Performance of nymphal and adult Nezara viridula on selected alternate host plants

Laboratory studies with the southern green stink bug, Nezara viridula (L.) (Hemiptera: Pentatomidae), feeding on seeds or fruits of some alternate host plants, demonstrated differences in nymphal survival and development time, and body weight at adult emergence. Lowest mortalities (25%) were observed for nymphs on fruits of dakota mustard [Brassica kaber (DC.)] and siberian motherwort (Leonurus sibiricus L.). On fruits of castor bean (Ricinus communis L.) nymph mortality was ca. 60%, and on mature seeds of L. sibiricus ca. 75%. Total development time from second instar to adult ranged from ca. 26 days on B. kaber to ca. 42 days on R. communis. Fresh body weight at adult emergence was greatest for nymphs on L. sibiricus and lowest on R. communis. Adult survival after 30 days and mean longevity was greater on L. sibiricus than on the remaining foods, being drastically reduced on R. communis. In general, reproductive performance on all foods was similar, except R. communis, on which no female reproduced. Female weight gain of N. viridula was generally greater on L. sibiricus On R. communis, adults lost weight during their first week and showed the lowest weight gain at the end of day 29. Field observations coupled with these laboratory studies suggest that in the Warta area (northern Paraná state, Brazil, 23° 11 S), L. sibiricus and B. kaber are important alternate food plants in the seasonal phenology of N. viridula. R. communis, however, is of poor nutritional quality, serving primarily as a refuge.

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Résumé L'alimentation au laboratoire de N. viridula avec les graines et les fruits de diverses plantes a entraîné différentes mortalités larvaires, différentes vitesses de développement et des différences dans les poids des adultes à l'émergence. Les mortalités les plus faibles (25%) on été observées avec les fruits de Brassica kaber DC. et de Leonurus sibiricus L.Avec les fruits de Ricinus communis L., la mortalité larvaire était de 60%; sur graines mûres de L. sibiricus, elle était de 75%. La durée du développement du second stade à l'adulte a varié de 26 jours sur B. kaber à 42 jours sur R. communis. Le poids frais des adultes à l'émergence était le plus élevé avec L. sibiricus et le plus faible avec de R. communis. La longévité des adultes au-delà de 30 jours était la plus élevée avec L. sibiricus, et était fortement réduite avec R. communis. Dans l'ensemble, la reproduction était de même importance avec les différents aliments, à l'exception de R. communis avec lequel les femelles ne se reproduisaient pas. Le gain de poids des femelles était généralement supérieur avec L. sibiricus; avec R. communis les adultes ont perdu du poids pendant la première semaine et ont présenté le plus faible gain à la fin du 29ème jour.Des observations dans la nature, associées à ces expériences de laboratoire, laissent supposer que dans la zone de Warta (au Nord de l'état de Paraná, 23° 11 S, au Bresil), L. sibiricus et B. kaber sont des plantes hôtes alternatives importantes dans la phénologie saisonnière de N. viridula. Cependant, R. communis, de faible valeur alimentaire, sert avant tout de refuge.

     

Effects of diet on growth, fecundity and duration of tethered flight of Nezara viridula

The effects of diet during the fifth stadium on performance of adult N. viridula were determined through comparing growth, fecundity, and duration of tethered flight of insects reared on four axenic diets: fresh green bean, raw-shelled peanut, a combination of the two and soaked soybean seed. Females reared on the combination diet were heavier than those reared on green beans and oviposited more than those reared on peanuts. Three-day-old adult females reared on peanuts flew longer than those reared on green beans. More females reared on diets containing mature seed showed a capacity for extended flight and had larger fat bodies at 34-days postemergence. Thus, inadequate diet during the fifth stadium irreversibly affected growth, development, reproduction and flight capacity.

