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Evaluation of susceptibility of varieties of cowpea to Callosobruchus maculatus (F.) and Callosobruchus subinnotatus (Pic.) (Coleoptera: Bruchidae)
J. stored Prod. Res. Vol. 29, No. 3, pp. 207-213, Printed in Great Britain. All rights reserved
1993
0022-474X/93 $6.00 + 0.00 Copyright cc 1993 Pergamon PressLtd
EVALUATION OF SUSCEPTIBILITY OF VARIETIES OF COWPEA TO CALLOSOBRUCHUS MACULATUS (F.) AND CALLOSOBRUCHUS SUBINNOTATUS (PIC.) (COLEOPTERA: BRUCHIDAE) GEORGE N. MBATA Institut fur Phytopathologie und Angewandte Zoologie, Vorratsschutx, Justus Liebig Universitat, Alter Steinbacher Weg 44, D6300 Giessen, Germany (Received for publication 11 March 1993) Ahatraet-Twenty-six varieties of cowpea some of which are among those distributed for international trials by the International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria were evaluated in comparison with Ife Brown, a known susceptible variety, for susceptibility to Caiiosobruchus maculatus (F.) and CaNosobruchussubinnotatus (Pk.). The most susceptible varieties to C. maculatus were TVu 310, TVu 2896, TVu 9944 and TVu 9836. Some varieties were found to be moderately susceptible to C. subinnotutus and these included TVu 18. TVu 36. TVu 2896 and IT81D-1020. Most varieties were found to be more susceptible to C. maculatus than to C. subinnotatus. However, the median developmental periods of the bruchids on the cowpea varieties were comparable in most cases. Some varieties were found to be consistently less susceptible to attack by the two bruchids and these included TVu 2027, IT81D-1064, ITIID-1157 and IT86-498. The weight losses due to infestatiou by the bruchids correlated very strongly to the susceptibility indices (SI) of the varieties to the bruchids. The losses in weight caused by C. subinnotatus were comparable with those caused by C. mnculatus even though the number of Fl adults of C. macufutus that emerged from the varieties was higher than those of C. subinnotatus. Key words-Bruchidae,
Cullosobruchus, cowpea, varieties, infestation, susceptibility.
INTRODUCTION
Cowpea, Vigna unguiculutu Walp. is the most important and widely consumed grain legume in Nigeria and indeed, most other African countries (Singh and Jackai, 1985). It provides more than half the plant protein in human diets and it is the key source of protein for the poorest sector of many developing countries (Rachie, 1985). Species of the genus Cullosobruchus are very damaging pests of stored legumes (Southgate, 1964; 1965; 1978). Apart from the most widely distributed species such as Callosobruchus macufutus (F.) and Callosobruchus chinensis (L.), others such as Callosobruchus rhodesianus (Pit.) and Callosobruchus subinnotatus (Pit.) are restricted to south Africa and west Africa, respectively, where they cause considerable damage to stored legumes (Prevett, 1966; Giga and Smith, 1987; Mbata, 1991a). C. maculatus and C. subinnotatus are widely distributed in subsahelian and humid west Africa where both species are found attacking cowpea and bambarra groundnuts (Booker, 1967; Prevett, 1966; Mbata, 1991a). The cowpea storage beetle, C. macularus is the most important pest of stored cowpea seeds. Often, after 3-5 months of storage, 100% infestation of cowpea by this bruchid is had observed observed in west Africa (Singh, 1977). Monge (personal communication) C. subinnotatus on cowpea while Mbata (1991b) established that cowpea can sustain the development of this bruchid. Ndlovu and Giga (1988) found that improved varieties of cowpea had varying susceptibilities to C. rhodesianus and that the level of resistance of these varieties to C. rhodesiunus was found to be lower than that of C. macufutus. It is not known if different varieties of cowpea would have different susceptibilities to C. subinnotatus, or if the susceptibilities of the cowpea varieties to C. subinnotatus would differ from those for C. maculatus. Equally Dick and Credland (1986a, 1986b) found that TVu 2027, a cowpea variety from which most of the improved varieties were derived was resistant to C. maculutus from several geographical locations. However, these C. macularus strains had different responses to the cowpea variety and the responses of the hybrids were found to be in between those of the parents. Dick and Credland (1986b) therefore, postulated that due to selection pressure that the population of the tolerant strain 207
GEORGE
208
N. MBATA
of the bruchid will continue to increase in subsequent generations. This calls for periodic re-evaluation of the cultivated varieties to the bruchid. Ofuya (1992) has suggested the possibility of existence of many strains of C. maculatus in Nigeria arising from the free trade within the economic community of west Africa and various ecological zones in Nigeria. The present study evaluated the susceptibility of cowpea varieties, some of which have pedigrees including ancestors which are known to be moderately resistant to the bruchid, C. maculatus. The present results were compared with some previous results to find out if there is a difference in tolerance between the strain of C. maculatus used in the present study and the insect used previously.
MATERIALS
AND METHODS
Source of experimental insects
A laboratory culture of C. subinnotatus was started from infested bambarra groundnuts obtained from the City market, Kano, Nigeria in 1987. C. maculatus was obtained from a laboratory culture maintained at the Institute for Vorratsschutz, Berlin, Germany but originated from the Teaching and Research Farm of the Federal University of Technology, Akure, Nigeria. The colonies were maintained in cooled incubators at 29 + 1°C and 70 + 2% r.h. The relative humidity was maintained with saturated solutions of sodium hydroxide (Madge, 1961) and monitored with cobalt thiocyanate as proposed by Solomon (1957). Both C. maculatus and C. subinnotatus were reared separately for one generation on Ife Brown cowpea variety which is known to be very susceptible to bruchids. This was done to make sure that the Fl adults used in the investigation had the same background. Source of cowpea varieties
Twenty-seven cowpea varieties were used for this study. These included both pure germplasm lines and improved varieties some of which are known to be at least moderately resistant to C. maculatus. The cowpea varieties were obtained from IITA, Ibadan, and included TVu 2027, TVu 11952, TVu 11953, TVu 18, TVu 36, TVu 310, TVu 2896, TVu 9836, TVu 9914, TVu 9929, TVu 9930, and TVu 9944. Others were IT8lD-994, IT8lD-1007, IT8lD-1020, ITSlD-1064, IT8lD1137, IT8 l D-1157, IT84D-449, IT84E- 1-108, IT84S-2246-4, IT86D-364, IT86D-498, IT87S- 1393, IT87S-1463B, IT87D-1827 and Ife Brown. All the IT8lD. lines have TVu 2027, a moderately bruchid resistant variety, as one of their ancestors while IT84S-2246-4 IT84D-449 IT86D-364 and IT86-498 were bred out from hybrid lines which had TVu 2027 within their pedigrees (Table 1). Some of these hybrid lines are also resistant to other pests such as thrips and aphids, and some, for instance IT84S-2246-4, have multiple disease resistance. The hybrid varieties used in the present study are included in the cowpea lines that are distributed by IITA for international trials. The TVu numbered lines are straight germplasm collections and as such do not have pedigrees. Surface areas of the cowpea seeds
The surface areas of the seeds were calculated based on the average areas of a cylinder and a sphere (Mitchell, 1990). The colour and the texture of the testa were noted for the various varieties (Table 1). The effect of the surface areas on the distribution of eggs by the females on the seeds was determined. Thirty seeds were placed in 8 cm dia. Petri dishes and 6 dishes were set up for each of the varieties. A pair (1 male : 1 female) of newly emerged adults (O-3 h old) reared one individual per seed following the protocol of Mitchell (1990) were placed in each of the dishes and allowed four days to mate and lay eggs. The number of eggs laid were determined and the number of eggs per seed calculated. These were related to the surface areas of the seeds. Median developmental periods of the bruchids on the cowpea varieties
Sound seeds with intact testa were selected from the cowpea seeds supplied. These were heated at 60°C in a fan-ventilated oven for 6 h to kill all insects and mites that might be present in them.
