Factors influencing the effectiveness of Trichosirocalus horridus (Panzer) in the control of Carduus thistles

CROP PROTECTION (1983) 2 (2), 143-151

Factors influencing the effectiveness of Trichosirocalus horridus (Panzer) in the control of Carduus thistles P. J.

SIEBURTH*,L,

T . KOK AND M. LENTNER

Department of Entomology, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA ABSTRACT. Trichosirocalushorridus (Panzer) (Coleoptera: Curculionidae), a European weevil introduced for the biological control of Carduusthistles, was initially released in 1974 and has become established in Virginia, USA. Studies were conducted to examine the impact of the weevil on Carduus thistles in the field in spring and winter, and in the laboratory. Variables included two thistle species, two thistle sizes and three larval densities. Response of the thistles to larval infestation was influenced by thistle growth, thistle size and larval density. Thistle rosettes grown in the laboratory had the greatest percentage of plants developing necrosis (990/0)with none recovering fully; the plant centre died in 89%, and 31% died completely, whereas no field-grown plants died. Spring rosettes developed the next largest percentage of necrosis (85%) and of death of centre (28°/0), but had the highest percentage of plants recovering (80%). Overwintering thistles developed the least amount of necrosis (43%) and of dead centres (2%), but had a lower recovery rate (61%) than spring thistle rosettes. Neither spring nor overwintering rosettes died even at the highest infestation level (50 larvae). Appearance of symptoms of infestation in overwintering thistle rosettes was influenced by plant species, plant size and rate of larval infestation. Thistle species was not a factor determining when laboratory rosettes manifested symptoms of infestation, and larval density did not influence spring rosettes. Small laboratory thistle roseties were killed by infestations of 30 larvae (42%) and 50 larvae (100%). Although T. horriduslarvae do not kill the majority of the thistles that they infest, they do weaken them by destroying crown tissues. T h e effectiveness of T. horridus may be greatly enhanced by the joint use of other stress factors.

Introduction

Trichosirocalus horridus (Panzer), (Coleoptera: Curculionidae) formerly Ceuthorhynchidius horridus (Panzer) ( T r u m b l e and Kok, 1980a), is a weevil which was * Current address: Department of Entomology, Clemson University, Clemson, SC 29631, USA. 0261-2194/83/02/0143-09503.00 © 1983 Butterworth & Co (Publishers) Ltd

144

T. horridus control of Carduus thistles

introduced into the USA from Europe (Kok, 1981a) for the biological control of Carduus thistles after host-specificity testing (Ward, Pienkowski and Kok, 1974; Kok, 1975). The weevils initially were released in Virginia in 1974 and had become well established by 1978 (Kok and Trumble, 1979). In Virginia, adult weevils emerge from May to June, feed until July and go into an aestival diapause. In September the adults resume feeding. Oviposition occurs from November to April; the eggs are inserted into the midribs and blades of the leaves of thistle rosettes. Emerging first instars burrow into the meristematic and crown tissue where they remain and feed. The larval stage is found from December to May and causes significant injury to the thistles (Trumble and Kok, 1979). After successful establishment of T. horridus, its value as a biological control agent could be assessed. Field observations indicated that the impact of T. horridus on Carduus thistles appeared to be a function of larval density, plant size , thistle species, and the environmental condition under which the thistle was growing. In the experiments described here, the effect of these variables was examined using rosettes infested (1) in the field in spring, (2) in the field in winter, or (3) in the laboratory. Materials and methods Musk thistle (Carduus thoermeri Weinmann) and plumeless thistle (Carduus acanthoides L.) rosettes were grown in the greenhouse in pots of capacity 3-8 fl for small plants and 7-6 f for large plants. Small plants were 23 + 3 cm in diameter (15 + 5 leaves) and large plants were 45 + 5 cm in diameter (28 + 6 leaves). They were transplanted into field plots for the spring and overwinter experiments. Larval densities of T. horridus were 0 (control), 10, 30 and 50 larvae per plant. Newly eclosed first instars from a laboratory colony were inoculated by the capillary tube technique (Ward, Pienkowski and Kok, 1974). Each treatment was replicated six times, giving a total of 96 plants per environmental condition. Plants were caged or bagged to catch emerging weevils. Emerged weevils, which were easy to detect on thistle foliage, were removed, counted, sexed and weighed. Plant symptoms recorded were the first indication of damaged or darkened tissue at the centre of the plant, the death of the plant centre (growth point), the development of secondary shoots, plant recovery, and plant death. A plant was considered to have recovered when necrotic tissue was no longer visible. Analysis of variance was used to analyse the data for each growth environment. A chi-square test of contrast (Grizzle, Starmer and Koch, 1969) using percentages was used to detect significant differences.