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Résumé Les effets du régime alimentaire pendant le 5ème stade sur les performances imaginales de N. viridula ont été déterminés en comparant la croissance, la fécondité, la durée du vol captif, et l'accumulation de corps gras chez des insectes élevés sur 4 régimes stérilisés. A partir de la formation des larves de 5ème stade, elles ont été alimentées sur haricots verts et arachides, ou transférées sur un régime formé exclusivement d'arachides, ou de haricots verts, ou de graines de soja trempées, cependant la durée du 5ème stade. Les insectes ont été transférés sur un régime de haricots verts et arachides à leur émergence.Les femelles élevées continuellement de haricots verts et arachides étaient plus lourdes (P<0,01) à l'émergence (161,6 mg) que celles élevées sur haricots verts (146,3 mg) pendant le 5ème stade, et pondent plus d'oeufs (P<0,08) pendant une durée de 34 jours (100,9 oeufs) que celles élevées sur arachides (54,7 oeufs). Plus de femelles élevées sur haricots verts et arachides (76,2%) pondent que celles élevées sur arachides (61,1%), soja trempé (62,5%), ou haricots verts (60,9%).Les femelles élevées sur arachides volaient plus longtemps (P < 0,01) (42,9 mn) que celles élevées sur haricots verts (19,8 mn) pendant des vols captifs le 3ème jour après l'émergence. 78% de toutes les femelles élevées sur arachides volaient >-30 mn, contre 50% pour celles élevées sur haricots verts et arachides, ou soja trempé, et 35% pour celles élevées sur haricots verts. Seulement 7% des femelles élevées sur haricots verts volaient à la 60ème minute à la fin du test, contre 25% pour les femelles soja trempé, 33% pour les femelles haricots verts et arachides, et 47% pour celles élevées sur arachides. Les femelles ont été disséquées 34 jours après l'émergence pour déterminer l'importance du corps gras. Les indices de corps gras (Kiritani, 1963) montraient que les femelles élevées sur arachides (1,8) et graines de soja (1,7) ont apparemment un corps gras plus développé que celles élevées sur haricots verts et arachides (1,5), our haricots verts (1,2), pendant le 5ème stade.Ces résultats montrent que l'alimentation pendant le dernier stade larvaire de N. viridula affecte irréversiblement la croissance, la fécondité et l'aptitude au vol des femelles. En estimant que des résultats semblables auraient été obtenus avec des insectes élevés sur ces 4 aliments depuis l'éclosion, haricots verts et arachides constituent le meilleur aliment pour l'élevage de N. viridula. Ce régime fournit une graine mûre, vraisemblablement nécessaire pour l'accumulation d'un corps gras suffisant et une aptitude au vol migratoire, et une plante verte qui fournit les aliments apparement essentiels à l'ovogenèse.

     

An evaluation of some natural enemies of >Nezara viridula in northern California

Natural enemies associated with eggs of >Nezara viridula (L.) (Heteroptera: Pentatomidae) wereevaluated by placing sentinel egg masses inweeds and cultivated tomato and bean crops innorthern California. Egg predation wasgenerally less than 10% and normally involvedpredators with chewing mouth parts. Predatorsseldom destroyed an entire egg mass, typicallyeating <40 eggs per exploited mass.Laboratory evaluation of >25 species ofpotential arthropod predators revealed that fewfed on >N. viridula eggs to any extent;however, numerous species fed on >N.viridula nymphs. Five species of eggparasites were recovered from sentinel eggmasses: >Trissolcus basalis (Wollaston),>Gryon obesum Masner and >Telenomuspodisi Ashmead (Scelionidae); and>Ooencyrtus californicus Girault and >O.johnsoni (Howard) (Encyrtidae). The major eggparasite was >T. basalis, the only exoticmember of the parasite guild; it typicallyparasitized 100% of the eggs in an exploitedegg mass. The results indicate thatparasitization of eggs and predation of smallnymphs can be important biotic mortalityfactors for >N. viridula populations innorthern California. It is suggested that acombination of factors – viz., eggparasitization, nymphal predation, regionalshortage of overwintering sites, and localshortages of suitable hosts – maintains thisexotic pest at relatively low levels in theregion.     