Cowpea
susceptibility
and C.
macularus
209
subinnotatus
Each varietal batch had 20 seeds selected randomly and dissected to ascertain that the seeds were free from internally developing beetles. Seven grams of each variety were weighed into plastic Petri dishes of 8 cm dia. in replicates of 10. A total of 270 dishes were placed with their top lids removed in desiccators maintained in incubators at 30 + 05°C and 70 + 2% r.h. for 1 week to allow for moisture content equilibration of the seeds. The relative humidity was maintained as described above. After the period of equilibration, the moisture content of the seeds rose from 7.8 to 10.2%. Two newly emerged adults (CL~h old), a male and a female, were introduced into each of the dishes. For each variety, 5 dishes were set up with C. maculatus while the remaining 5 dishes contained C. subinnotatus. After the introduction of the adult beetles, the lids of the dishes were replaced. The dishes were thereafter, put back in the incubators. After 5 days, these adults were removed from the Petri dishes which were then replaced in the incubators. In the dishes where the females failed to lay eggs, fresh adults were used to replace the old ones. As from the 20th day of infesting the seeds, the samples were examined daily for the emergence of Fl adults which were removed and counted. Inspection was continued until 4 weeks following the emergence of the last adult from each dish. Median developmental period, was taken as the duration from the middle of the oviposition period to the time taken for 50% of the adults to emerge (Dobie, 1974). Susceptibility indices (23) The index of susceptibility is given after Howe (197 1) and Dobie (1974): Log,Fl Index of susceptibility = D
x loo
where Fl is the total number of emerging adults and D is the median developmental period. The susceptibility indices were compared among the varieties and related hybrids having similar pedigrees. Losses associated with infestation of the cowpea varieties Quantitative losses arising from the portions of cowpea consumed by the larval stages of the bruchids were assessed. This was obtained by finding the difference in the weights of the samples before and after infestation, after removing the frass and adjusting for moisture. The data arising Table I. Characteristics
of the cowpea varieties
and distributions
Cowpea IifleS
Ife brown TVu I8 TVu 36 TVu 310 TVu 11952 TVu 11953 TVu 2027 TVu 2896 TVu 9836 TVu 9914 TVu 9929 TVU 9930 TVU 9944 IT8 1D-994 178lD-1007 IT81D-1020 IT8lD-1064 IT8lD-II37 ITSID-I 157 IT84D-449 IT84Ll-108 IT84S-2246-4 IT86D-364 IT86D-498 IT87S. 1393 IT87S-1463B IT87D- I827
Pedigree Pure germplasm Pure germplasm Pure germplasm Pure germplasm Pure germplasm Pure germplasm Pure germplasm Pure germplasm Pure germplasm Pure germplasm Pure germplasm Pure germplasm (TVu 1190xTVu76)x (TVu 2027 x TVu 625) TVx I l93-9F x TVu 2027 TVx I l93-9F x TVu 2027 TVx Il93-9F x TVu 2027 TVx I l93-7D x TVu 2027 TVx I193-7D x TVu 2027 IT82D-716 x IT8lD-1020 TVu 3000 x IT82E-60 IT82D-716 x IT81D-1020 (IT82D-889 x IT83D-442) x IT82D-716 (IT84E-124 x (TKx 133-l6D-2 x IT8lD-988)
of the eggs by Callosobruchus spp. on the seeds
Mean surface area of seeds (mm’) 161.5 k 3.0 122.5 2 3.7 120.3 f 5.5 III.1 k3.9 292.7 & 6.2 297.3 k 2. I 276.6 + 4.3 106.6 f 2.2 129.6* 1.3 118.2 ri: 3.9 121.6 i 4.2 139.1 f 4.7 137.1 + 6.3 186.7 f 4.7 145.8 167.3 157.8 192.9 192.7 156.6 167.5 158.9 149.5 123.6 127.6 160.9 161.2
f f f f i i f f + k + f f
5.9 3.9 6.7 5.3 6.7 5.6 7.2 6.2 6.5 3.1 3.7 5.4 5.9
No. of eggs/seed ___ C. ntarularus C. mbinnorurus 2.4 f 0.2 2.3 it 0.2 2.5 k 0.1 I.8 k 0.2 2.8 + 0.1 2.7 * 0.3 2.8 + 0.2 1.8kO.l 2.2 + 0.3 2.0 * 0.2 2.2 f 0.2 1.6+0.1 I.7 * 0.2 2.7 i 0. I
I .8 ?r 0.6 I.2 * 0.4 2.3 + 0.6 0.3 + 0.04 I.8 & 0.5 I .9 * 0.3 2.0 + 0.6 0.9 i_ 0.3 I .o * 0.2 1.3 f 0.5 0.7 * 0.2 1.3 f0.2 0.8 + 0.2 0.9 i 0.2
2.2 2.1 2.7 2.4 2.6 2.2 2.6 2.3 2.1 2.0 2.1 2.4 2.4
I .9 * 0.3 I.7 f 0.4 2.2 * 0.3 I.8 + 0.4 2.0 + 0.3 1.4 * 0.6 1.0 f 0.3 1.4 + 0.5 I.1 LO.4 0.7 * 0.3 I .o + 0.4 1.3 + 0.4 0.8 +_0.3
+ 0.2 kO.1 f 0.2 +_0.2 + 0.2 f 0.2 2 0.2 + 0.3 * 0.3 * 0.2 * 0.2 f 0.2 +_0.2
GEORGE N. MBATA
210 Table 2. The median Cowpea lines Ife brown TVu 18 TVu 36 TVu 310 TVu 11952 TVu 11953 TVu 2027 TVu 2896 TVu 9836 TVu 9914 TVu 9929 TVu 9930 TVu 9944 IT8 I D-994 IT8lD-1007 IT8lD-1020 IT8lD-1064 IT8lD-II37 IT8lD-1157 IT84D-449 IT84E-l-108 IT84S-2246-4 IT86D-364 IT86D-498 IT87S-1393 IT87S-1463B IT87D-1827
developmental periods (days) of Callosobrurhus sm. on the cowoea varieties
Developmental periods of the bruchids (Mean f SE) c. maculalus C. subinnotarus 29.2 f 0.4a’ 32.6 f 0.2b 30.4 + 0.3ab 31.5 +0.3ab 35.8 + 0.6cd 36.6 + 0.8d 43.8 _+ l.3f 30.2 + O&b 30.5 rt 0.2ab 31.0k0.3ab 32.6 + 0.3b 32.0 + 0.7ab 31.1 +0.4ab 36.5 + 0.5d 41.0 f 2.0ef 37.3 + 0.6d 52.8 + 0.9h 35.4 f 0.3cd 49.3 + 0.4g 35. I + 0.2cd 32.8 f 0.2bc 37.5 + 0.7d 37.1 + 0.5d 41.9 * 0.7e 40.0 + 0.5e 34.7 f O&d 35.8 +_0.5cd
II * I *
N.S.' N.S. N.S. * l * I I * *
N.S. N.S. N.S. I
N.S. 1 1
N.S. t
N.S. N.S. * *
35.8 40.4 35. I 42.5 37.2 37.4 45.3 33.5 41.8 43.9 37.8 45.1 42.6 43.6 44.4 37.2 53.7 43.9 48.7 43.8 43. I 39.6 43.0 41.7 43.5 45.2 40. I
f 1.8ab + 0.7cd f 0.6ab f 0.8cde ?r 0.5ab * 0.9abc * I.& f 0.3a + I&de f 0.5de + O.Sabc + 0.9e f O.Ade k 0.6d f I &de f 0.5ab f I .3g + 0.9de f 0.6f f I .8de + 0.3de + 0.9bcd It 0.7de + I.lcd + I .4de +_ l.5e i 0.4bcd
‘Vertical values followed by the different letters are significantly different (P < 0.05). ‘Horizontal values having asterisks between them are significantly different (P < 0.05). ‘N.S. = not significant.
from the study were analysed statistically using analysis of variance, Duncan’s multiple range test and Student’s T-test.
