maintained on butternut squash were significantly more fecund than those reared on hosts from other host plants, for all female age groups evaluated. Those reared on hosts maintained on weeping fig had significantly lower egg loads for all female age groups (Fig. 3). Among wasps reared on mealybugs maintained on the same host plants, three-day-old females had the highest egg load for all rearing host plants (Fig. 3). When egg load was regressed against female body size, it increased linearly for all host plants except pigeon pea (Fig. 4).
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squash, with 0.147 ± 0.033 females/female/day and 1.158 ± 0.052 females/female/day, respectively.
4. Discussion
Fig. 2. Survival curve of ovipositing females of Anagyrus pseudococci reared on different mealybug-infested host plants.
Table 3 Mean ( ± SEM) of longevity (in days) of host-deprived A. pseudococci wasps reared from five different host plant species and fed with pure honey. Host plant
M. indica C. moschata P. aculeata C. cajan F. benjamina F df P
Longevity (days)
Statistics ($ and #)
Female
Male
F
df
38.3 ± 2.44bcA 47.2 ± 1.74aA 41.8 ± 2.03abA 35.6 ± 2.84bcA 32.0 ± 3.04cA 5.60 4, 45 0.0010
34.4 ± 2.43aA 31.7 ± 2.51aB 31.1 ± 2.33aB 30.2 ± 2.64aA 17.6 ± 1.33bB 8.15 4, 45 <0.0001
1.28 25.75 11.97 1.94 18.84
1, 1, 1, 1, 1,
P 18 18 18 18 18
0.2721 <0.0001 0.0028 0.1808 0.0004
Means within the same column followed by the same lower case letters and within the same row followed by the same upper case letters are not significantly different (Student–Newman–Keuls test, a = 0.05).
3.4. Demographic growth parameters R. iceryoides host plants had a strong influence on various growth parameters of A. pseudococci (Table 4). Host plants significantly affected the intrinsic rate of natural increase (rm), population doubling time (Td) and finite rate of increase (k). Net reproductive rate (Ro) was 1.7, 1.4, 2.2 and 4.0 times higher for females reared and allowed to oviposit on butternut squash than those on mango, Jerusalem thorn, pigeon pea and weeping fig plants, respectively. Similarly, the intrinsic rate of natural increase (rm) was 1.3, 1.4, 1.7 and 2.6 times higher for females reared and allowed to oviposit on butternut squash than those on mango, Jerusalem thorn, pigeon pea and weeping fig plants, respectively. Population doubling time (Td) on butternut squash was 61.23% shorter than that on weeping fig, and the mean generation time (GT) was 2–9 days less on butternut squash compared to the other host plants. For example, the net reproductive rate (Ro) decreased from 21.753 ± 0.137 females/female/generation on butternut squash to 5.476 ± 0.066 females/female/generation on weeping fig, while the mean generation time (GT) increased from 20.95 ± 0.331 days on butternut squash to 29.83 ± 0.279 days on weeping fig. Also, doubling time (Td) increased from 4.715 ± 0.106 days on butternut squash to 12.16 ± 3.762 days on weeping fig. Both the intrinsic rate of natural increase (rm) and the finite rate of increase (k) reached their maximum on butternut
The effect of host plant on the preference and performance of the parasitoid has been well documented for many host/parasitoid systems (e.g., Barbosa et al., 1982; Souissi and Le Rü, 1997, 1998). Among the widely reported aspects of the host plant–parasitoid interaction is the effect of host plant on various phases of the parasitization process (Vet and Dicke, 1992; Vinson and Williams, 1991). In our study, although A. pseudococci females accepted R. iceryoides regardless of their host plants, they showed a marked preference for those reared on butternut squash, Jerusalem thorn and mango. Similar results of differential host acceptability were reported for related encyrtids in association with other mealybug species when cultured on different plant species (Souissi and Le Rü, 1997, 1998). The low acceptability of mealybugs reared on weeping fig in this study might have been related to the low quantity and quality of the volatiles (kairomones) emitted from the plant as a result of low feeding activity of the mealybugs on it. Another possible explanation is the inferior quality of the mealybugs in terms of their small size (C. Tanga, unpub.) as well as their poor nutritional quality, or a combination of the two factors. These factors either separately, or in combination, have been reported to have great influence on host acceptability by several other parasitoid species (e.g., Hulspas-Jordaan and van Lenteren, 1978). The differential host acceptability for oviposition by A. pseudococci recorded in this study was also reflected on the suitability of R. iceryoides for the parasitoid’s immature development in terms of time to mummification, and percentage of mummified nymphs and eclosed parasitoids. On weeping fig mealybugs (least accepted) not only was the average percentage of wasp eclosion lowest, but also only slightly more than one-third (38.6%) of the mummified nymphs formed on this plant produced wasps, suggesting that the majority of the parasitoid offspring were unable to complete their development in the mealybugs maintained on this host plant. This may have been due to the immune reaction mounted by the host against the parasitoid immature stages, although dissection revealed neither egg nor larval encapsulation. However, other forms of cellular or hormonal defenses may have been involved in mealybugs reared on this plant leading to mortality of immature stage of the wasps. A differential effect of host plant on mealybug suitability for parasitoid development was also reported by Souissi and Le Rü (1997) for Apo. lopezi when reared on cassava mealybug. The authors reported that the number of mummies/female parasitoid, number of emerged parasitoids, wasp survival and development time varied considerably among mealybug host plants. In a separate study using the same tritrophic system, Souissi et al. (1998) found that the percentage of mummified nymphs as well as the percentage of emerged parasitoids was significantly affected by the host plant of P. manihoti. The suitability of R. iceryoides maintained on butternut squash, Jerusalem thorn and mango was also evident in the higher number of female progeny produced when A. pseudococci was reared on these host plants. Female wasps were presumably able to assess host quality and deposited more fertilized eggs in more suitable hosts or, alternatively, mortality of female offspring was higher in lower quality hosts (those maintained on weeping fig). However, no dissection of uneclosed mummies was made to ascertain this. Host size is used by female parasitoids to predict the host-stage suitability for parasitoid offspring, and to adjust the sex ratio among different stages of the same host fed on the same host plant. For example, production of offspring female-biased sex ratio by A. pseudococci in larger hosts of other mealybug species has been reported by several
C.M. Tanga et al. / Biological Control 65 (2013) 43–52
49
Fig. 3. Effect of host plant of the rearing host of Anagyrus pseudococci on the parasitoid egg load of females of different age groups. Bars with the same lower case letters are not significantly different among host plants for the same age group, and bars with the same upper case letters are not significantly different among age groups for the same host plant.