Nezara viridula parasitism by the tachinid fly Trichopoda pennipes ten years after its accidental introduction into Italy from the New World

Percent parasitism of Nezara viridula (L.) (Heteroptera:Pentatomidae) by Trichopoda pennipes F.(Diptera: Tachinidae) ten years after itsfortuitous introduction was evaluated in someareas of Italy. The parasitoid is acclimatisedin central Italy with a parasitism in 1998 of21.3% in the west coast area (Lazio) and of24.8% in the inland area (Umbria). Host adultsof both sexes were attacked with the same ratein 1998, whereas this was higher in males thanfemales in 1991 and 1992. Parasitism of nymphswas lower in all three years. In 1998parasitized bugs showed 1 (54.1%) to a maximumof 15 (0.4%) macrotype eggs per individual,distributed preferentially on the thorax ascompared with other body parts. None of theother pentatomid species collected in the sameareas were parasitized by T. pennipes,indicating that there have not been indirecteffects of this fortuitous introduction tillnow.     

Mate location in the southern green stink bug,Nezara Viridula (Heteroptera: Pentatomidae), mediated through substrate-borne signals on ivy

We observed mate location behavior in Nezara viridulaon the ivy Hedera helix.The female stays motionless on the plant stem, calling with rhythmic repetitions of her calling song. The male walks on the plant, answering with his calling and courtship songs. On encountering petioles or branching points, the male stops and straddles his legs across the fork. Only after another repetition of the female's call does the male proceed to walk again. This behavior may enable the male to compare the vibrations in the two substrates and choose the stronger one to follow. On plants where visual and airborne acoustic communication can be prevented by leaves, substrateborne vibrational signals perceived by receptors in legs would enhance the likelihood of mate location in N. viridula.     

Density dependence and regulation in the systemNezara viridula (L.) (Hemiptera: Pentatomidae), host andTrichopoda giacomellii (Blanchard) (Diptera: Tachinidae), parasitoid

Summary The system formed byN. viridula and its almost specific parasitoidT. giacomellii has been studied in the Buenos Aires province, Argentina for 26 generations of the former and 20 generations of the latter. An intensive study over 9 host generations and key factor analysis made possible the identification and quantification of different mortality factors. The main factors determining the fluctuations in total generation mortality were: egg mortality due mainly to parasitism byT. basalis; nymphal predation on 1^st, 2^nd and 3^rd instar nymphs and migration; and adult mortality and reduction on potential natality due to parasitism byT. giacomellii. Adult mortality and reduction of potential natality, due to parasitism byT. giacomellii, was density dependent. This factor is capable of maintaining the simulations generated by a simple model based on key factor analysis, within the range of observed densities. These results suggest that parasitism byT. giacomellii can regulate the population, even in a small area of 450 m².     

Performance of nymphal and adult southern green stink bug on an overwintering host plant and impact of nymph to adult food-switch

Performance of nymphs and adults of the southern green stink bug,Nezara viridula (L.) (Heteroptera: Pentatomidae) feeding on immature fruits of radish,Raphanus raphanistrum L. (Brassicaceae) was poor compared with feeding on immature soybean [Glycine max (L.) Merrill] pods, but a change from radish to soybean from nymph to adult alleviated some of the detrimental effects of this food on adult performance. Over 50% of nymphs died on radish, while only 15% died on soybean. To complete nymph development (stadia 2 through 5), male and female nymphs required 9.5 and 13.3 days longer on radish compared to soybean. Fresh body weight of 1-d-old adult females and males was ≅65% greater for those reared as nymphs on soybean than on radish, with females being significantly heavier than males on both foods. Survivorship and longevity of females, however, was greater for those fed on radish as nymphs and as adults, as compared to those that fed on soybean or radish followed by soybean. Males lived longer when fed exclusively on soybean. In general, reproductive performance and percentage gain in adult body weight was greatest for insects fed on soybean only, and were better for those whose food was switched from nymph to adult, as compared to those insects fed on radish solely.     