RESULTS Surface area of seeds and the distribution of eggs on them The surface areas of the seeds and distribution of eggs on them is given in the Table 1. The surface area of the seeds were found to vary among the varieties. The following varieties: TVu 2027, TVu 11952 and TVu 11953 had the largest surface areas while TVu 310, TVu 2896 and TVu 9914 had the smallest surface areas. Most of the hybrid varieties had surface areas mid-way between the two extremes. The number of eggs laid per seed correlated with the surface areas but this correlation was far stronger for C. maculutus (r = 0.7089) than for C. subinnotatus(r = 0.5126). C. subinnotatus laid fewer eggs on cowpea seeds than when provided with bambarra groundnuts (Mbata, 1990). Median developmental period of the bruchids on the cowpea varieties The range of the median developmental periods of C. maculatus was between 29.2 days on Ife brown and 52.8 days on IT8lD-1064 while those of C. subinnotatus ranged between 33.5 days on TVu 2986 and 53.7 days on IT8 1D- 1064 (Table 2). The developmental periods of C. maculatus were generally shorter than those of C. subinnotatus. However, the developmental periods were not significantly different on 10 of the bruchid resistant varieties (Table 2). The median developmental periods of C. maculutus on Ife brown, a susceptible variety, was generally shorter than on all the other varieties but were not significantly different from many of the TVu straight germplasm collections. The median developmental period of C. subinnotatus was longer on 18 varieties than on Ife brown and most of these varieties were improved varieties derived from TVu 2027. It was observed that Fl adults of both bruchids emerged from TVu 2027 continuously over a period of time and their longevity overlapped to encourage production of further generations and maintenance of infestation.
Cowpea
susceptibility
to C. maculaius
211
and C. subinnotatus
Number of first filial adults that emerged from the varieties
The numbers of Fl adults emerging from the varieties of cowpea are given in Fig. 1. The number of these adults were affected by the varieties and response of the two species were significantly affected. Double classification analysis variance of the number of Fl adults showed that the difference between the two species was significant (P < 0.01). More emergents of C. maculatus than C. subinnotatus were observed on all varieties except ITSlD-1064 and IT8lD-1157 (Fig. 1). Very high numbers of C. maculatus were observed on Ife brown, IT84E-l-108, TVu 36, and TVu 2896 and TVu 9944 while the highest number of Fl adults of C. subinnotatus were observed on TVu 36, IT8lD-1064, IT84D-449 and IT84E-l-108. The numbers of C. subinnotatus that emerged from IT8lD-1064 and IT8 lD-1157 were significantly higher than those of C. macufatus (P < 0.05). The susceptibility indices (SZ) of cowpea varieties to C. maculatus and C. subinnotatus
The SI of the cowpea varieties to the two bruchids are given in Table 3. Generally, the cowpea varieties were more susceptible to C. maculatus than to C. subinnotatus. The SI of the varieties to C. maculatus ranged from 4.4 on IT8lD-1064 to 14.8 on Ife brown while the SI of the varieties of C. subinnotatus ranged between 4.5 on IT84S-2246-4, IT86D-364, and IT86D-498 to 9.8 on Ife brown. All the TVu numbered lines with the exception of TVu 2027, TVu 11952 and TVu 11953 had very high SI to C. maculatus. Most of the improved varieties, especially those bred out from TVu 2027, a moderately resistant variety to bruchids, were found to have SI to C. maculatus not significantly different from that of TVu 2027. Even though the SI of cowpea varieties to C. subinnotatus were lower than those of C. macufatus, the different varieties had varying susceptibilities to the bruchid. Most of the straight germplasm lines, with the exception of TVu 2027, TVu 11952 and TVu 11953 were more susceptible than the improved varieties to C. subinnotatus. The surface areas of the seeds did not correlate with the SI. This shows that the number of emergents from the cowpea varieties and median developmental periods were not dependent on the surface areas of the seeds.
I
_C
.mac&tus
W
C.sut)innotatus
50 r h
h
gh bl
g
I_
f
f
de
Varieties of cowpea Fig. 1. The number
of FI adults
of 2 bruchids
on different
varieties
of cowpea.
GF.QRGE N. MBATA
212 Table
3. The
susceptibilities indices (SI) of cowpea infestation by CaNosobrucbus spp.
Cowpea lines
Index of susceptibilities c. macularus
Ife brown TVu 18 TVu 36 TVU 310 TVU 11952 TVU 11953 TVu 2027 TVu 2896 TVu 9836 TVu 9914 TVu 9929 TVU 9930 TVU 9944 IT8 I D-994 IT8lD-1007 IT8lD-1020 IT81D-1064 lT8lD-1137 IT81D-1157 IT84D-449 IT84E-l-108 1188-2246-4 IT86D-364 IT86D-498 IT87S-1393 IT87S-1463B IT87D-1827
14.8 k 0.2h’ 10.7 & 0.3f 1 I .2 * 0.3fg 11.3* 0.3fg 8.5 + 0.5~ 8.6 f 0.3cd 7.4 f 0.4bc 12.2 * 0.3g 11.9+0.2g I I .2 * 0.3fg 10.8 f 0.2f I 1.o * 0.5fg 11.7f0.4fg 9.2 k 0.2de 7.7 f 0.5c 7.9 f 0.2c 4.4 f 0.4a 9.5 f 0.3e 5.3 *0.3a 9.5 f 0.2e 1I .4 f 0.3fg 8.5 + 0.3~ 8.3 f 0.5~ 6.5 + 0.4b 8.6 f 0.2cd 8.1 + 0.7~ 8.Y f O.Zd
* l l
N.S.' N.S. N.S. l
* I * * .
N.S. N.S. N.S. *
N.S. l I * *
N.S. l l l
to
(Mean + SE) C. subimotatus
$2
l
varieties
9.8 7.5 8.7 4.9 6.8 7.4 5.7 8.6 6.0 4.9 7.4 6.2 5.3 5.9 7.1 7.5 5.8 7.0 6.1 7.0 6.3 4.5 4.5 4.5 7.1 4.6 5.0
+ 0.7f + 0.2d f 0.4e * 0.5ab f 0.8cd f 0.4d + 0.9b f 0.4e f 0.2bc + 0.3ab & 0.4d f 0.4bc i 0.5ab i 0.3bc f 0.4cd + 0.3d f 0.3g f 0.5cd + 0.4bc ; 0.6cd + 0.1~ + 0.3a f 0.3a + 0.5a +_0.4cd + 0.2a f 0.4ab
’ Vertical values followed by different letters arc significantly different (P < 0.05). 2Hotizontal values having asterisks between them are significantly different (P < 0.05). ‘N.S. = not significant.