authors (e.g. Daane et al., 2004; Güleç et al., 2007). However, the sex ratio of Anagyrus kamali Moursi (Hymenoptera: Encyrtidae) reared on Maconellicoccus hirsutus Green (Homoptera: Pseudococcidae) was not affected by the host plants on which it was reared, although host body size varied significantly among the host plants (Persad and Khan, 2007). Studies have shown that the duration of parasitoid developmental time varies with the host’s nutritional history (e.g., Kouamé and Mackauer, 1991; Souissi and Le Rü, 1997). Our results agree with these findings as A. pseudococci developmental time varied with host plant. Similar results of the effect of host plant on the duration of the parasitoid developmental time were also reported for other encyrtid species such as Apo. lopezi (Souissi and Le Rü, 1997) and A. kamali (Persad and Khan, 2007). The difference in developmental time of A. pseudococci reported in our study may have been caused by variation in the quality of mealybugs reared on different host plants. Also host size has a significant effect on parasitoid developmental time. A positive correlation between host size and parasitoid development time was reported in several host–parasitoid associations (e.g., Ruberson et al., 1989; Vinson and Iwantsch, 1980), especially for idiobionts. However, for koinobionts, there is no general pattern between parasitoid developmental time and host size (Godfray, 1994). In the present study, A. pseudococci developmental time was shorter on larger hosts (those reared on butternut squash), than on smaller hosts (those reared on weeping fig). Contrary to our results, Souissi and Le Rü (1998) reported a positive correlation between developmental time and host size for Apo. lopezi when reared on P. manihoti maintained on different host plants. Shorter developmental time is a desirable trait. In the field it shortens the duration of exposure of parasitized mummies to predation and hyperparasitism, while in
the laboratory it enhances the mass rearing of parasitoids intended for augmentative releases. Adult wasp size is a strong indicator of parasitoid fitness (for review see Jervis and Copland, 1996), influencing the usefulness of the parasitoid for biological control (Godfray, 1994; van Lenteren et al., 2002). For example, wasp size of the closely related species A. kamali was found to be positively correlated with longevity, mating preference, fecundity, reproductive longevity, progeny emergence and sex-ratio (Sagarra et al., 2001). Parasitoid size is largely dependent on its host (e.g., Barratt and Johnstone, 2001; López et al., 2009), as it dictates the amount of nutrients available for the parasitoid’s developing larvae. Host size in turn, is dependent on the food quality of its host plant (Barbosa et al., 1982). Variation in wasp size of A. pseudococci reported in the current study can be explained by the host size differences, reflecting their nutritional history, being larger on butternut squash, mango and Jerusalem thorn, and smaller on weeping fig (C. Tanga, unpub.). Several factors influence parasitoid fecundity among which are female body size (Bernal et al., 2001) and age, and type and availability of food for female parasitoids, both during immature development and as adults (Baggen and Gurr, 1998; Idris and Grafius, 1995), as well as host food plant (Eben et al., 2000). In our study, host plant, female age, and their interaction strongly influenced parasitoid egg load. Host plant can affect parasitoid egg load either through its effect on host size, which consequently affects parasitoid size, or due to the absence of essential nutritional compounds or presence of toxic compounds. Similar observations of effect of host plant on parasitoid fecundity were reported for other mealybug parasitoid species such as Apo. lopezi (Souissi and Le Rü, 1997). However, A. kamali fecundity was not affected by the host plant of its host (Persad and Khan, 2007).
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C.M. Tanga et al. / Biological Control 65 (2013) 43–52
Fig. 4. Influence of the size of Anagyrus pseudococci females (tibia length) reared from Rastrococcus iceryoides maintained on different host plants on their egg loads. GLM based on Poisson distribution for 3-day-old females.
Table 4 Life table parameters of the parasitoid Anagyrus pseudococci ovipositing on 3rd nymphal instar Rastrococcus iceryoides reared on five host plant species. Host plant
rm
Ro
GT
Td
k
M. indica C. moschata P. aculeata C. cajan F. benjamina
0.111 ± 0.016b 0.147 ± 0.033a 0.108 ± 0.015c 0.086 ± 0.006d 0.057 ± 0.011e
13.018 ± 0.181c 21.753 ± 0.137a 15.453 ± 0.206b 09.851 ± 0.064d 5.476 ± 0.066e
23.12 ± 0.206d 20.95 ± 0.331e 25.35 ± 0.264c 26.60 ± 0.202b 29.83 ± 0.279a
6.245 ± 0.094d 4.715 ± 0.106e 6.418 ± 0.101c 8.060 ± 0.304b 12.16 ± 3.762a
1.117 ± 0.022b 1.158 ± 0.052a 1.114 ± 0.020b 1.090 ± 0.006c 1.059 ± 0.011d
Means ± SE within column followed by the same letter are not significantly different (Student–Newman–Keuls test, a = 0.05). rm, Jackknife estimate of the intrinsic rate of increase (female eggs/female/day); Ro, net reproductive rate (female offspring/female/generation); GT, mean generation time (days); Td = doubling time (days); k, finite rate of increase for population (female offspring/female/day).
Another important parameter of the parasitoid fitness that varied across host plants used in the current study was the longevity. One of the most important factors that determine parasitoid lon-
gevity is wasp size, which is a function of host size. Other factors include a wasp’s access to the host for oviposition. In the present study, longevity of host-deprived A. pseudococci females varied
C.M. Tanga et al. / Biological Control 65 (2013) 43–52
among host plants. The indirect effect of host plant on parasitoid longevity, mediated through the direct effect of the host plant on the quality of the parasitoid’s host, has also been reported for related parasitoids (Persad and Khan, 2007; Souissi and Le Rü, 1997). Interestingly, for the oviposting females observed in this study, longevity was shortest and longest when A. pseudococci females were offered hosts on butternut squash and weeping fig, respectively. Females offered a host maintained on butternut squash actively oviposited, leading to an average lifetime fecundity of 35.1 eggs/female, while those offered hosts maintained on weeping fig had the lowest lifetime fecundity (16.2 eggs/female). Presumably, female wasps perceive R. iceryoides on weeping fig to be low quality hosts, resulting in egg resorption, the energy and materials of which are used to increase the female’s longevity. Increased longevity due to egg resorption has been reported for other parasitoid species (Ramadan et al., 1995). High reproductive potential is among the criteria used for the selection of natural enemies (van Lenteren, 1986). Based on the demographic parameters of A. pseudococci obtained in the current study, wasps reared on butternut squash were superior to those of weeping fig. Similar findings of the effect of host plant on parasitoid’s demographic parameters were reported for Apo. lopezi (Souissi and Le Rü, 1997). It has been postulated that a parasitoid is considered to be an efficient biocontrol agent if, among other criteria, its intrinsic rate of natural increase is the same as or greater than that of the pest (Bigler, 1989; van Lenteren and Woets, 1988). Both the net reproductive rate and the finite rate of increase of A. pseudococci reported in the current study were lower than those reported for its target pest, R. iceryoides, when the latter was reared on the same host plant used in this study (C. Tanga, unpub.). Also, the parasitoid generation and doubling times were longer than those of the pest (C. Tanga, unpub.). This suggests that although A. pseudococci could contribute to the suppression of this invasive pest, it will not be able to control the expanding population of R. iceryoides when alone. This calls for exploration for co-evolved natural enemies from the pest’s aboriginal home of India for introduction into Africa, an activity that has been undertaken and the results of which are reported in C. Tanga (unpub.). In light of our results, A. pseudococci reared on hosts maintained on butternut squash, mango, and Jerusalem thorn is more profitable in terms of mass rearing of this parasitoid, in terms of all the fitness traits evaluated. Also, the same host plants were found to be suitable for development of the host, R. iceryoides (C. Tanga, unpub). However, mango and Jerusalem thorn are leafy plants and require exposure to direct sunlight for their maintenance. Thus screen houses will be needed if the host and the parasitoid are to be reared on these plants. On the other hand, butternut squash fruit can be maintained in the laboratory and does not require a large space, either for R. iceryoides or parasitoid colony maintenance. Therefore, butternut squash fruit is an ideal host for the parasitoid mass production. The finding also has significant implications for management of this pest on the target host plant, mango. Jerusalem thorn is an important ornamental shade plant used by growers in the vicinity of mango. Therefore, this plant will serve as a refuge to conserve and augment the A. pseudococci population ahead of infestation build-up of R. iceryoides in mango orchards.