Performance and seasonal abundance of the neotropical brown stink bug, shape Euschistus heros nymphs and adults on a novel food plant (pigeonpea) and soybean

Pigeonpea, Cajanus cajan (L.) Millsp. (Leguminosae) was found as a novel food plant of the neotropical brown stink bug, Euschistus heros (F.), in Paraná State, Brazil. Laboratory studies indicated nymphs performing better when fed on immature pods of soybean, Glycine max (L.) Merrill (Leguminosae), than on immature pods of pigeonpea. Female nymphs required 1.7 d less to complete development and tended to have a lower mortality on soybean (17.5%) than on pigeonpea (25.0%). At emergence, fresh body weight was greater on soybean than on pigeonpea. Adult E. heros performed better on soybean than on pigeonpea. After 120 d, 35% of adults were alive on soybean and 30% on pigeonpea; 93.3% of females oviposited on soybean, and 73.3% on pigeonpea, with an oviposition delay of 17 days on pigeonpea compared to soybean. Fecundity was 63% greater on soybean than on pigeonpea, and adult body weight gain for females and males at different ages was 6–36% greater on soybean than on pigeonpea. E. heros colonized soybean during January–March, from the pod-setting and filling stages (R3–R6) to maturity (R7–R8), and pigeonpea during April–June. These results indicate that E. heros has expanded its host range to pigeonpea in Brazil; pods of this plant are suitable food, and can support the insect during autumn-early winter, after soybean harvest, and before it enters into partial hibernation.     

Seasonal abundance of the neotropical brown stink bug, Euschistus heros, in overwintering sites, and the breaking of dormancy

The seasonal abundance of the neotropical brown stink bug, Euschistus heros (Heteroptera: Pentatomidae), in overwintering sites in northern Paraná state, Brazil (latitude 23°11 S, longitude 51°11 W) was monitored from September 1994 to August 1995. The breaking of dormancy (oligopause) was studied in the laboratory by comparing the feeding activity and reproduction of adults collected in the field under different physiological conditions (i.e., dormant and non-dormant). No bugs were found in overwintering sites during the summer (December to February) and during early autumn (March). From mid-autumn to winter (April – August), the number of E. heros captured in these sites gradually increased, decreasing thereafter with the start of spring in September. Dormant and non-dormant E. heros taken to the laboratory and maintained at 25 ± 1 °C , 65± 5% r.h., and L14: D10 photoperiod, and given suitable food (soybean pods or seeds), began feeding immediately. The number of stylet sheaths deposited/day on the food was greater for non-dormant than for dormant adults. Feeding activity was greater on immature pods than on mature seeds of soybean. Dormant females placed under suitable biotic and abiotic conditions took ca. 2 weeks to start reproduction, in contrast to non-dormant females, which reproduced immediately.     

The Influence of Substrate on Male Responsiveness to the Female Calling Song in Nezara viridula

The female calling song (FCS) of the southern green stink bug Nezara viridula is composed of vibrational pulse trains that include either short pulses and a long pulse (FCS-1) or just short pulses (FCS-2). Their function in communication was studied by investigating male vibratory responses to natural and artificial signals on artificial and natural substrates. On a loudspeaker membrane, FCS-1 triggered in males from a Slovene and a French population significantly more courtship songs (MCrS) than FCS-2. Experiments with artificial signals showed that male responses are modulated by the duration of pulse trains and pulse repetition time. On a bean plant, males of both populations responded in the same way to the two types of female calling song pulse trains. Moreover, a laser vibrometer study of the transmission of different natural and artificial vibratory signals through the bean plant showed that the pulses of a high repetition rate are prolonged and fused at distances from the emitter. We conclude that female calling song pulse trains of different temporal structure have the same function in vibrational communication of the species. The temporal and spectral structures of the female calling songs of N. viridula are discussed in terms of effective transmission through plants.     

The role of host semiochemicals in parasitoid specificity: a case study with Trissolcus brochymenae and Trissolcus simoni on pentatomid bugs

The role of semiochemicals on host specificity of two egg parasitoid species, the European Trissolcus simoni and the American Trissolcus brochymenae (Hymenoptera: Scelionidae), was studied in an olfactometer and in different arenas. Cues from two allopatric pests of cabbage, the European Eurydema ventrale and the American Murgantia histrionica (Heteroptera: Pentatomidae), and from the polyphagous and cosmopolitan Nezara viridula were tested. Both T. simoni and T. brochymenae responded to volatile and contact cues from their co-evolved hosts E. ventrale and M. histrionica, respectively, thus confirming the role of host semiochemicals in host location and recognition. When cues of non-co-evolved hosts were presented, a partial “new association” was obtained, as T. simoni probed and oviposited in M. histrionica eggs and some adult emergence occurred. However, this association is unlikely to occur in the field because T. simoni did not respond to volatile cues of M. histrionica. Instead T. brochymenae partially responded to volatile and contact cues from E. ventrale, but eggs were rarely accepted and parasitoids did not develop in this host. When N. viridula was tested, T. simoni responded only to contact cues, whereas T. brochymenae partially responded to volatile and contact cues, but N. viridula eggs were not suitable for development. Therefore, the N. viridula–T. brochymenae association reported from the literature appears unreliable. Understanding the mechanisms that result in host specificity may help increase parasitoid safety and predict their efficacy in biological control with old or new associations.     