Table 4. Weight (g) losses of cowpea varieties due to infestation Callosobruchus spp. Cowpea lines Ife brown TVu I8 TVu 36 TVu 310 TVu II952 TVu 11953 TVu 2027 TVu 2896 TVu 9836 TVu 9914 TVu 9929 TVU 9930 TVu 9994 IT8 1D-994 IT81D-1007 IT81D-1020 IT8lD-1064 IT81D-1137 IT81D-I 157 IT84D-449 IT84E-I-108 IT848-2246-4 IT86D-364 IT86D-498 IT87S- 1393 IT87S-1463B IT87D-I 827
by
Weight(g) losses due to infestation (Mean + SE) C. maculatus C. subinnotatus I .9 f 0.2c’ I.2 & 0.3bc I .4 + 0.2bc I .4 f 0.4bc 0.5 fO.la 0.4-?_O.la 0.6 k 0.2ab I.1 +0.3b I.1 ;0.4b 0.9 ? 0.2b I .O & 0.2b 0.8 & 0.2ab 1.4 k 0.3bc 0.6 + O.lab 0.6 + 0.2ab 0.6 f 0.2ab 0.4 +O.la 0.9 f 0.2b 0.6 ir 0.2ab 0.8 + 0.3ab 1.8f 0.3~ 0.6 + 0.2ab 0.7 f 0.2ab 0.3 +0.1a I.0 f 0.3b 0.5 1?_0.la 0.6 f 0.2ab
N.S.’ N.S. N.S. lZ N.S. N.S. N.S. * * * N.S. N.S. * N.S. N.S. N.S. I N.S. N.S. N.S. I N.S. N.S. N.S. N.S. N.S. N.S.
2.2 f 0.4c 0.8 f 0.2ab 1.0 + 0.2b 0.4+0.1a 0.5 + O.la 0.7 + O.Oab 0.5 f 0.2a 0.5 2 O.Za 0.5 f 0.2a 0.4 f O.la 0.6 + 0.3ab 0.7 & 0.2ab 0.4iO.la 0.3 * O.la 0.9 + 0.3b 0.8 + 0.3ab 0.9 k 0.2b 0.7 _t 0.2ab 0.9 k 0.3b 0.7 + 0.2ab 0.9 + 0.2b 0.4 + 0.2a 0.6 + 0.2ab 0.3+0.la 0.9 + 0.3b 0.6 k 0.2ab 0.6 f 0.2ab
‘Verticalvalues
followed by different letters are significantly different (P < 0.05). ZHorizontal values having asterisks in between them are significantly different (P < 0.05) ‘N.S. = not significant.
Weight losses of cowpea varieties to the bruchids Losses in weight of cowpea seeds due to infestation are given in Table 4. Most of the varieties sustained from moderate to fairly high losses in weight to both C. maculatus and C. subinnotatus. Even though the SI of most of the varieties were higher for C. maculatus than for C. subinnotatus, the differences in weight losses of most of the varieties due to the food eaten by the developing beetles were not significant. Ife brown, most of the straight germplasm collections and IT84E-1-108 sustained very high losses to C. maculatus. Equally, Ife brown sustained a very high loss in weight to C. subinnotatus. Only two of the improved varieties sustained losses to both bruchids which are significantly different (P < 0.05). The weight losses correlated very strongly with susceptibility indices (P < 0.01); for C. maculatus the coefficient of correlation, r = 0.8194, while for C. subinnotatus the coefficient of correlation, r = 0.7089. DISCUSSION Both Callosobruchus spp. were able to develop and produce progeny on the cowpea varieties. Generally, C. maculutus did better than C. subinnotutus. Ndlovu and Giga (1988) found that some of the improved varieties they screened were more susceptible to C. rhodesiunus than to C. maculutus. In the present study, none of the varieties screened had a significantly higher susceptibility to C. subinnotatus than to C. maculatus. Several varieties including TVu 2027, TVu 11952, TVu 11953 and most of improved varieties that had TVu 2027 within their pedigrees were consistently poor diets for both bruchids. These varieties had the lowest susceptibility indices and were able to support fewer larvae. It is probable that the mechanism for resistance of these varieties to the two bruchids are the same. Since the performance of C. subinnotatus on most of the varieties was comparable to that of C. maculatus, it is probable that given longer period of contact between C. subinnotatus and most of the varieties their susceptibilities to this bruchid will increase. Dick and Credland (1986a) had observed an improvement in the ability of subsequent generations of some strains of C. maculatus to develop on TVu 2027. Also Singh and Singh (1990) observed that emergence of Fl adults of C. maculutus from TVu 2027 was protracted and sparse, making it impossible for the variety to sustain further generations. It is probable that strains of C. macularus that are tolerant to TVu 2027 are already
Cowpea susceptibility to C. macularus and C. subinnotatus
213
in existence especially as the Fl adults observed in the present study emerged close enough to mate and lay eggs. Even though fewer Fl adults of C. subinnotatus emerged from most of the cowpea varieties, the losses in weight due to infestation by C. subinnotatus were comparable with those caused by C. maculatus. This is probably because C. subinnotatus is larger than C. maculatus. This also suggests that C. subinnotatus has the potential to become a serious pest of cowpea. The status of C. subinnotatus as a pest of cowpea should therefore be re-evaluated. C. ~ubinnofu~us is now being found on cowpea (Monge, personal communication). This calls for fresh surveys in areas where the two species occur. In one of the improved varieties, IT81D-1137, derived from TVu 2027, the number of Fl adults and SI were significantly higher than for other varieties that had similar pedigrees. Equally C. maculutus had fairly high SI on ITSID-994. It is probable that the bruchids had had greater contact with these varieties. Dick and Credland (1986b) had earlier warned on the selection pressure imposed by resistant varieties on the bruchid, C. macularus. One way of avoiding this is by periodic screening of resistant varieties and by breeders to keep at least a step ahead of the bruchids. author is grateful to the International Foundation for Science, Grev Turegatan, Stockholm, Sweden, for partially funding this investigation and to Dr G. 0. Myers, IITA, Ibadan, Nigeria, for supplying the cowpea varieties and information on their pedigrees. This work was started at Imo State University, Ok&we, Nigeria, and completed at Vorratsschutz, Geissen, Germany. The author is grateful to Professor Dr W. Stein for providing laboratory space at Geissen. Acknowledgements-The
REFERENCES Booker R. H. (1967) Observations on three bruchids associated with cowpea in Nigeria. J. stored Prod. Res. 3, l-5. Dick K. M. and Credland P. F. (1986a) Variation in response of Callosobruchus macuirus (F:) to a resistant variety of cowpea. J. stored Prod. Res. 22, 4348. Dick K. M. and Credland P. F. (1986b) Changes in the response of Caiiosobruchus macuiatus (F.) (Coleoptera: Bruchidae) to a resistant variety of cowpea. J. stored Prod. Res. 22, 227-233. Dobie P. (1974) The laboratory assessment of the inherent susceptibility of maize to post harvest infestation by Sitophilus zeamais Motsch. (Coleptera: Curculionidae). J. stored Prod. Res. 10, 183-197. Giga D. P. and Smith R. H. (1987) Egg production and development of Caliosobruchus rhodesianus (Pit.) and CaBosobruchus maculatus (F.) (Coleoptera: Bruchidae) on several commodities at two different temperatures. J. stored Prod. Res. 23, 9-15.
Howe R. W. (1971) A parameter for expressing the suitability of environment for insect development. J. stored Prod. Res. 7, 6345.
Madge D. S. (1961) the control of relative humidity with aqueous solutions of sodium hydroxide. Enromol. exp. Applic. 4, 251-353.