Acknowledgments This research was supported by a grant from the German Federal Ministry for Economic Cooperation and Development (BMZ) to the icipe African Fruit Fly Programme (AFFP) and a student fellowship to the senior author by the German Academic Exchange Service (DAAD). We are grateful to Ms Dolorosa Osogo, Dr R. Cope-
51
land and Prof. Clarke H. Scholtz for their comments on the earlier version of the manuscript.
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Contents lists available at SciVerse ScienceDirect
Biological Control journal homepage: www.elsevier.com/locate/ybcon
Effect of host plant on bionomic and life history parameters of Anagyrus pseudococci (Hymenoptera: Encyrtidae), a parasitoid of the mango mealybug Rastrococcus iceryoides (Homoptera: Pseudococcidae) Chrysantus M. Tanga a,b, Samira A. Mohamed a,⇑, P. Govender b, Sunday Ekesi a a b
International Centre of Insect Physiology and Ecology (ICIPE), P.O. Box 30772-00100, Nairobi, Kenya Department of Zoology and Entomology, University of Pretoria, Pretoria, South Africa
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
" Mealybug acceptability by Anagyrus
a r t i c l e
i n f o
Article history: Received 17 December 2011 Accepted 19 November 2012 Available online 28 November 2012 Keywords: Host plants Anagyrus pseudococci Host acceptability Host suitability Life table parameters
Wasp’s larva.
Wasp’s eggs.
Mummified mealybug with exit hole.
Emerge adult wasp Time = 24 h: Wasps are reproductively active, ready to oviposit.
Parasitized mealybug.
Survival from egg to adult.
Time = 72 h: Wasps are released and females oviposit on mealybug-infested plants.
Cord
100
80
Handle
a ab
ab b c
Sleeve
% parasitism
pseudococci mirrored that of the host suitability. " Performance of A. pseudococci varied significantly with the host plant species. " Life table parameters were superior in parasitoids reared on mealybugs maintained on butternut squash. " These parameters were inferior in parasitoids reared on mealybugs maintained on weeping fig.
60
40
20 Cage
Infested plants (five host plant species were used, see graph on the right) in the screen house. Time = 24 h: Parasitoids removed monitored for parasitism.
0 C. moschata P. aculeata
M. indica Host plant
C. cajan
F. benjamina
Percentage parasitism on different host plants.
a b s t r a c t Anagyrus pseudococci Girault is a solitary koinobiont endoparasitoid of several mealybug species. The effect of five host plants (Mangifera indica L., Cucurbita moschata Duchesne, Parkinsonia aculeata L., Cajanus cajan L. and Ficus benjamina Roxb.) on host acceptability for oviposition and suitability for immature development of this parasitoid in the invasive mango mealybug Rastrococcus iceryoides Green were investigated. Effect of host plant on fitness traits (parasitoid size, egg load and longevity) and life table parameters were also assessed. Although A. pseudococci accepted the mealybug regardless of the host plant, the level of acceptability varied significantly. Percentage of parasitized nymphs was higher on C. moschata, followed by P. aculeata and M. indica, while it was lowest on F. benjamina. Host suitability was also strongly affected by the host plant and largely mirrored host acceptability for all the parameters evaluated. Female wasps reared on mealybugs maintained on C. moschata and P. aculeata were bigger and more fecund, while those reared from mealybugs maintained on F. benjamina were of inferior quality with regard to all fitness parameters evaluated. A. pseudococci achieved a greater intrinsic rate of natural increase (rm), net reproductive rate (Ro) and finite rate of increase (k) on mealybugs maintained on C. moschata and P. aculeata. In addition, the wasp had a shorter mean generation time (G) and population doubling time (Td) on mealybugs maintained on C. moschata. The reverse was true for those maintained on F. benjamina. The findings are discussed in view of improvement of laboratory mass rearing, as well as field enhancement of the parasitoid performance. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction ⇑ Corresponding author. Fax: +254 20 8632001. E-mail address: [email protected] (S.A. Mohamed). 1049-9644/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.biocontrol.2012.11.003
The mango mealybug, Rastrococcus iceryoides Green (Hemiptera: Pseudococcidae), an important pest of mango, Mangifera
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C.M. Tanga et al. / Biological Control 65 (2013) 43–52
indica L. (Anacardiaceae), is native to southern Asia. The pest was accidentally introduced into Africa (mainly Tanzania, Kenya and northern Malawi) where it has become a serious pest of mango, several other food crops, ornamental plants and forest trees (Luhanga and Gwinner, 1993; Williams, 1989). In Tanzania, Kenya and Malawi, mango losses can range from 30% to complete crop failure in unmanaged orchards (CABI, 2000; C. Tanga, unpub.). Both adults and nymphs cause direct damage by sucking sap from fruits, tender leaves, petioles, inflorescences and young shoots. Indirect damage is caused by excretion of honeydew on which sooty mould develops reducing the photosynthetic activity of the infested plant. Heavily infested leaves turn yellow and gradually dry out while infestation on inflorescences results in their excessive shedding, resulting in reduced fruit set. Severely infested fruits drop prematurely, and the few that reach maturity will be contaminated with honeydew and sooty mould, which greatly reduces their market value. Additionally, honeydew excreted by mealybugs attracts ants, which may interfere with oviposition by the fruit fly parasitoids Fopius arisanus (Sonan) and Diachasmimorpha longicaudata (Ashmead) that were introduced into Africa for biological control of fruit flies and other major pests of mango. In Tanzania where R. iceryoides is widely distributed across several agroecological zones, this pest has become a major target for insecticidal sprays on mango, in addition to pruning and burning of infested plant parts (C. Tanga, unpub.; Willink and Moore, 1988). Apart from health and environmental hazards, chemical pesticide applications do not provide adequate control of mealybugs and the high cost or unavailability of pesticides have led some African mango growers to cut down mango trees, or completely abandon their cultivation. Mealybug populations may increase rapidly, causing severe damage when introduced to new areas where natural enemies are scarce. For example, the mango mealybug, Rastrococcus invadens Williams (Homoptera: Pseudococcidae), a species closely related to R. iceryoides, caused a marked decrease in mango production across West and Central Africa. However, it was brought under control by release of two encyrtid wasps, Gyranusoidea tebygi Noyes and Anagyrus mangicola Noyes, from India (Bokonon-Ganta et al., 2002; Neuenschwander et al., 1994). Another example is the South American cassava mealybug, Phenacoccus manihoti Matile-Ferrero (Homoptera: Pseudococcidae) that threatened the livelihoods of millions of Africans relying on cassava as one of their staple foods (Herren and Neuenschwander, 1991; Zeddies et al., 2000). This pest has been suppressed and yields increased by the introduction and release of coevolved parasitoids (e.g., Apoanagyrus lopezi De Santis (Hymenoptera: Encyrtidae)) from South America (Herren et al., 1987; Neuenschwander, 2001). Yet another example is Planococcus kenyae Le Pelley that was a major pest of arabica coffee in Kenya during the 1920s and 1930s. The introduction of several parasitoid species (especially Anagyrus sp. near kivuensis Compere) from Uganda in 1938 reduced the status of this mealybug to a minor pest (Le Pelley, 1943), and has remained so ever since. For R. iceryoides, classical biological control is likely the most suitable management option in Africa, considering that the pest is of no economic significance to mango production in its native home range, where several natural enemies cause up to 40% parasitism (Narasimham and Chacko, 1988; Tandon and Srivastava, 1980). However, the composition and efficacy of indigenous natural enemies that may have formed new associations with this pest in Kenya and Tanzania, need to be established before considering any such introduction. A survey carried out between 2008–2009 in Kenya and Tanzania (C. Tanga, unpub.) reported six Encyrtid parasitoid species (Anagyrus aegyptiacus Moursi, Leptomastix dactylopii Howard, L. tecta Prinsloo, Agarwalencyrtus citri Agarwal, Aenasius longiscapus Compere and Anagyrus pseudococci Girault) attacking R. iceryoides on different host plants, with A. pseudococci