Host handling behavior of the egg parasitoid Telenomus triptus to the egg mass of the stink bug Piezodorus hybneri

Host handling behavior of Telenomus triptus Nixon (Hymenoptera: Scelionidae) to an egg mass of Piezodorus hybneri Gmelin (Heteroptera: Pentatomidae) was studied in the laboratory. Five distinct behavioral events could be distinguished in the host handling behavior: drumming, ovipositor-insertion, marking, walking, and resting. Female wasps showed two types of behavioral sequence in an ovipositional bout. One consisted of drumming, ovipositor-insertion, and marking, and the other drumming, ovipositor-insertion, walking, and resting. Females did not seem to lay an egg when ovipositor-insertion was not followed by marking. This was observed frequently in the early oviposition bout, on average 2.9 times per female. Females finally succeeded in parasitizing all the eggs in a host egg mass in most cases. The durations of drumming, ovipositor-insertion, and marking on an egg mass were nearly constant, while the total time spent by a female on and beside a host egg mass varied considerably as a result of variable durations of walking and resting. Self-superparasitism occurred when almost all the eggs in a host egg mass were parasitized. Females laid the first male egg within the first four eggs; this could be an adaptation to small egg masses or single egg.     

Effect of predators and host phenology on the maternal and reproductive behaviors ofGargaphia lace bugs (Hemiptera: Tingidae)

Lace bugs of the genus Gargaphia specialize on taxonomically and ecologically diverse host plants. To examine the impact of predation pressure and host phenology on Gargaphia subsocial and reproductive behavior, we compared G. tiliae, a woodland species restricted to riparian trees in the genus Tilia, and G. solani, a specialist on Solanum in early successional habitats. Both lace bug species exhibit maternal care of eggs and nymphs. Predator densities associated with Solanum were higher than those associated with Tilia and were correlated with higher levels of maternal aggression exhibited by G. solani. In both habitats, artificial exclusion of predators significantly increased nymphal survivorship over maternally guarded broods. Both species reduced the costs associated with maternal care by ovipositing into the egg masses of conspecifics. Gargaphia solani and G. tiliae differed most strikingly in voltinism and thus the potential for iteroparity. Leaf age determined the reproductive future of G. tiliae nymphs independent of photoperiod or temperature. Feeding on plants with young foliage was sufficient to trigger oviposition. Constraints imposed by host plants on life-history patterns and maternal options are discussed.     

Chemoreceptors of adult Mexican bean beetles: External morphology and role in food preference

The Mexican bean beetle, Epilachna varivestis Mulsant (Coleoptera; Coccinellidae), is an oligophagous insect that accepts leaves of soybean, Glycine max (L.) Merrill, as food but exhibits distinct preferences among varieties. It is believed that the basis for discrimination between the resistant PI 229358 and the susceptible cv. Williams is chemical. By the ablation of chemosensilla on antennae, maxillary palpi, labial palpi, and foretarsi, followed with leaf disc preference tests, we found antennal and maxillary palp chemosensilla to be almost entirely responsible for Mexican bean beetles' discrimination between foliage of the resistant and susceptible genotypes. All appendages, and the epipharynx, were observed by scanning electron microscopy and, except for foretarsi, were found to bear several types of presumably olfactory and gustatory sensilla, which are figured and described. Since behavioral observations show that olfaction is usually sufficient to cause rejection of PI 229358, we conclude that olfactory sensilla mainly on antennae and maxillary palpi are critical in food discrimination in this system.