Mbata G. N. (1990) Studies on the developmental biology of Callosobruchus subinnotatus (Pit.) (Coleoptera: Bruchidae) a pest of stored bambarra groundnuts. Insect Sci. Appiic. 11, 245-251. Mbata G. N. (1991a) The seasonal incidence and abundance of insects pests of stored bambarra groundnuts. Proc. I.F.S.-C.T.A. seminar on ‘The inflwnce of climatic factor on the tropical crop production’, Ouagadougau, pp. 452459. Mbata G. N. (1991b) Studies on the biology of two congeneric species of Callosobruchus. Proc. Fifth Int. Working Conf Stored Prod. Protection, Bordeaux, 1990, pp. 125-133. Mitchell R. (1990) Behavioral ecology of Callosobruchus macuiatus. In Bruchids and Legumes: Economics, Ecology and Coeuolution (Edited by Fujii K., Gatehouse A. M. R., Johnson C. D., Mitchell R. and Yoshida T.), pp. 317-330. Kluwer, London Ndlovu T. M. and Giga D. P. (1988) Studies on varietal resistance of cowpeas to the cowpea weevil, CaBosobruchus rhodesianus (Pit.) Insect Sci. Applic. 9, 123-128. Ofuya T. (1992) Aspects of oviposition and strategies in a Nigerian strain of Callosobruchus macularus (F.) (Coleoptera: Bruchidae). Proc. XIX Int. Congr. Entomoi., Beiiing, China, D. 584. Prevett P. F. .(1966) Observations on the biology of-six species of Bruchidae (Coleptoera) in northern Nigeria. Entomol. Mon. Msg.
102, 174-180.
Rachie K. 0. (1985) Introduction. In Cowpea Research, Producrion and Utilization (Edited by Singh S. R. and Rachie K. 0.). Wiley, London. Singh B. B. and Singh S. R. (1990) Breeding for Bruchid resistance in cowpea. In BruchidF and Legumes: Economics, Ecology and Coevolution (Edited by Fujii K., Gatehouse A. M. R., Johnson C. D., Mitchell R. and Yoshida T.), pp. 219-228. Kluwer, London. Singh S. R. (1977) Cowpea cultivars resistant to insect pests in world germplasm collection. Trop. grain legume BUN. 9, 3-7. Singh S. R. and Jackai L. E. N. (1985) Insect pests of cowpea in Africa: Their life cycle, economic importance and potential for control. In Cowpea Research, Production ond Utilization (Edited by Singh S. R. and Rachie K. 0.). Wiley, London. Solomon M. E. (1957) Estimation of humidity with cobalt thiocyanate papers and permanent colour standards. Bull. enc. Res. 168, 489-506.
Southgate B. J. (1964) Distribution and host of certain Bruchidae in Africa. Trop. stored Prod. Inf. 7. 277-279. Southgate B. J. (1965) Pulse Bruchids of Africa. Proc. 12th Int. Congress Ent, London 1964, p. 642. Southgate B. J. (1978) The importance of bruchidae as pests of grain legumes, their distribution and control. In Pests of Grain Legumes, Ecology and Control (Edited by Singh S. R., Emden H. F. van and Taylor A.), pp. 219-229. Academic press, London.
1993
0022-474X/93 $6.00 + 0.00 Copyright cc 1993 Pergamon PressLtd
EVALUATION OF SUSCEPTIBILITY OF VARIETIES OF COWPEA TO CALLOSOBRUCHUS MACULATUS (F.) AND CALLOSOBRUCHUS SUBINNOTATUS (PIC.) (COLEOPTERA: BRUCHIDAE) GEORGE N. MBATA Institut fur Phytopathologie und Angewandte Zoologie, Vorratsschutx, Justus Liebig Universitat, Alter Steinbacher Weg 44, D6300 Giessen, Germany (Received for publication 11 March 1993) Ahatraet-Twenty-six varieties of cowpea some of which are among those distributed for international trials by the International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria were evaluated in comparison with Ife Brown, a known susceptible variety, for susceptibility to Caiiosobruchus maculatus (F.) and CaNosobruchussubinnotatus (Pk.). The most susceptible varieties to C. maculatus were TVu 310, TVu 2896, TVu 9944 and TVu 9836. Some varieties were found to be moderately susceptible to C. subinnotutus and these included TVu 18. TVu 36. TVu 2896 and IT81D-1020. Most varieties were found to be more susceptible to C. maculatus than to C. subinnotatus. However, the median developmental periods of the bruchids on the cowpea varieties were comparable in most cases. Some varieties were found to be consistently less susceptible to attack by the two bruchids and these included TVu 2027, IT81D-1064, ITIID-1157 and IT86-498. The weight losses due to infestatiou by the bruchids correlated very strongly to the susceptibility indices (SI) of the varieties to the bruchids. The losses in weight caused by C. subinnotatus were comparable with those caused by C. mnculatus even though the number of Fl adults of C. macufutus that emerged from the varieties was higher than those of C. subinnotatus. Key words-Bruchidae,
Cullosobruchus, cowpea, varieties, infestation, susceptibility.
INTRODUCTION
Cowpea, Vigna unguiculutu Walp. is the most important and widely consumed grain legume in Nigeria and indeed, most other African countries (Singh and Jackai, 1985). It provides more than half the plant protein in human diets and it is the key source of protein for the poorest sector of many developing countries (Rachie, 1985). Species of the genus Cullosobruchus are very damaging pests of stored legumes (Southgate, 1964; 1965; 1978). Apart from the most widely distributed species such as Callosobruchus macufutus (F.) and Callosobruchus chinensis (L.), others such as Callosobruchus rhodesianus (Pit.) and Callosobruchus subinnotatus (Pit.) are restricted to south Africa and west Africa, respectively, where they cause considerable damage to stored legumes (Prevett, 1966; Giga and Smith, 1987; Mbata, 1991a). C. maculatus and C. subinnotatus are widely distributed in subsahelian and humid west Africa where both species are found attacking cowpea and bambarra groundnuts (Booker, 1967; Prevett, 1966; Mbata, 1991a). The cowpea storage beetle, C. macularus is the most important pest of stored cowpea seeds. Often, after 3-5 months of storage, 100% infestation of cowpea by this bruchid is had observed observed in west Africa (Singh, 1977). Monge (personal communication) C. subinnotatus on cowpea while Mbata (1991b) established that cowpea can sustain the development of this bruchid. Ndlovu and Giga (1988) found that improved varieties of cowpea had varying susceptibilities to C. rhodesianus and that the level of resistance of these varieties to C. rhodesiunus was found to be lower than that of C. macufutus. It is not known if different varieties of cowpea would have different susceptibilities to C. subinnotatus, or if the susceptibilities of the cowpea varieties to C. subinnotatus would differ from those for C. maculatus. Equally Dick and Credland (1986a, 1986b) found that TVu 2027, a cowpea variety from which most of the improved varieties were derived was resistant to C. maculutus from several geographical locations. However, these C. macularus strains had different responses to the cowpea variety and the responses of the hybrids were found to be in between those of the parents. Dick and Credland (1986b) therefore, postulated that due to selection pressure that the population of the tolerant strain 207
GEORGE
208
N. MBATA
of the bruchid will continue to increase in subsequent generations. This calls for periodic re-evaluation of the cultivated varieties to the bruchid. Ofuya (1992) has suggested the possibility of existence of many strains of C. maculatus in Nigeria arising from the free trade within the economic community of west Africa and various ecological zones in Nigeria. The present study evaluated the susceptibility of cowpea varieties, some of which have pedigrees including ancestors which are known to be moderately resistant to the bruchid, C. maculatus. The present results were compared with some previous results to find out if there is a difference in tolerance between the strain of C. maculatus used in the present study and the insect used previously.