accounting for 95% parasitism. However, information regarding the effects of host plant on the preference and performance of A. pseudococci is lacking. The objective of the present study was therefore to: (1) evaluate the effect of five host plants on acceptability for oviposition and suitability of R. iceryoides for the immature development of A. pseudococci, and (2) assess the effect of the host plant on the fitness traits, and the life table parameters of the parasitoid. 2. Materials and methods 2.1. Host plants Seedlings of four host plants, namely mango (M. indica L.) (Anacardiaceae), Jerusalem thorn (Parkinsonia aculeata L.) (Fabaceae), weeping fig (Ficus benjamina Roxb.) (Moraceae), and pigeon pea (Cajanus cajan (L.) Millsp. (Fabaceae), in addition to the rearing host butternut squash fruit (Cucurbita moschata Duchesne) (Cucurbitaceae) were used in this study. These plants were selected because of their known association with the invasive pest in the field (C. Tanga, unpub.). The plant seedlings were purchased from a commercial nursery and maintained in screen house cage (287 cm height 256 cm length 252 cm width) free from other pests and pesticides at the International Centre of Insect Physiology and Ecology (icipe), Nairobi, Kenya. In the screen house the plants were maintained in plastic containers (35 cm height 29 cm top diameter 20 cm bottom diameter or 19 cm height 21 cm top diameter 12.8 cm bottom diameter) and were fertilized with farmyard manure and watered every two days. 2.2. Host insect The colony was initiated from a cohort of 300 adult R. iceryoides that were collected from mango orchards in coastal Kenya in February 2008. In the laboratory, the insects were reared on mature fruit of butternut squash for approximately 20 generations before the start of the experiment. For colony maintenance, weekly or biweekly infestation of 10–20 butternut squash fruits was carried out, and after every 6 months, wild mealybugs from the field were introduced to maintain genetic diversity. Prior to the experiments, 30 adult female mealybugs with wellformed ovisacs obtained from the stock colony were placed on each host plant maintained in large cages (30 cm length 30 cm width 60 cm height) with screened sides and glass tops. Cultures were maintained under screen house conditions at 22.3 ± 1.07 °C, 40–75% relative humidity (RH), and a photoperiod of 12L: 12D for at least 3 generations to allow them to adapt to their new host. 2.3. Parasitoids The parasitoid colony was initiated from a cohort of 93 A. pseudococci wasps (72 $ and 21 #), which were collected from the same host plant and location as that of the host insect, and brought to the laboratory at icipe. They were reared on third instar nymphs and adults of R. iceryoides maintained on butternut squash fruits in Perspex cages (30 cm length 30 cm width 30 cm height). The parasitoids were provided with streaks of pure honey as food and the colony was maintained at ambient condition (26– 28 °C, 45–60% RH, and photoperiod of 12L:12D). Preceding the experiment, wasps were collected from the stock colony and maintained under screen house conditions using the same procedure as described for R. iceryoides above. Every two weeks, 20 A. pseudococci adult females were released into Perspex clip cages (12 cm length 12 cm width 12 cm height) contain-
C.M. Tanga et al. / Biological Control 65 (2013) 43–52
ing a host plant infested with an average of 250 third nymphal instar R. iceryoides. Two small holes (1 cm in diameter) were made on the top and bottom of the cages to accommodate the plant stem. Prior to the experiment, parasitoids were maintained on a host plant species for at least three generations (from egg to adult) to allow them to adapt and to remove maternal effects (Lacey, 1998), thus minimizing the effects of associative learning. At the start of the experiment, 12 days after the release of the parasitoids, mummies were collected from the host plants and individually placed in gelatin capsules. Mummies (<24 h) were observed twice daily and newly-emerged parasitoids were used for the experiments. 2.4. Bioassays 2.4.1. Effect of host plant on R. iceryoides acceptability for oviposition by A. pseudococci Prior to the introduction of the parasitoid wasps, seedlings of test host plants and butternut squash fruits infested with mealybugs were obtained from the stock culture and placed in test cages (30 cm length 30 cm width 30 cm height). Only 100 R. iceryoides nymphs of the suitable host stage (third nymphal instar) were retained on each host plant. Thereafter, 10 mated, naive and fed 3day-old wasps (5$ and 5#) were introduced into the cages. The parasitoids were left to forage and oviposit for 24 h at the ambient conditions described above, after which, the parasitoids were removed and the exposed mealybugs allowed to continue feeding on their respective host plant for two days. All surviving R. iceryoides were later dissected in phosphate buffer solution under a stereomicroscope and the number of nymphs containing at least one parasitoid egg recorded. The number of superparasitized nymphs and encapsulated eggs was also noted and the percentage of parasitized nymphs determined. This experiment was replicated eight times for each plant species. 2.4.2. Effect of host plant on R. iceryoides suitability for development of immature A. pseudococci To further test for the effect of host plant on R. iceryoides suitability for development of immature A. pseudococci, additional sets of 100 third nymphal instars of R. iceryoides were exposed to the parasitoids. Number and status of the parasitoid used, and the duration and conditions of the exposure were the same as those described above for host acceptability, except in this experiment the hosts were allowed to develop till mummification and parasitoid eclosion. Mummies from each host plant were collected, counted and stored separately in transparent plastic cups (4 cm height, 5.5 cm base diameter and 7.5 cm top diameter) until parasitoid emergence. The number and sex of emerging wasps were recorded and the length of their left hind tibia measured. Percentage of mummified nymphs was computed based on the initial number of exposed hosts (100 nymphs) while the percent parasitoid emergence was computed based on the number of the mummified nymphs for each host plant. The female offspring were computed as a percentage of the emerging females over the total number of emerging wasps. The experiment was replicated eight times for each host plant. 2.5. Effect of host plant on some fitness traits of A. pseudococci 2.5.1. Female body size and egg load Mated A. pseudococci females, newly emerged from R. iceryoides on different host plants, were held separately with access to pure honey and water in Perspex cages (15 cm length 12 cm width 12 cm height). Females were dissected at ages 0, 1, 3 and 9 days, and egg loads were recorded. Each female wasp was placed in a drop of phosphate buffer solution on a glass slide.