Thistle rosettes infested in spring The field plot in which the thistles were planted was part ofa hayfield-cowpasture, a typical thistle site, near Blacksburg, Virginia. It was lightly cultivated with a rotary machine during the first week in May 1979 and randomly planted with 96 rosettes grown initially in pots. The rosettes were given a week to acclimatize, before being inoculated with first instars of the weevil. Rosettes were caged as soon as they had been planted, in order to prevent wild T. horridus adults from ovipositing on the thistles and, later, to catch emerging adults. Cages were constructed of yard edging (green plastic 2 mm thick and 15 cm wide) stapled to form a circle 48 cm in diameter for small plants and 58 cm in diameter for the large plants. Stapled tubes of nylon

P. J. SIEBURTH,L. T. KOK ANDM. LENTNER

145

organza were sewn to each circle. The cages were inserted 20 cm into the ground and soil was banked along the sides. The top of each cage was closed by a twist-tie at the top of the organza tube and placed so that its bulk would not shade the plant. The rosettes were checked daily during the first 2 weeks and every other'day thereafter until the end of the experiment.

Thistle rosettes infested during winter The same field that had been used for the spring thistle rosettes was cultivated in September 1979 and randomly planted with thistle rosettes. Rosettes were inoculated with T. horridus larvae on 16-17 December 1979, and caged in March 1980 as described above. These rosettes were not caged as soon as they were planted; this was to permit normal snow cover during winter. Plants were checked weekly during the winter and every other day in April until the end of the experiment in July.

Laboratory thistle rosettes Thistle rosettes in pots were inoculated with larvae and randomly arranged on laboratory tables under fluorescent lights (40 W). The room was maintained at 21°C and 10 h photoperiod during the experimental period between September 1978 and June 1980. The plants were examined every day for the first 2 weeks after inoculation and subsequently every other day until no new weevils had emerged for 2 weeks. Rosettes were covered with plastic bags 5 weeks after they were inoculated, to catch emerging adults. Bags larger than the rosettes were used so that the leaves would not touch the sides of the bags. The bags were inverted every other day to reduce moisture accumulation. Results

Necrosis and death of plant centre For spring rosettes, the length of time until necrosis was first visible was influenced by thistle species and thistle size (Table 1). Plumeless thistle rosettes developed necrosis in an average of 12 days and musk thistle in 15 days. Necrosis appeared sooner in small thistle rosettes (11 days) than in large thistle rosettes (16 days); 51.7% of the former had dead centres compared with only 6-3% of the latter. In overwintered rosettes, thistle species interacted with larval density to influence the length of time until necrosis was first visible (Table 1). Musk thistle rosettes inoculated with 30 larvae developed necrosis (107 days) before those inoculated with 10 and 50 larvae (113 and 112 days, respectively). Corresponding times for development of necrosis in plumeless thistle rosettes were 117, 99 and 108 days, respectively. Only one plant, a small plumeless thistle rosette, had a dead centre. Almost all the laboratory-grown infested thistle rosettes developed necrosis (98.6%). Thistle size and larval density both influenced the length of time until necrosis was first visible (Table 2). Small rosettes showed necrosis in 5 days, and large rosettes in 8 days. Rosettes inoculated with 30 larvae showed necrosis sooner than those with 10 larvae (5 and 8 days, respectively), but there was no significant difference between these and the rosettes with 50 larvae (6 days).

T. horridus control of Carduus thistles

146

TABLE 1. Factors influencing the rate of development of necrosis and shoots in thistle rosettes inoculated with T. horridus larvae in the spring and winter Significance levels for rejection of null hypothesis 1 Spring Winter Influencing factor

Necrosis

Shoots

Necrosis

Shoots

Species Size Species- Size Larval density Species • Larval density Size" Larval density Species • Size" Larval density