MATERIALS
AND METHODS
Source of experimental insects
A laboratory culture of C. subinnotatus was started from infested bambarra groundnuts obtained from the City market, Kano, Nigeria in 1987. C. maculatus was obtained from a laboratory culture maintained at the Institute for Vorratsschutz, Berlin, Germany but originated from the Teaching and Research Farm of the Federal University of Technology, Akure, Nigeria. The colonies were maintained in cooled incubators at 29 + 1°C and 70 + 2% r.h. The relative humidity was maintained with saturated solutions of sodium hydroxide (Madge, 1961) and monitored with cobalt thiocyanate as proposed by Solomon (1957). Both C. maculatus and C. subinnotatus were reared separately for one generation on Ife Brown cowpea variety which is known to be very susceptible to bruchids. This was done to make sure that the Fl adults used in the investigation had the same background. Source of cowpea varieties
Twenty-seven cowpea varieties were used for this study. These included both pure germplasm lines and improved varieties some of which are known to be at least moderately resistant to C. maculatus. The cowpea varieties were obtained from IITA, Ibadan, and included TVu 2027, TVu 11952, TVu 11953, TVu 18, TVu 36, TVu 310, TVu 2896, TVu 9836, TVu 9914, TVu 9929, TVu 9930, and TVu 9944. Others were IT8lD-994, IT8lD-1007, IT8lD-1020, ITSlD-1064, IT8lD1137, IT8 l D-1157, IT84D-449, IT84E- 1-108, IT84S-2246-4, IT86D-364, IT86D-498, IT87S- 1393, IT87S-1463B, IT87D-1827 and Ife Brown. All the IT8lD. lines have TVu 2027, a moderately bruchid resistant variety, as one of their ancestors while IT84S-2246-4 IT84D-449 IT86D-364 and IT86-498 were bred out from hybrid lines which had TVu 2027 within their pedigrees (Table 1). Some of these hybrid lines are also resistant to other pests such as thrips and aphids, and some, for instance IT84S-2246-4, have multiple disease resistance. The hybrid varieties used in the present study are included in the cowpea lines that are distributed by IITA for international trials. The TVu numbered lines are straight germplasm collections and as such do not have pedigrees. Surface areas of the cowpea seeds
The surface areas of the seeds were calculated based on the average areas of a cylinder and a sphere (Mitchell, 1990). The colour and the texture of the testa were noted for the various varieties (Table 1). The effect of the surface areas on the distribution of eggs by the females on the seeds was determined. Thirty seeds were placed in 8 cm dia. Petri dishes and 6 dishes were set up for each of the varieties. A pair (1 male : 1 female) of newly emerged adults (O-3 h old) reared one individual per seed following the protocol of Mitchell (1990) were placed in each of the dishes and allowed four days to mate and lay eggs. The number of eggs laid were determined and the number of eggs per seed calculated. These were related to the surface areas of the seeds. Median developmental periods of the bruchids on the cowpea varieties
Sound seeds with intact testa were selected from the cowpea seeds supplied. These were heated at 60°C in a fan-ventilated oven for 6 h to kill all insects and mites that might be present in them.
Cowpea
susceptibility
and C.
macularus
209
subinnotatus
Each varietal batch had 20 seeds selected randomly and dissected to ascertain that the seeds were free from internally developing beetles. Seven grams of each variety were weighed into plastic Petri dishes of 8 cm dia. in replicates of 10. A total of 270 dishes were placed with their top lids removed in desiccators maintained in incubators at 30 + 05°C and 70 + 2% r.h. for 1 week to allow for moisture content equilibration of the seeds. The relative humidity was maintained as described above. After the period of equilibration, the moisture content of the seeds rose from 7.8 to 10.2%. Two newly emerged adults (CL~h old), a male and a female, were introduced into each of the dishes. For each variety, 5 dishes were set up with C. maculatus while the remaining 5 dishes contained C. subinnotatus. After the introduction of the adult beetles, the lids of the dishes were replaced. The dishes were thereafter, put back in the incubators. After 5 days, these adults were removed from the Petri dishes which were then replaced in the incubators. In the dishes where the females failed to lay eggs, fresh adults were used to replace the old ones. As from the 20th day of infesting the seeds, the samples were examined daily for the emergence of Fl adults which were removed and counted. Inspection was continued until 4 weeks following the emergence of the last adult from each dish. Median developmental period, was taken as the duration from the middle of the oviposition period to the time taken for 50% of the adults to emerge (Dobie, 1974). Susceptibility indices (23) The index of susceptibility is given after Howe (197 1) and Dobie (1974): Log,Fl Index of susceptibility = D
x loo
where Fl is the total number of emerging adults and D is the median developmental period. The susceptibility indices were compared among the varieties and related hybrids having similar pedigrees. Losses associated with infestation of the cowpea varieties Quantitative losses arising from the portions of cowpea consumed by the larval stages of the bruchids were assessed. This was obtained by finding the difference in the weights of the samples before and after infestation, after removing the frass and adjusting for moisture. The data arising Table I. Characteristics
of the cowpea varieties
and distributions
Cowpea IifleS
Ife brown TVu I8 TVu 36 TVu 310 TVu 11952 TVu 11953 TVu 2027 TVu 2896 TVu 9836 TVu 9914 TVu 9929 TVU 9930 TVU 9944 IT8 1D-994 178lD-1007 IT81D-1020 IT8lD-1064 IT8lD-II37 ITSID-I 157 IT84D-449 IT84Ll-108 IT84S-2246-4 IT86D-364 IT86D-498 IT87S. 1393 IT87S-1463B IT87D- I827
Pedigree Pure germplasm Pure germplasm Pure germplasm Pure germplasm Pure germplasm Pure germplasm Pure germplasm Pure germplasm Pure germplasm Pure germplasm Pure germplasm Pure germplasm (TVu 1190xTVu76)x (TVu 2027 x TVu 625) TVx I l93-9F x TVu 2027 TVx I l93-9F x TVu 2027 TVx Il93-9F x TVu 2027 TVx I l93-7D x TVu 2027 TVx I193-7D x TVu 2027 IT82D-716 x IT8lD-1020 TVu 3000 x IT82E-60 IT82D-716 x IT81D-1020 (IT82D-889 x IT83D-442) x IT82D-716 (IT84E-124 x (TKx 133-l6D-2 x IT8lD-988)
of the eggs by Callosobruchus spp. on the seeds
Mean surface area of seeds (mm’) 161.5 k 3.0 122.5 2 3.7 120.3 f 5.5 III.1 k3.9 292.7 & 6.2 297.3 k 2. I 276.6 + 4.3 106.6 f 2.2 129.6* 1.3 118.2 ri: 3.9 121.6 i 4.2 139.1 f 4.7 137.1 + 6.3 186.7 f 4.7 145.8 167.3 157.8 192.9 192.7 156.6 167.5 158.9 149.5 123.6 127.6 160.9 161.2
f f f f i i f f + k + f f
5.9 3.9 6.7 5.3 6.7 5.6 7.2 6.2 6.5 3.1 3.7 5.4 5.9
No. of eggs/seed ___ C. ntarularus C. mbinnorurus 2.4 f 0.2 2.3 it 0.2 2.5 k 0.1 I.8 k 0.2 2.8 + 0.1 2.7 * 0.3 2.8 + 0.2 1.8kO.l 2.2 + 0.3 2.0 * 0.2 2.2 f 0.2 1.6+0.1 I.7 * 0.2 2.7 i 0. I
I .8 ?r 0.6 I.2 * 0.4 2.3 + 0.6 0.3 + 0.04 I.8 & 0.5 I .9 * 0.3 2.0 + 0.6 0.9 i_ 0.3 I .o * 0.2 1.3 f 0.5 0.7 * 0.2 1.3 f0.2 0.8 + 0.2 0.9 i 0.2