45
The specimen was dissected and observed at 35 with a Leica EZ4D stereomicroscope with an integral digital camera [Leica Microsystems (Switzerland) Limited]. Only mature oocytes with a well-defined neck connecting the two bulbs were counted. After the number of mature eggs was recorded, the wasps’ left hind tibia (LHT) was measured to the nearest 0.0025 mm under the same magnification.
2.5.2. Effect of host plant on parasitoid adult longevity of nonovipositing wasps Four groups of 20 parasitoid wasps (10$, 10#) that had emerged from mealybug-infested host plants on the same day (08:00– 11:00 h) and fed on pure honey were aspirated and maintained in a cage (20 20 20 cm) for each host plant separately. The cages were kept under the same ambient conditions as above and the parasitoids monitored daily. Dead wasps were removed and their longevity recorded. Thereafter, their left hind tibia lengths were measured as described above. Individuals that drowned in excess solution were excluded.
2.6. Life table experiment and calculation of demographic parameters One pair of newly emerged and fed A. pseudococci adults originating from each host plant culture was introduced into a test Perspex clip cage described above, containing 20 nymphs of the suitable host stage on their respective host plants. Parasitoids were left to forage and oviposit for 24 h. Adult female parasitoids were removed and transferred to another infested host plant with 20 new mealybugs each day for the entire life span of the parasitoid female. These observations were replicated using 10 females for each host plant. Upon dying, the left hind tibia of each female was measured as described above. For each exposure, mummified mealybugs were carefully removed and transferred into transparent plastic cups (4 cm height, 5.5 cm base diameter and 7.5 cm top diameter). Mummies were checked twice a day for parasitoid emergence. Parameters recorded during this experiment were days to mummification, developmental time, the number of offspring emerging, total number of offspring for each parasitoid female during its lifetime, and female longevity. Data obtained from this experiment were used to generate demographic growth parameters: the intrinsic rate of natural increase (rm), net reproductive rate (Ro), mean generation time (G), population doubling time (DT) and finite rate of increase (k).
2.7. Statistical analysis The percentages of parasitized nymphs, mummified mealybugs and emerged adult wasps, female offspring and lifetime fecundity were analyzed using one-way analysis of variance using a general linear model (SAS Institute, 2010). Developmental time, adult wasp size and longevity of host-deprived wasps were analyzed using two-way ANOVA, with host plant and sex as factors. Also egg load and longevity of ovipositing females were analyzed using two-way ANOVA with host and female age as factors. Count data were log10 transformed, while the percentages were arcsine transformed before statistical analysis (Sokal and Rohlf, 1981). When the ANOVA was significant, means were separated using the Student–Newman–Keuls (SNK) test. Demographic parameters—intrinsic rate of increase (rm), net reproductive rate (Ro), mean generation time (GT), doubling time (Td) and finite rate of increase (k)—were calculated using the Jackknife procedure described by Hulting et al. (1990).
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3. Results 3.1. Effect of host plant on R. iceryoides acceptability for oviposition and suitability of the immature development of A. pseudococci An acceptable host was defined as a host containing at least one parasitoid egg. A. pseudococci females accepted R. iceryoides regardless of the host plant on which it was cultured. However, acceptability varied significantly among host plants, with the highest percentage of parasitized nymphs occurring on mealybugs cultured on butternut squash, Jerusalem thorn and mango (Table 1). Superparasitism was recorded only on hosts maintained on butternut squash, Jerusalem thorn and mango, with an average percentage superparasitism of 2.4 ± 1.02, 1.7 ± 0.77 and 1.1 ± 0.73, respectively. No encapsulation of eggs or larvae by R. iceryoides was recorded on any of the tested host plants. Host suitability (as measured by number of days to mummification, percentage of mummified hosts, percentage eclosion of adult parasitoids, percentage female offspring and preimaginal developmental time) were strongly influenced by host plant, and the trends observed largely mirrored host acceptability (Table 1). However, sex ratio of the parasitoid offspring was female-biased, irrespective of the host plant (Table 1). A. pseudococci completed development on R. iceryoides maintained on all of the host plant species. However, the duration of preimaginal development time varied considerably across host plants (Table 2). Both sexes took significantly less time to emerge from butternut squash-reared mealybugs, and longer from weeping fig-reared mealybugs (Table 2), and the same trend was observed for the overall mean developmental time of the parasitoid (Table 2). Females took significantly longer to develop than males
for all host plants tested except on butternut squash (Table 2). Males emerged approximately one day earlier than females for all host plants except Jerusalem thorn (Fig. 1). 3.2. Effect of the host plant on parasitoid fitness parameters 3.2.1. Parasitoid adult size Body size of both male and female wasps, as measured by left hind tibia length, was strongly influenced by the host plant (Table 1). Both sexes attained a significantly larger body size when the hosts were maintained on butternut squash and Jerusalem thorn, while those that emerged from hosts maintained on weeping fig were significantly smaller (Table 1). The data also revealed that female wasps emerging from R. iceryoides were significantly larger than males for all host plants tested (Table 1). 3.3. Parasitoid adult longevity 3.3.1. Longevity of ovipositing female (host provided), and lifetime fecundity The survival of female wasps reared on, and offered, hosts maintained on any of the host plants followed a type I survivorship curve (Fig. 2). However, the overall mean life span of A. pseudococci females was significantly longer when the parasitoid was reared on, and offered, hosts maintained on weeping fig (16.8 ± 0.66 day) and pigeon pea (15.2 ± 0.36 day) and it was shorter when the parasitoid was reared on, and offered, hosts maintained on butternut squash (11.4 ± 0.72 days). However, female parasitoids reared on and offered hosts maintained on mango and Jerusalem thorn had a similar life expectancy.