0"0085 ** 0"0001 ** 0-5108 ns 0-2694 ns 0-2572 ns 0"2770 ns 0-5587 ns

0"0152 * 0"2020 ns 0"0046"* 0"4901 ns 0'5762 ns 0"9124 ns 0.2433 ns

0"2055 ns 0"5502 ns 0"1020 ns 0"6085 ns 0"0062 ** 0"0606 ns --

0-0001 ** 0"0001 ** 0-0077"* 0"1763 ns 0-9440 ns 0"1470 ns 0"1935ns

1 . p < 0"05, **P < 0"01, ns = not significantly different at P < 0"05, analysis of variance. Laboratory-grown rosettes had the greatest percentage of necrosis and death of centre, followed by spring rosettes and overwintered rosettes (Table 3). In laboratory rosettes, death of centre was influenced by plant size and larval density. Small and large rosettes had dead centres in 16 and 22 days, respectively. Rosettes with 50 larvae had dead centres in 15 days compared with 21 days for those rosettes with 10 and 30 larvae. For spring thistles, species as well as size were main effects, but in overwintered thistles too few plants had dead centres for this to be attributable to any influencing factor.

Shoot production In the spring, thistle species influenced the length of time for shoot formation, and was also an influence as an interaction with rosette size (Table 1). Infested plumeless thistle rosettes produced shoots sooner than musk thistle rosettes (averaging 24 and 27 days, respectively). Small and large plumeless thistle rosettes produced shoots at 25 and 23 days, respectively. Corresponding times for small and large musk thistle rosettes to produce shoots were 24 and 30 days. Overwintered thistle shoot production was influenced by thistle species and size (Table 1). Plumeless thistle rosettes produced side shoots at 111 days compared with musk thistle rosettes at 119 days. In contrast to the spring rosettes, the large overwintered rosettes produced side shoots before small rosettes (at 11l and 117 days). Thistle species and size also interacted to influence the time when shoots became visible. Large plumeless thistle rosettes produced shoots sooner than small plumeless thistle rosettes (at 106 and 116 days, respectively), but no difference was found between small and large musk thistle rosettes. F o r laboratory rosettes, plant size and larval density interacted to influence the time of shoot production (Table 2). Most of the rosettes which did not die, developed shoots. Small rosettes with 30 larvae produced shoots later than the small rosettes with 10 and 50 larvae (40, 27 and 27 days, respectively), whereas large rosettes with

Necrosis 0"0769 0.0001 0.5262 0.0015 0"7435 0'0746 0.4270

ns ** ns ** ns ns ns

0.2637 0.4199 0"1564 0"2208 0"6567 0"0288 0"8157

ns ns ns ns ns * ns

0.5327 0.8514 0.1917 0.6272 0-9888 0"7674 --

ns ns ns ns ns ns

0"5074 0"0944 0-2193 0.0001 0"0002 0"0788 0"1955

ns ns ns ** ** ns ns

0'5413 0"2666 0"8685 0"0030 0.0224 0"0738 0-0219

ns ns ns ** * ns *

Significance levels for rejection of null hypothesis 1 Male weevils F e m a l e weevils D e a t h o f centre Shoots D e a t h o f plant emerged emerged

1 , p < 0'05, * * P < 0.01, n s = n o t significantly different at P < 0"05, analysis of variance.

Species 0"9944 ns Size 0.0001 ** Species" Size 0"7719 ns Larval density 0"0355 * Species" Larval density 0.6923 ns Size" Larval density 0.6614 ns Species" Size • Larval density 0"6349 ns

Influencing factor

TABLE 2. Factors influencing the rate of d e v e l o p m e n t of necrosis, death o f centre, shoots, death o f plant and weevil emergence from thistle rosettes inoculated with T. horridus larvae in the laboratory

-7.

z z

z

©

z

Symptom 81) 41) 66) 66)

(1.2, 2.0)

(89, (15, (94, (94,

p)3 43b 2b 61b 0 36 2.6

Mean%

(4"9, 0'4)

(56, 30) ( 0, 3) (50, 72)

(M, P) 99c 89c 0c 31 49 4"9

(4.4, 5'4)

(31, 31)

(97, 100) (97, 81)

(M, P)

Laboratory

Mean~o

Environmental condition Overwinter

I . p < 0.05, chi-square test of contrasts; ns = not significantly different at P < 0-05. 2 Different letters for means within the same row indicate statistical difference at P < 0.05 3 M=musk thistle, P=plumeless thistle. 4 Out of 2160 first instars inoculated for each environmental condition.