2.2 2.1 2.7 2.4 2.6 2.2 2.6 2.3 2.1 2.0 2.1 2.4 2.4
I .9 * 0.3 I.7 f 0.4 2.2 * 0.3 I.8 + 0.4 2.0 + 0.3 1.4 * 0.6 1.0 f 0.3 1.4 + 0.5 I.1 LO.4 0.7 * 0.3 I .o + 0.4 1.3 + 0.4 0.8 +_0.3
+ 0.2 kO.1 f 0.2 +_0.2 + 0.2 f 0.2 2 0.2 + 0.3 * 0.3 * 0.2 * 0.2 f 0.2 +_0.2
GEORGE N. MBATA
210 Table 2. The median Cowpea lines Ife brown TVu 18 TVu 36 TVu 310 TVu 11952 TVu 11953 TVu 2027 TVu 2896 TVu 9836 TVu 9914 TVu 9929 TVu 9930 TVu 9944 IT8 I D-994 IT8lD-1007 IT8lD-1020 IT8lD-1064 IT8lD-II37 IT8lD-1157 IT84D-449 IT84E-l-108 IT84S-2246-4 IT86D-364 IT86D-498 IT87S-1393 IT87S-1463B IT87D-1827
developmental periods (days) of Callosobrurhus sm. on the cowoea varieties
Developmental periods of the bruchids (Mean f SE) c. maculalus C. subinnotarus 29.2 f 0.4a’ 32.6 f 0.2b 30.4 + 0.3ab 31.5 +0.3ab 35.8 + 0.6cd 36.6 + 0.8d 43.8 _+ l.3f 30.2 + O&b 30.5 rt 0.2ab 31.0k0.3ab 32.6 + 0.3b 32.0 + 0.7ab 31.1 +0.4ab 36.5 + 0.5d 41.0 f 2.0ef 37.3 + 0.6d 52.8 + 0.9h 35.4 f 0.3cd 49.3 + 0.4g 35. I + 0.2cd 32.8 f 0.2bc 37.5 + 0.7d 37.1 + 0.5d 41.9 * 0.7e 40.0 + 0.5e 34.7 f O&d 35.8 +_0.5cd
II * I *
N.S.' N.S. N.S. * l * I I * *
N.S. N.S. N.S. I
N.S. 1 1
N.S. t
N.S. N.S. * *
35.8 40.4 35. I 42.5 37.2 37.4 45.3 33.5 41.8 43.9 37.8 45.1 42.6 43.6 44.4 37.2 53.7 43.9 48.7 43.8 43. I 39.6 43.0 41.7 43.5 45.2 40. I
f 1.8ab + 0.7cd f 0.6ab f 0.8cde ?r 0.5ab * 0.9abc * I.& f 0.3a + I&de f 0.5de + O.Sabc + 0.9e f O.Ade k 0.6d f I &de f 0.5ab f I .3g + 0.9de f 0.6f f I .8de + 0.3de + 0.9bcd It 0.7de + I.lcd + I .4de +_ l.5e i 0.4bcd
‘Vertical values followed by the different letters are significantly different (P < 0.05). ‘Horizontal values having asterisks between them are significantly different (P < 0.05). ‘N.S. = not significant.
from the study were analysed statistically using analysis of variance, Duncan’s multiple range test and Student’s T-test.
RESULTS Surface area of seeds and the distribution of eggs on them The surface areas of the seeds and distribution of eggs on them is given in the Table 1. The surface area of the seeds were found to vary among the varieties. The following varieties: TVu 2027, TVu 11952 and TVu 11953 had the largest surface areas while TVu 310, TVu 2896 and TVu 9914 had the smallest surface areas. Most of the hybrid varieties had surface areas mid-way between the two extremes. The number of eggs laid per seed correlated with the surface areas but this correlation was far stronger for C. maculutus (r = 0.7089) than for C. subinnotatus(r = 0.5126). C. subinnotatus laid fewer eggs on cowpea seeds than when provided with bambarra groundnuts (Mbata, 1990). Median developmental period of the bruchids on the cowpea varieties The range of the median developmental periods of C. maculatus was between 29.2 days on Ife brown and 52.8 days on IT8lD-1064 while those of C. subinnotatus ranged between 33.5 days on TVu 2986 and 53.7 days on IT8 1D- 1064 (Table 2). The developmental periods of C. maculatus were generally shorter than those of C. subinnotatus. However, the developmental periods were not significantly different on 10 of the bruchid resistant varieties (Table 2). The median developmental periods of C. maculutus on Ife brown, a susceptible variety, was generally shorter than on all the other varieties but were not significantly different from many of the TVu straight germplasm collections. The median developmental period of C. subinnotatus was longer on 18 varieties than on Ife brown and most of these varieties were improved varieties derived from TVu 2027. It was observed that Fl adults of both bruchids emerged from TVu 2027 continuously over a period of time and their longevity overlapped to encourage production of further generations and maintenance of infestation.
Cowpea
susceptibility
to C. maculaius
211
and C. subinnotatus
Number of first filial adults that emerged from the varieties
The numbers of Fl adults emerging from the varieties of cowpea are given in Fig. 1. The number of these adults were affected by the varieties and response of the two species were significantly affected. Double classification analysis variance of the number of Fl adults showed that the difference between the two species was significant (P < 0.01). More emergents of C. maculatus than C. subinnotatus were observed on all varieties except ITSlD-1064 and IT8lD-1157 (Fig. 1). Very high numbers of C. maculatus were observed on Ife brown, IT84E-l-108, TVu 36, and TVu 2896 and TVu 9944 while the highest number of Fl adults of C. subinnotatus were observed on TVu 36, IT8lD-1064, IT84D-449 and IT84E-l-108. The numbers of C. subinnotatus that emerged from IT8lD-1064 and IT8 lD-1157 were significantly higher than those of C. macufatus (P < 0.05). The susceptibility indices (SZ) of cowpea varieties to C. maculatus and C. subinnotatus
The SI of the cowpea varieties to the two bruchids are given in Table 3. Generally, the cowpea varieties were more susceptible to C. maculatus than to C. subinnotatus. The SI of the varieties to C. maculatus ranged from 4.4 on IT8lD-1064 to 14.8 on Ife brown while the SI of the varieties of C. subinnotatus ranged between 4.5 on IT84S-2246-4, IT86D-364, and IT86D-498 to 9.8 on Ife brown. All the TVu numbered lines with the exception of TVu 2027, TVu 11952 and TVu 11953 had very high SI to C. maculatus. Most of the improved varieties, especially those bred out from TVu 2027, a moderately resistant variety to bruchids, were found to have SI to C. maculatus not significantly different from that of TVu 2027. Even though the SI of cowpea varieties to C. subinnotatus were lower than those of C. macufatus, the different varieties had varying susceptibilities to the bruchid. Most of the straight germplasm lines, with the exception of TVu 2027, TVu 11952 and TVu 11953 were more susceptible than the improved varieties to C. subinnotatus. The surface areas of the seeds did not correlate with the SI. This shows that the number of emergents from the cowpea varieties and median developmental periods were not dependent on the surface areas of the seeds.
I
_C
.mac&tus
W
C.sut)innotatus
50 r h
h
gh bl
g
I_
f
f
de
Varieties of cowpea Fig. 1. The number
of FI adults
of 2 bruchids
on different
varieties
of cowpea.
GF.QRGE N. MBATA
212 Table
3. The
susceptibilities indices (SI) of cowpea infestation by CaNosobrucbus spp.
Cowpea lines
Index of susceptibilities c. macularus
Ife brown TVu 18 TVu 36 TVU 310 TVU 11952 TVU 11953 TVu 2027 TVu 2896 TVu 9836 TVu 9914 TVu 9929 TVU 9930 TVU 9944 IT8 I D-994 IT8lD-1007 IT8lD-1020 IT81D-1064 lT8lD-1137 IT81D-1157 IT84D-449 IT84E-l-108 1188-2246-4 IT86D-364 IT86D-498 IT87S-1393 IT87S-1463B IT87D-1827
14.8 k 0.2h’ 10.7 & 0.3f 1 I .2 * 0.3fg 11.3* 0.3fg 8.5 + 0.5~ 8.6 f 0.3cd 7.4 f 0.4bc 12.2 * 0.3g 11.9+0.2g I I .2 * 0.3fg 10.8 f 0.2f I 1.o * 0.5fg 11.7f0.4fg 9.2 k 0.2de 7.7 f 0.5c 7.9 f 0.2c 4.4 f 0.4a 9.5 f 0.3e 5.3 *0.3a 9.5 f 0.2e 1I .4 f 0.3fg 8.5 + 0.3~ 8.3 f 0.5~ 6.5 + 0.4b 8.6 f 0.2cd 8.1 + 0.7~ 8.Y f O.Zd
* l l
N.S.' N.S. N.S. l
* I * * .