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Résumé E. varivestis Mulsant, insecte oligophage, accepte les feuilles de Glycine max (L.) Merrill (soja) comme aliment, mais présente des préférences marquées parmi les variétés. On considère que l'origine de la discrimination entre le cultivar sensible Williams et PI 229358 résistant, est chimique. Des expériences de choix avec des rondelles de feuilles, après ablation des sensilles chimiques des antennes et des palpes maxillaires et labiaux, et des tarses antérieurs, ont montré que les sensilles chimiques étaient presque entièrement responsables de la discrimination entre les feuillages de génotypes résistant et sensible. Tous les appendices ont été observés au microscope à balayage, et, à l'exception des tarses antérieurs, ils portaient plusieurs types de sensilles gustatives et olfactives, qui ont été représentées et décrites. Puisque les observations comportementales montrent que l'olfaction est normalement suffisante pour provoquer le rejet de PI 229358, nous concluons que les sensilles olfactives des antennes et des palpes maxillaires jouent un rôle crucial dans le choix de l'aliment.

     

Fecundity and Longevity of Green Vegetable Bug,Nezara viridula, Following Parasitism byTrichopoda giacomellii

The effects of parasitism by the ArgentinianTrichopoda giacomellii(Blanchard) on reproduction and longevity of its host,Nezara viridula(L.) are reported. Parasitoid larvae suppress egg maturation, reducing by 70% the fecundity of mature female hosts during the period of larval development. Egg viability was not affected, but mating frequency was reduced by approximately 50%. When parasitized as newly eclosed adults, 84% of females fail to reproduce. In male hosts, fertility and mating frequency were not affected during the period of larval parasitoid development. In male and reproductively immature female hosts, death was coincident with, or occurred shortly after parasitoid emergence (2–4 days); in mature females, death occurred on average 2 weeks after larval parasitoid emergence. Host mortality occurred as a consequence of tissue damage incurred as the parasitoid larvae emerged from the host. Some individuals survived parasitism though no further reproductive activity (mating or oviposition) occurred. The effectiveness ofT. giacomelliias a biological control agent is discussed in relation to its impact on reproduction and survival of its host and contrasted with the action of otherTrichopodaspecies.     

The role of maternal effects in adaptation to different diets

The environmental influences of mothers on offspring traits, or maternal effects, often arise from dietary differences experienced by mothers. However, few studies have explored if and how maternal effects facilitate adaptation to new host plants. To address this, we compared the maternal and direct effects arising from dietary differences in two populations of the large milkweed bug, Oncopeltus fasciatus that live on and feed on the seeds from different hosts. We compared a laboratory population, which has been reared for over 400 generations on sunflower seeds and is now adapted to use these as a host, to the wild population, which is adapted to the ancestral diet of toxic milkweed seeds. We first tested for changes in maternal effects, and then examined offspring performance and survivorship. We found evidence for evolution of the maternal effect facilitating the use of a novel host. However, the strongest effects were population differences and direct dietary effects for all traits. Offspring performance was more strongly influenced by diet than maternal effects. Survivorship depended on population and offspring diet, and their interaction, but was unaffected by maternal diet or other interactions. In the artificially evolved population, diet breadth was increased rather than evolving specialization. Our results suggest changes in maternal effects are likely to be weak compared to direct effects of host plants and other adaptations in adaptation to a novel host. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 114 , 202–211.     

The role of host species, age and defensive behaviour on ovipositional decisions in a solitary specialist and gregarious generalist parasitoid (Cotesia species)