85a 28a 80a 0 35 1.6

Mean~o 2 (M,

Spring

ns

ns

g¢

Significance 1

Response by thistle rosette s to infestation by T. horridus under different environmental conditions

Rosettes developing necrosis Necrotic plants in which the centres died Necrotic rosettes which recovered Infested plants which died Plants producing weevils Weevils emerging 4

TABLE 3.

e~

-4~ O~

P. J. SIEBURTH,L. T. KoK ANDM. LENTNER

149

30 larvae produced shoots sooner than the large rosettes with 10 and 50 larvae (24, 33 and 28 days).

Recovery~death of plant Mean recovery for all spring plants was 80%. Sixty-two per cent of the small necrotic rosettes and 97% of the large necrotic rosettes recovered. There was no significant difference between the recovery times for small and large rosettes. For the overwintered plants, mean recovery for the plumeless thistles was 73~o and 50% for musk thistles. Of the necrotic plumeless thistles, more of the small rosettes (100%) recovered than the large rosettes (57°/0). For musk thistles, recovery of the necrotic rosettes decreased with increasing larval density, with the large rosettes showing better recovery than the small rosettes. About one-third of all infested laboratory plants died (31%). The proportion of plants dying increased with increasing larval density for each plant size. None of the rosettes with 10 larvae died, but 33°/0 of the large rosettes and 100°/0 of small rosettes with 50 larvae died. There was no difference in the time for plants to die due to thistle species, rosette size, or larval density (Table 2). Laboratory rosettes did not recover and were the only ones which died completely (31%); nevertheless, many of the plants developed shoots and continued to grow. None of the spring and overwintered thistles died, and more necrotic spring than overwintered thistles recovered (Table 3).

Newly emerged adults The percentage of plants grown under the three different environmental conditions and which produced weevils was not significantly different. Less than 5% of the inoculated larvae emerged as adults, most of them from the laboratory rosettes (Table 3). New adults did not emerge from all the plants which had necrotic tissue, and some adults were collected from plants which did not show any evidence of necrosis. Larval density influenced the length of time for weevil emergence (Table 2) but not the weight of the weevils. Mean emergence time decreased with increasing larval density, ranging from 70 days for rosettes with 10 larvae, to 64 days for rosettes with 30 larvae and 57 days for rosettes with 50 larvae. Emergence time was influenced by an interaction with thistle species for both sexes. Musk thistle rosettes with 10 and 30 larvae produced weevils at approximately the same time (73 and 72 days), whereas those with 50 larvae produced adults much sooner (55 days). Plumeless thistle rosettes with 30 and 50 larvae produced adults at approximately the same time (60 and 59 days), whereas rosettes with 10 larvae produced adults much later (67 days). With regard to the emergence of female weevils, there was also a three-way interaction with thistle size, thistle species and larval density. Discussion The data summarized in Tables 1 and 2 confirm that thistle species, thistle size, larval density and/or interactions between them are major factors influencing the weeds' response to weevil infestation. The response of thistle species to infestation is probably related to the size of the crown tissue where the larvae of T. horridus first burrow and feed. The crown meristematic tissue in plumeless thistle rosettes is