Table 1 Effect of five host plants on biological parameters of Anagyrus pseudococci produced from 3rd nymphal instar Rastrococcus iceryoides (values represent mean ± SE). Host plant
M. indica C. moschata P. aculeata C. cajan F. benjamina F df P
Parasitized nymph (%)
Time to mummification (days)
Host mummified (%)
Adult eclosion Female (%) offspring (%)
Tibia length [mm] Female
Male
F
df
73.13 ± 2.48ab 79.13 ± 1.19a 74.25 ± 1.52ab 70.75 ± 1.52b 58.75 ± 2.17c 16.36 4, 35 <0.0001
10.47 ± 0.48bc 9.40 ± 0.38c 9.93 ± 0.36bc 11.20 ± 0.38b 12.73 ± 0.34a 11.03 4, 70 <0.0001
81.5 ± 3.02ab 89.75 ± 1.46a 83.75 ± 3.73ab 77.38 ± 3.91b 48.38 ± 2.67c 23.06 4, 35 <0.0001
68.25 ± 2.17ab 83.0 ± 1.72a 74.50 ± 4.47ab 67.25 ± 5.04b 38.63 ± 2.60c 26.72 4, 35 <0.0001
0.443 ± 0.002bA 0.452 ± 0.002aA 0.450 ± 0.003abA 0.443 ± 0.003bA 0.400 ± 0.002cA 76.47 4, 118 <0.0001
0.371 ± 0.002cB 0.415 ± 0.006aB 0.385 ± 0.004bB 0.360 ± 0.003cB 0.304 ± 0.003 dB 100.64 4, 46 <0.0001
393.74 56.0 172.94 263.46 523.61
1, 1, 1, 1, 1,
59.64 ± 1.01ab 65.79 ± 1.45a 62.25 ± 2.75ab 59.53 ± 1.70b 57.07 ± 2.33c 2.9 4, 35 0.0359
Statistics ($ and #) P 31 37 30 33 33
<0.0001 <0.0001 <0.0001 <0.0001 <0.0001
Means within the same column followed by the same lower letters and within the same row followed by the same upper case letters are not significantly different (Student– Newman–Keuls test, a = 0.05).
Table 2 Egg-adult developmental time and lifetime fecundity (mean ± SE) of female A. pseudococci on third instar R. iceryoides reared on five different host plant species. Host plant
M. indica C. moschata P. aculeata C. cajan F. benjamina F df P
Mean developmental time (in days ± s.e.)
Statistics ($ and #)
#
$
F
df
18.81 ± 0.19bB 15.52 ± 0.25dA 17.22 ± 0.33cB 19.41 ± 0.17bB 21.50 ± 0.38aB 67.77 4, 79 < 0.0001
19.83 ± 0.12cA 15.76 ± 0.14eA 19.08 ± 0.14dA 20.68 ± 0.21bA 23.73 ± 0.17aA 329.14 4, 166 < 0.0001
21.86 0.80 37.04 15.90 38.59
1, 1, 1, 1, 1,
Combined developmental time (#$)
Number of life time progeny production
19.51 ± 0.12c 15.68 ± 0.13e 18.47 ± 0.19d 20.23 ± 0.17b 23.03 ± 0.24a 253.63 4, 250 < 0.0001
27.6 ± 1.00bc 34.6 ± 1.10a 30.1 ± 1.66ab 25.2 ± 1.50bc 22.2 ± 3.17c 6.50 4, 45 0.0003
P 49 61 53 46 36
< 0.0001 0.3759 < 0.0001 0.0002 < 0.0001
Means within the same column followed by the same lower letters and within the same row followed by the same upper case letters do not differ significantly by StudentNewman-Keuls test (P < 0.05).
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Male
M. indica
14
Female
Mean number of emerged wasps
10
Female
8 6 4 2
26
27
12 10 8 6 4 2
0
0 19
20
21
22
23
24
25
26
27
20
21
22
Days from oviposition
23
24
25
Days from oviposition
C. moschata
9
Male
F. benjamina
Male
Female
Female
8 Mean number of emerged wasps
18 16 Mean number of emerged wasps
Male
14
12 Mean number of emerged wasps
C. cajan
16
14 12 10 8 6 4
7 6 5 4 3 2 1
2
0
0 15
16
17
18
19
20
21
22
23
22
24
23
24
25
26
27
28
29
Days from oviposition
Days from oviposition
P. aculeata
Male
Female
16
mean number of emerged wasps
14 12 10 8 6 4 2 0 17
18
19
20
21
22
23
24
25
26
Days from oviposition
Fig. 1. Anagyrus pseudococci emergence pattern on various host plants.
3.3.2. Longevity of non-ovipositing A. pseudococci female (hostdeprived) Host plant and parasitoid sex had a strong influence on the life expectancy of wasps (Table 3). Females lived significantly longer when maintained on hosts reared on butternut squash and Jerusalem thorn (Table 3). Longevity of males was comparable across all the host plants, except for weeping fig on which it was shorter. When males and females were reared on the same host plant, females lived significantly longer than males when reared on hosts maintained on butternut squash, Jerusalem thorn and weeping fig. Longevity was comparable for both sexes when the wasps were reared on mango and pigeon pea (Table 3).
3.3.3. Effect of host plant and female age on egg load Host plant of R. iceryoides and the parasitoid female age had a significant effect on parasitoid egg load, as did their interaction (F = 6.86; df = 8, 322; P < 0.0001) (Fig. 3). Females reared on hosts maintained on butternut squash were significantly more fecund than those reared on hosts from other host plants, for all female age groups evaluated. Those reared on hosts maintained on weeping fig had significantly lower egg loads for all female age groups (Fig. 3). Among wasps reared on mealybugs maintained on the same host plants, three-day-old females had the highest egg load for all rearing host plants (Fig. 3). When egg load was regressed against female body size, it increased linearly for all host plants except pigeon pea (Fig. 4).
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squash, with 0.147 ± 0.033 females/female/day and 1.158 ± 0.052 females/female/day, respectively.
4. Discussion
Fig. 2. Survival curve of ovipositing females of Anagyrus pseudococci reared on different mealybug-infested host plants.
Table 3 Mean ( ± SEM) of longevity (in days) of host-deprived A. pseudococci wasps reared from five different host plant species and fed with pure honey. Host plant
M. indica C. moschata P. aculeata C. cajan F. benjamina F df P
Longevity (days)
Statistics ($ and #)
Female
Male
F
df
38.3 ± 2.44bcA 47.2 ± 1.74aA 41.8 ± 2.03abA 35.6 ± 2.84bcA 32.0 ± 3.04cA 5.60 4, 45 0.0010
34.4 ± 2.43aA 31.7 ± 2.51aB 31.1 ± 2.33aB 30.2 ± 2.64aA 17.6 ± 1.33bB 8.15 4, 45 <0.0001
1.28 25.75 11.97 1.94 18.84
1, 1, 1, 1, 1,
P 18 18 18 18 18
0.2721 <0.0001 0.0028 0.1808 0.0004
Means within the same column followed by the same lower case letters and within the same row followed by the same upper case letters are not significantly different (Student–Newman–Keuls test, a = 0.05).