The main objective of this study was to determine the extent to which host acceptance behaviour as related to host species, age, and defensive behaviour might explain the differences in host use that exist between two congeneric and sympatric species of parasitic wasps. Cotesia glomerata (L.) (Hymenoptera: Braconidae) is gregarious and generalist on several species of Pieridae, whereas C. rubecula (Marshall) is solitary and specific to Pieris rapae (L.). Cotesia species differed in their responses to host species (P. brassicae (L.), P. napi (L.) and P. rapae) and developmental stage (early and late 1st, 2nd and 3rd instars). In no-choice tests, host acceptance by C. rubecula was higher for p. rapae and females did not distinguish among the 6 host ages. In contrast, when foraging for P. brassicae and P. napi, C. rubecula females more readily attacked early first instar. Cotesia glomerata showed a higher degree of behavioural plasticity towards acceptance of Pieris host species and host age than did C. rubecula. Cotesia glomerata females parasitized the three Pieris species and showed higher acceptance of first and second instars over third instar. Oviposition success was also influenced by host defensive behaviour. The frequency and the effectiveness of defensive behaviour rose with increasing age of the host, P. brassicae being the most aggressive Pieris species. Furthermore, the mean duration of C. glomerata oviposition was significantly reduced by the defensive reactions of P. brassicae, which would likely affect parasitoid fitness as oviposition time is positively correlated to clutch size in C. glomerata. Acceptance frequencies corresponded well to field reports of Pieris-Cotesia associations and to patterns of parasitoid larval performance, suggesting that the acceptance phase might be used as a reliable indicator of Cotesia host-specificity.     

Flight responses in a wind tunnel of the parasitoid Microplitis croceipes to three spring and three summer host plants of Heliothis spp.

Flight responses of the parasitoid, Microplitis croceipes (Cresson) (Hymenoptera: Braconidae) to two phenological stages of spring host plants (crimson clover, Trifolium incarnatum L., hairy vetch, Vicia villosa Corbiere, and cutleaf geranium, Geranium dissectum L.) and summer host plants (cotton, Gossypium hirsutum L., sorghum, Sorghum bicolor L., and soybean Glycine max (L.) Merr.) of Heliothis virescens (F.), were measured in a dual choice situation in a wind tunnel. Preflowering clover was more attractive than preflowering geranium, followed by preflowering vetch. Clover was preferred over geranium in flowering stages as well. No within species differences were detected for the two phenological stages. Preflowering and flowering cotton and soybean were preferred over preflowering and flowering sorghum. The findings could be important in pre-determining release sites for M. croceipes in the field.     

The influence of plant species on attraction and host acceptance inCotesia glomerata (Hymenoptera: Braconidae)

Females of the larval parasitoidCotesia glomerata (L.) use plant-associated cues to locate their lepidopteran host,Pieris rapae L. In this study we investigated the influence of four host plant species,Brassica oleracea var.acephala (‘Vates’ kale),Tropaeolum majus (nasturtium),Lunaria annua (honesty), andCleome spinosa (spider flower), on two components of the host selection process inC. glomerata, namely, attraction and host acceptance. Choice tests in a flight tunnel showed that parasitoids were attracted to some host plant species more than to others in the absence of host larvae.B. oleracea was the most attractive plant species, followed byL. annua, T. majus, andC. spinosa. In previous studies it was shown thatB. oleracea carries highly suitable hosts forC. glomerata and that, in the field, parasitization rates on this plant were the highest. When host larvae were reared on the four host plant species and then transferred to a common substrate (B. oleracea var.capitata, cabbage), plant species that had served as diet for the hosts did not have a significant effect on acceptance for parasitization. Thus, parasitoids were attracted to host plant species differentially, but they did not discriminate among host larvae based on the dietary history of their hosts. ForC. glomerata, it appears that phytochemistry mediates host selection more by influencing parasitoid attraction than it does by affecting host acceptance.     

Influence of olfactory stimulants on resistance/susceptibility of pearl millet, Pennisetum americanum to the rice weevil, Sitophilus oryzae

The orientation responses of the rice weevil, Sitophilus oryzae (L.) (Coleoptera: Curculionidae) as influenced by the olfactory stimulants emanating from either resistant or susceptible pearl millet, Pennisetum americanum, kernels were studied using a Y-tube olfactometer. The rice weevils were more attracted towards odours from millet grains than to blank controls and to susceptible entries more than to resistant entries, in general. However, there were exceptions. Odours from some resistant entries caused statistically the same, or even more positive orientations than those from some susceptible entries, and this suggests that the relative resistance of an entry and its olfactory attractiveness may not be strongly related, or that the resistance to infestation might be influenced by factors other than attractiveness. The partial damage of the millet kernel pericarp by abrasion slightly increased the orientational response of the weevils to odours from some susceptible and some resistant entries alike.