N.S. N.S. N.S. *
N.S. l I * *
N.S. l l l
to
(Mean + SE) C. subimotatus
$2
l
varieties
9.8 7.5 8.7 4.9 6.8 7.4 5.7 8.6 6.0 4.9 7.4 6.2 5.3 5.9 7.1 7.5 5.8 7.0 6.1 7.0 6.3 4.5 4.5 4.5 7.1 4.6 5.0
+ 0.7f + 0.2d f 0.4e * 0.5ab f 0.8cd f 0.4d + 0.9b f 0.4e f 0.2bc + 0.3ab & 0.4d f 0.4bc i 0.5ab i 0.3bc f 0.4cd + 0.3d f 0.3g f 0.5cd + 0.4bc ; 0.6cd + 0.1~ + 0.3a f 0.3a + 0.5a +_0.4cd + 0.2a f 0.4ab
’ Vertical values followed by different letters arc significantly different (P < 0.05). 2Hotizontal values having asterisks between them are significantly different (P < 0.05). ‘N.S. = not significant.
Table 4. Weight (g) losses of cowpea varieties due to infestation Callosobruchus spp. Cowpea lines Ife brown TVu I8 TVu 36 TVu 310 TVu II952 TVu 11953 TVu 2027 TVu 2896 TVu 9836 TVu 9914 TVu 9929 TVU 9930 TVu 9994 IT8 1D-994 IT81D-1007 IT81D-1020 IT8lD-1064 IT81D-1137 IT81D-I 157 IT84D-449 IT84E-I-108 IT848-2246-4 IT86D-364 IT86D-498 IT87S- 1393 IT87S-1463B IT87D-I 827
by
Weight(g) losses due to infestation (Mean + SE) C. maculatus C. subinnotatus I .9 f 0.2c’ I.2 & 0.3bc I .4 + 0.2bc I .4 f 0.4bc 0.5 fO.la 0.4-?_O.la 0.6 k 0.2ab I.1 +0.3b I.1 ;0.4b 0.9 ? 0.2b I .O & 0.2b 0.8 & 0.2ab 1.4 k 0.3bc 0.6 + O.lab 0.6 + 0.2ab 0.6 f 0.2ab 0.4 +O.la 0.9 f 0.2b 0.6 ir 0.2ab 0.8 + 0.3ab 1.8f 0.3~ 0.6 + 0.2ab 0.7 f 0.2ab 0.3 +0.1a I.0 f 0.3b 0.5 1?_0.la 0.6 f 0.2ab
N.S.’ N.S. N.S. lZ N.S. N.S. N.S. * * * N.S. N.S. * N.S. N.S. N.S. I N.S. N.S. N.S. I N.S. N.S. N.S. N.S. N.S. N.S.
2.2 f 0.4c 0.8 f 0.2ab 1.0 + 0.2b 0.4+0.1a 0.5 + O.la 0.7 + O.Oab 0.5 f 0.2a 0.5 2 O.Za 0.5 f 0.2a 0.4 f O.la 0.6 + 0.3ab 0.7 & 0.2ab 0.4iO.la 0.3 * O.la 0.9 + 0.3b 0.8 + 0.3ab 0.9 k 0.2b 0.7 _t 0.2ab 0.9 k 0.3b 0.7 + 0.2ab 0.9 + 0.2b 0.4 + 0.2a 0.6 + 0.2ab 0.3+0.la 0.9 + 0.3b 0.6 k 0.2ab 0.6 f 0.2ab
‘Verticalvalues
followed by different letters are significantly different (P < 0.05). ZHorizontal values having asterisks in between them are significantly different (P < 0.05) ‘N.S. = not significant.
Weight losses of cowpea varieties to the bruchids Losses in weight of cowpea seeds due to infestation are given in Table 4. Most of the varieties sustained from moderate to fairly high losses in weight to both C. maculatus and C. subinnotatus. Even though the SI of most of the varieties were higher for C. maculatus than for C. subinnotatus, the differences in weight losses of most of the varieties due to the food eaten by the developing beetles were not significant. Ife brown, most of the straight germplasm collections and IT84E-1-108 sustained very high losses to C. maculatus. Equally, Ife brown sustained a very high loss in weight to C. subinnotatus. Only two of the improved varieties sustained losses to both bruchids which are significantly different (P < 0.05). The weight losses correlated very strongly with susceptibility indices (P < 0.01); for C. maculatus the coefficient of correlation, r = 0.8194, while for C. subinnotatus the coefficient of correlation, r = 0.7089. DISCUSSION Both Callosobruchus spp. were able to develop and produce progeny on the cowpea varieties. Generally, C. maculutus did better than C. subinnotutus. Ndlovu and Giga (1988) found that some of the improved varieties they screened were more susceptible to C. rhodesiunus than to C. maculutus. In the present study, none of the varieties screened had a significantly higher susceptibility to C. subinnotatus than to C. maculatus. Several varieties including TVu 2027, TVu 11952, TVu 11953 and most of improved varieties that had TVu 2027 within their pedigrees were consistently poor diets for both bruchids. These varieties had the lowest susceptibility indices and were able to support fewer larvae. It is probable that the mechanism for resistance of these varieties to the two bruchids are the same. Since the performance of C. subinnotatus on most of the varieties was comparable to that of C. maculatus, it is probable that given longer period of contact between C. subinnotatus and most of the varieties their susceptibilities to this bruchid will increase. Dick and Credland (1986a) had observed an improvement in the ability of subsequent generations of some strains of C. maculatus to develop on TVu 2027. Also Singh and Singh (1990) observed that emergence of Fl adults of C. maculutus from TVu 2027 was protracted and sparse, making it impossible for the variety to sustain further generations. It is probable that strains of C. macularus that are tolerant to TVu 2027 are already
Cowpea susceptibility to C. macularus and C. subinnotatus
213
in existence especially as the Fl adults observed in the present study emerged close enough to mate and lay eggs. Even though fewer Fl adults of C. subinnotatus emerged from most of the cowpea varieties, the losses in weight due to infestation by C. subinnotatus were comparable with those caused by C. maculatus. This is probably because C. subinnotatus is larger than C. maculatus. This also suggests that C. subinnotatus has the potential to become a serious pest of cowpea. The status of C. subinnotatus as a pest of cowpea should therefore be re-evaluated. C. ~ubinnofu~us is now being found on cowpea (Monge, personal communication). This calls for fresh surveys in areas where the two species occur. In one of the improved varieties, IT81D-1137, derived from TVu 2027, the number of Fl adults and SI were significantly higher than for other varieties that had similar pedigrees. Equally C. maculutus had fairly high SI on ITSID-994. It is probable that the bruchids had had greater contact with these varieties. Dick and Credland (1986b) had earlier warned on the selection pressure imposed by resistant varieties on the bruchid, C. macularus. One way of avoiding this is by periodic screening of resistant varieties and by breeders to keep at least a step ahead of the bruchids. author is grateful to the International Foundation for Science, Grev Turegatan, Stockholm, Sweden, for partially funding this investigation and to Dr G. 0. Myers, IITA, Ibadan, Nigeria, for supplying the cowpea varieties and information on their pedigrees. This work was started at Imo State University, Ok&we, Nigeria, and completed at Vorratsschutz, Geissen, Germany. The author is grateful to Professor Dr W. Stein for providing laboratory space at Geissen. Acknowledgements-The
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