3.4. Demographic growth parameters R. iceryoides host plants had a strong influence on various growth parameters of A. pseudococci (Table 4). Host plants significantly affected the intrinsic rate of natural increase (rm), population doubling time (Td) and finite rate of increase (k). Net reproductive rate (Ro) was 1.7, 1.4, 2.2 and 4.0 times higher for females reared and allowed to oviposit on butternut squash than those on mango, Jerusalem thorn, pigeon pea and weeping fig plants, respectively. Similarly, the intrinsic rate of natural increase (rm) was 1.3, 1.4, 1.7 and 2.6 times higher for females reared and allowed to oviposit on butternut squash than those on mango, Jerusalem thorn, pigeon pea and weeping fig plants, respectively. Population doubling time (Td) on butternut squash was 61.23% shorter than that on weeping fig, and the mean generation time (GT) was 2–9 days less on butternut squash compared to the other host plants. For example, the net reproductive rate (Ro) decreased from 21.753 ± 0.137 females/female/generation on butternut squash to 5.476 ± 0.066 females/female/generation on weeping fig, while the mean generation time (GT) increased from 20.95 ± 0.331 days on butternut squash to 29.83 ± 0.279 days on weeping fig. Also, doubling time (Td) increased from 4.715 ± 0.106 days on butternut squash to 12.16 ± 3.762 days on weeping fig. Both the intrinsic rate of natural increase (rm) and the finite rate of increase (k) reached their maximum on butternut
The effect of host plant on the preference and performance of the parasitoid has been well documented for many host/parasitoid systems (e.g., Barbosa et al., 1982; Souissi and Le Rü, 1997, 1998). Among the widely reported aspects of the host plant–parasitoid interaction is the effect of host plant on various phases of the parasitization process (Vet and Dicke, 1992; Vinson and Williams, 1991). In our study, although A. pseudococci females accepted R. iceryoides regardless of their host plants, they showed a marked preference for those reared on butternut squash, Jerusalem thorn and mango. Similar results of differential host acceptability were reported for related encyrtids in association with other mealybug species when cultured on different plant species (Souissi and Le Rü, 1997, 1998). The low acceptability of mealybugs reared on weeping fig in this study might have been related to the low quantity and quality of the volatiles (kairomones) emitted from the plant as a result of low feeding activity of the mealybugs on it. Another possible explanation is the inferior quality of the mealybugs in terms of their small size (C. Tanga, unpub.) as well as their poor nutritional quality, or a combination of the two factors. These factors either separately, or in combination, have been reported to have great influence on host acceptability by several other parasitoid species (e.g., Hulspas-Jordaan and van Lenteren, 1978). The differential host acceptability for oviposition by A. pseudococci recorded in this study was also reflected on the suitability of R. iceryoides for the parasitoid’s immature development in terms of time to mummification, and percentage of mummified nymphs and eclosed parasitoids. On weeping fig mealybugs (least accepted) not only was the average percentage of wasp eclosion lowest, but also only slightly more than one-third (38.6%) of the mummified nymphs formed on this plant produced wasps, suggesting that the majority of the parasitoid offspring were unable to complete their development in the mealybugs maintained on this host plant. This may have been due to the immune reaction mounted by the host against the parasitoid immature stages, although dissection revealed neither egg nor larval encapsulation. However, other forms of cellular or hormonal defenses may have been involved in mealybugs reared on this plant leading to mortality of immature stage of the wasps. A differential effect of host plant on mealybug suitability for parasitoid development was also reported by Souissi and Le Rü (1997) for Apo. lopezi when reared on cassava mealybug. The authors reported that the number of mummies/female parasitoid, number of emerged parasitoids, wasp survival and development time varied considerably among mealybug host plants. In a separate study using the same tritrophic system, Souissi et al. (1998) found that the percentage of mummified nymphs as well as the percentage of emerged parasitoids was significantly affected by the host plant of P. manihoti. The suitability of R. iceryoides maintained on butternut squash, Jerusalem thorn and mango was also evident in the higher number of female progeny produced when A. pseudococci was reared on these host plants. Female wasps were presumably able to assess host quality and deposited more fertilized eggs in more suitable hosts or, alternatively, mortality of female offspring was higher in lower quality hosts (those maintained on weeping fig). However, no dissection of uneclosed mummies was made to ascertain this. Host size is used by female parasitoids to predict the host-stage suitability for parasitoid offspring, and to adjust the sex ratio among different stages of the same host fed on the same host plant. For example, production of offspring female-biased sex ratio by A. pseudococci in larger hosts of other mealybug species has been reported by several
C.M. Tanga et al. / Biological Control 65 (2013) 43–52
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Fig. 3. Effect of host plant of the rearing host of Anagyrus pseudococci on the parasitoid egg load of females of different age groups. Bars with the same lower case letters are not significantly different among host plants for the same age group, and bars with the same upper case letters are not significantly different among age groups for the same host plant.
authors (e.g. Daane et al., 2004; Güleç et al., 2007). However, the sex ratio of Anagyrus kamali Moursi (Hymenoptera: Encyrtidae) reared on Maconellicoccus hirsutus Green (Homoptera: Pseudococcidae) was not affected by the host plants on which it was reared, although host body size varied significantly among the host plants (Persad and Khan, 2007). Studies have shown that the duration of parasitoid developmental time varies with the host’s nutritional history (e.g., Kouamé and Mackauer, 1991; Souissi and Le Rü, 1997). Our results agree with these findings as A. pseudococci developmental time varied with host plant. Similar results of the effect of host plant on the duration of the parasitoid developmental time were also reported for other encyrtid species such as Apo. lopezi (Souissi and Le Rü, 1997) and A. kamali (Persad and Khan, 2007). The difference in developmental time of A. pseudococci reported in our study may have been caused by variation in the quality of mealybugs reared on different host plants. Also host size has a significant effect on parasitoid developmental time. A positive correlation between host size and parasitoid development time was reported in several host–parasitoid associations (e.g., Ruberson et al., 1989; Vinson and Iwantsch, 1980), especially for idiobionts. However, for koinobionts, there is no general pattern between parasitoid developmental time and host size (Godfray, 1994). In the present study, A. pseudococci developmental time was shorter on larger hosts (those reared on butternut squash), than on smaller hosts (those reared on weeping fig). Contrary to our results, Souissi and Le Rü (1998) reported a positive correlation between developmental time and host size for Apo. lopezi when reared on P. manihoti maintained on different host plants. Shorter developmental time is a desirable trait. In the field it shortens the duration of exposure of parasitized mummies to predation and hyperparasitism, while in
the laboratory it enhances the mass rearing of parasitoids intended for augmentative releases. Adult wasp size is a strong indicator of parasitoid fitness (for review see Jervis and Copland, 1996), influencing the usefulness of the parasitoid for biological control (Godfray, 1994; van Lenteren et al., 2002). For example, wasp size of the closely related species A. kamali was found to be positively correlated with longevity, mating preference, fecundity, reproductive longevity, progeny emergence and sex-ratio (Sagarra et al., 2001). Parasitoid size is largely dependent on its host (e.g., Barratt and Johnstone, 2001; López et al., 2009), as it dictates the amount of nutrients available for the parasitoid’s developing larvae. Host size in turn, is dependent on the food quality of its host plant (Barbosa et al., 1982). Variation in wasp size of A. pseudococci reported in the current study can be explained by the host size differences, reflecting their nutritional history, being larger on butternut squash, mango and Jerusalem thorn, and smaller on weeping fig (C. Tanga, unpub.). Several factors influence parasitoid fecundity among which are female body size (Bernal et al., 2001) and age, and type and availability of food for female parasitoids, both during immature development and as adults (Baggen and Gurr, 1998; Idris and Grafius, 1995), as well as host food plant (Eben et al., 2000). In our study, host plant, female age, and their interaction strongly influenced parasitoid egg load. Host plant can affect parasitoid egg load either through its effect on host size, which consequently affects parasitoid size, or due to the absence of essential nutritional compounds or presence of toxic compounds. Similar observations of effect of host plant on parasitoid fecundity were reported for other mealybug parasitoid species such as Apo. lopezi (Souissi and Le Rü, 1997). However, A. kamali fecundity was not affected by the host plant of its host (Persad and Khan, 2007).