150

T. horridus control 0fCarduus thistles

smaller than that in musk thistles: destruction of a smaller amount of tissue is sufficient for necrosis to be visible in plumeless thistle. Plant size is important because small thistles have fewer stored reserves to resist infestation and less meristematic tissues to be destroyed before necrosis becomes visible. The rate at which the meristematic tissue is destroyed is a direct function of larval density and feeding. Interactions of thistle species, thistle size and larval density complicate the influence of individual factors. Confounding all these is the type of conditions for thistle growth. Thus, when the plants are grown under favourable conditions in spring, larval density (10-50/plant) does not influence the time for plant symptoms to appear, but thistle species and thistle size do. For those thistles grown under less favourable (laboratory) conditions, thistle species does not influence the rate at which symptoms of weevil infestation appear, but thistle size and larval density do. Plant species, larval density and plant size all influence the time at which symptoms appear in overwintering rosettes. The environmental growth conditions determine plant vigour and hence response to infestation with T. horridus. The large percentage of laboratory thistle rosettes which had dead centres and the relatively large proportion of plants which died (31%) indicate that plants which are grown under unfavourable conditions are unable to overcome the damage by larval feeding. In contrast, the ability of field-grown plants to survive infestation with T. horridus attests to the importance of plant vigour, as none of the field plants died from larvae inoculated into the rosettes. The relationship between plant vigour and weevil efficacy in controlling thistles is emphasized by the data on weevil emergence. The small number of adults emerging (1-6% from spring rosettes) may be related to plant vigour which could prevent successful development of the weevils in the infested tissues. This is supported by our field observations at release sites where large larvae were sometimes found dead in infested rosettes which had recovered. Some rosettes with new growth contained trapped single dead larvae, but no infested plants were found with several dead larvae in them, indicating that large numbers of larvae can overcome plant resistance better than small infestations. The shorter time to weevil emergence at higher larval densities also suggests that larvae feed more rapidly during crowding, because of nutrient limitations. The reaction ofplumeless thistle to larval infestation, and the number of emerging adults, indicate that musk thistle is a better host than plumeless thistle for T. horridus. Relevant factors may include the smaller amounts ofmeristematic tissue in plumeless thistle, as well as its characteristic of sending out secondary shoots even without an injury stimulus. This suggests that T. horridus gives poorer control of plumeless thistles than of musk thistles. Although T. horridus larvae do not kill the majority of the thistle rosettes that they infest, they attack the weed at a critical point in its life cycle, just before stem elongation to produce the flower stalk. Plant reserves in the meristematic and crown tissues, which should be utilized in flowering and seed production, are instead being consumed by the weevil or are used in resisting infestation. Asthe ratio of weevils to thistles increases, the side shoots as well as the main meristems will be infested (as observed at our established release sites), causing further delay in plant development. The impact of T. horridus is more pronounced when the plants are subjected to additional stress, which may be caused by unfavourable growth conditions or other thistle-feeding insects as well as the use of herbicides. Stressed thistles are susceptible and can be killed when larval density is high. Because T. horridus is

P. J. SIEBURTH,L. T. Kog AND M. LENTNER

151

compatible with Rhinocyllus conicus Froelich (a thistle-head weevil) and the herbicide 2,4-D ( T r u m b l e and Kok, 1980b; Kok, 1981b), the efficacy of T. horridus as a biological control agent of Carduus thistles will be greatly enhanced when it is used jointly with the other stress factors.

Acknowledgement T h i s study was 12-14-1001-1204.

supported

by

USDA

Cooperative

Research

Grant

No.

References GRIZZLE,J.E., STARMER,C.F. AND KOCH, G.G. (1969). Analysis of categorical data by linear models. Biometrics 25, 489-504. KOK, L.T. (1975). Host specificity studies on Ceuthorhynchidius horridus (Panzer) (Coleoptera: Curculionidae) for the biocontrol of musk and plumeless thistle. Weed Research 15, 21-25. KOK, L.T. (198la). Status of two European weevils for the biological control of Carduus thistles in the USA. Acta Phytopathologica Academiae Scientiarum Hungaricae 16, 139 142. KOK, L.T. (1981b). Compatibility of Rhinocyllus conicus, Trichosirocalus horridus and 2,4-D for Carduus thistle control. In: Proceedings, V International Symposium on the Biological Control of Weeds, Brisbane, Australia, 1980, p. 441445. KOK, L.T. AND TREMBLE,J.T. (1979). Establishment of Ceuthorhynchidius horridus (Coleoptera: Curculionidae), an imported thistle-feeding weevil, in Virginia. Environmental Entomology 8, 221-223. TREMBLE,J.T. AND KOK, L.T. (1979). Ceuthorhynchidius horridus (Coleoptera: Curculionidae): Life cycle and development on Carduus thistles in Virginia. Annals of the Entomological Society of America 72, 563-564. TRUMBLE, J.T. AND KOK, L.T. (1980a). A bibliography of Ceuthorhynchidius horridus (Panzer) (= Trichosirocalus horridus Panzer), an introduced weevil for the biological control of Carduus thistles. Bulletin of the Entomological Society of America 26, 464-467. TRUMBLE,J.T. ANDKOK,L.T. (1980b). Impact of 2,4-D on Ceuthorhynchidius horridus (Coleoptera: Curculionidae) and their compatibility for the integrated control of Carduus thistles. Weed Research 20, 73-75. WARD, R.H., P~ENKOWSKI,R.L. AND KOK, L.T. (1974). Host specificity of the first instar of Ceuthorhynchidius horridus, a weevil for the biological control of thistles. Journal of Economic Entomology 67, 735-737. Accepted 16 October 1982