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Fig. 4. Influence of the size of Anagyrus pseudococci females (tibia length) reared from Rastrococcus iceryoides maintained on different host plants on their egg loads. GLM based on Poisson distribution for 3-day-old females.
Table 4 Life table parameters of the parasitoid Anagyrus pseudococci ovipositing on 3rd nymphal instar Rastrococcus iceryoides reared on five host plant species. Host plant
rm
Ro
GT
Td
k
M. indica C. moschata P. aculeata C. cajan F. benjamina
0.111 ± 0.016b 0.147 ± 0.033a 0.108 ± 0.015c 0.086 ± 0.006d 0.057 ± 0.011e
13.018 ± 0.181c 21.753 ± 0.137a 15.453 ± 0.206b 09.851 ± 0.064d 5.476 ± 0.066e
23.12 ± 0.206d 20.95 ± 0.331e 25.35 ± 0.264c 26.60 ± 0.202b 29.83 ± 0.279a
6.245 ± 0.094d 4.715 ± 0.106e 6.418 ± 0.101c 8.060 ± 0.304b 12.16 ± 3.762a
1.117 ± 0.022b 1.158 ± 0.052a 1.114 ± 0.020b 1.090 ± 0.006c 1.059 ± 0.011d
Means ± SE within column followed by the same letter are not significantly different (Student–Newman–Keuls test, a = 0.05). rm, Jackknife estimate of the intrinsic rate of increase (female eggs/female/day); Ro, net reproductive rate (female offspring/female/generation); GT, mean generation time (days); Td = doubling time (days); k, finite rate of increase for population (female offspring/female/day).
Another important parameter of the parasitoid fitness that varied across host plants used in the current study was the longevity. One of the most important factors that determine parasitoid lon-
gevity is wasp size, which is a function of host size. Other factors include a wasp’s access to the host for oviposition. In the present study, longevity of host-deprived A. pseudococci females varied
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among host plants. The indirect effect of host plant on parasitoid longevity, mediated through the direct effect of the host plant on the quality of the parasitoid’s host, has also been reported for related parasitoids (Persad and Khan, 2007; Souissi and Le Rü, 1997). Interestingly, for the oviposting females observed in this study, longevity was shortest and longest when A. pseudococci females were offered hosts on butternut squash and weeping fig, respectively. Females offered a host maintained on butternut squash actively oviposited, leading to an average lifetime fecundity of 35.1 eggs/female, while those offered hosts maintained on weeping fig had the lowest lifetime fecundity (16.2 eggs/female). Presumably, female wasps perceive R. iceryoides on weeping fig to be low quality hosts, resulting in egg resorption, the energy and materials of which are used to increase the female’s longevity. Increased longevity due to egg resorption has been reported for other parasitoid species (Ramadan et al., 1995). High reproductive potential is among the criteria used for the selection of natural enemies (van Lenteren, 1986). Based on the demographic parameters of A. pseudococci obtained in the current study, wasps reared on butternut squash were superior to those of weeping fig. Similar findings of the effect of host plant on parasitoid’s demographic parameters were reported for Apo. lopezi (Souissi and Le Rü, 1997). It has been postulated that a parasitoid is considered to be an efficient biocontrol agent if, among other criteria, its intrinsic rate of natural increase is the same as or greater than that of the pest (Bigler, 1989; van Lenteren and Woets, 1988). Both the net reproductive rate and the finite rate of increase of A. pseudococci reported in the current study were lower than those reported for its target pest, R. iceryoides, when the latter was reared on the same host plant used in this study (C. Tanga, unpub.). Also, the parasitoid generation and doubling times were longer than those of the pest (C. Tanga, unpub.). This suggests that although A. pseudococci could contribute to the suppression of this invasive pest, it will not be able to control the expanding population of R. iceryoides when alone. This calls for exploration for co-evolved natural enemies from the pest’s aboriginal home of India for introduction into Africa, an activity that has been undertaken and the results of which are reported in C. Tanga (unpub.). In light of our results, A. pseudococci reared on hosts maintained on butternut squash, mango, and Jerusalem thorn is more profitable in terms of mass rearing of this parasitoid, in terms of all the fitness traits evaluated. Also, the same host plants were found to be suitable for development of the host, R. iceryoides (C. Tanga, unpub). However, mango and Jerusalem thorn are leafy plants and require exposure to direct sunlight for their maintenance. Thus screen houses will be needed if the host and the parasitoid are to be reared on these plants. On the other hand, butternut squash fruit can be maintained in the laboratory and does not require a large space, either for R. iceryoides or parasitoid colony maintenance. Therefore, butternut squash fruit is an ideal host for the parasitoid mass production. The finding also has significant implications for management of this pest on the target host plant, mango. Jerusalem thorn is an important ornamental shade plant used by growers in the vicinity of mango. Therefore, this plant will serve as a refuge to conserve and augment the A. pseudococci population ahead of infestation build-up of R. iceryoides in mango orchards.
Acknowledgments This research was supported by a grant from the German Federal Ministry for Economic Cooperation and Development (BMZ) to the icipe African Fruit Fly Programme (AFFP) and a student fellowship to the senior author by the German Academic Exchange Service (DAAD). We are grateful to Ms Dolorosa Osogo, Dr R. Cope-
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land and Prof. Clarke H. Scholtz for their comments on the earlier version of the manuscript.
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