Influence of pre-emergence experience on response to host and host plant odours in the larval parasitoidEupelmus vuilleti

The response to different host and plant species odours was investigated inEupelmus vuilleti (Crw). This hymenopteran is a solitary ectoparasitoid of several species of bruchids developing inside Leguminosae seeds. The locomotor behaviour of females reared onBruchidius atrolineatus (Pic) larvae developing inVigna unguiculata (Walp) seeds was analysed using a tubular olfactometer. Females showed a specific sensitivity to the semiochemicals emanating from the host and the seed species on which they had developed. Odours fromV. unguiculata seeds were attractive to the parasitoid and stimulated their locomotor activity. Odours fromVigna radiata (Wil) seeds had no effect on the locomotor behaviour. Odours fromB. atrolineatus larvae were attractive to the females whereas odours fromCallosobruchus maculatus (Fab), another bruchid species, had no effect. By isolating the females from the seed and the host at different developmental stages, we found that the specific sensitivity observed resulted from an early adult learning. This learning which occurs before the emergence from the seed while the imago is in the larval chamber of its host is dependent on contact with the seed and the host larval remains.     

Response of the parasitoidEupelmus vuilleti to the odours of the phytophagous host and its host plant in an olfactometer

Eupelmus vuilleti (CRW) is a solitary ectoparasitoid which, under our experimental conditions, developed at the expense of the larvae and pupae ofBruchidius atrolineatus (Pic) (Coleoptera, Bruchidae). The larvae of this beetle are found inside the seeds of cowpea,Vigna unguiculata (Walp). Using a tubular olfactometer, we determined the olfactory stimuli involved in the orientation of females of the hymenopterous speciesE. vuilleti towards one of its hosts;B. atrolineatus. Several odours were tested including those from host plants ofB. atrolineatus, from non-host plants, from the host itself and from the waste products of the host. The odours of pods or dry seeds ofV. unguiculata (host plant of the phytophagous insect) affected the locomotor behaviour of females ofE. vuilleti. The odours from the plant in the vegetative stage had no effect. Kairomones from larvae, pupae or faeces ofB. atrolineatus were attractive to the parasitoid females. Odours from adult male or femaleB. atrolineatus, as well as the sex pheromone emitted by females were not attractive toE. vuilleti females. This sex pheromone did however stimulate the locomotor activity of the parasitoid. The adaptive significance of these olfactory stimuli is discussed.

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Résumé Eupelmus vuilleti (CRW) est un ectoparasito?de solitaire qui se développe dans nos conditions expérimentales aux dépens des larves et des nymphes deBruchidius atrolineatus (Pic) (Coléoptère:Bruchidae). Les larves de ce Coléoptère se trouvent à l'intérieur des graines d'une LégumineuseVigna unguiculata (Walp). Grace à un olfactomètre tubulaire, nous avons déterminé les différents stimuli olfactifs intervenant dans l'orientation des femelles de cet hyménoptère, vers l;un de ses h?tes;Bruchidius atrolineatus. Plusieurs odeurs émanant soit de plantes-h?tes ou nonh?tes deB. atrolineatus, soit de l'h?te lui-même ou résultant de son métabolisme ont été testées. Les odeurs de gousses ou de graines sèches deVigna unguiculata Walp (plante-h?te du phytophage) influencent le comportement locomoteur des femelles. Par contre, les odeurs de plantes au stade végétatif n'ont pas d'effet. Les kairomones émanant de larves, nymphes ou fèces de larves deB. atrolineatus exercent un effect attractif sur les femelles parasito?des. Les odeurs émanant d'adultes males ou femelles deB. atrolineatus ainsi que la phéromone sexuelle émise par les femelles ne sont pas attractives pour les femelles deE. vuilleti. Cette phéromone sexuelle, par contre, stimule l'activité locomotrice des parasito?des. La signification adaptative de ces différents stimuli olfactifs est discutée.

     

Analysis of multiparasitism by Eupelmus vuilleti (Craw) (Eupelmidae) and Dinarmus basalis (Rond) (Pteromalidae) in the presence of one of their common hosts,Bruchidius atrolineatus (Pic) (Coleoptera Bruchidae)

Bruchidius atrolineatus (Pic) is a tropical beetle (Coleoptera Bruchidae) that develops during the larval and pupal stages in the seeds of a legume Vigna unguiculata (Walp). Two species of Hymenoptera, Dinarmus basalis (Rond) and Eupelmus vuilleti (Craw), solitary ectoparasitoids of the larvae and pupae of B. atrolineatus, were introduced successively in the presence of their hosts, varying the interval between the two introductions. When D. basalis females were introduced 24 h, 3 days or 7 days after E. vuilleti, multiparasitism was low. The females had low fecundity, and their eggs were not distributed randomly over the different available hosts. When E. vuilleti females were introduced second, they oviposited on the different hosts availabe and did not avoid multiparasitism. The presence of hosts already parasitised by D. basalis increased the reproduction of E. vuilleti, and the fecundity of the females was higher than in control batches with E. vuilleti alone. E. vuilleti seems capable of detecting the ovipositor shafts drilled by the D. basalis females, and by introducing its own ovipositors killing the D. basalis eggs or larvae. When interspecific competition was occurring the number of E. vuilleti adults emerging from the seeds was no different from that observed in control batches with E. vuilleti alone, and there were always fewer D. basalis adults than in control batches (D. basalis alone). This interspecific competition reduces the influence of the two parasitoids in the biological control of bruchid populations.     

Sex ratio regulation in the waspEupelmus vuilleti (crwf.), an ectoparasitoid on bean weevil larvae (Hymenoptera: Pteromalidae)

In Africa, the seeds and/or pods ofVigna unguiculata andVigna radiata (Papilionacea) are attacked in fields and storage structures by bruchid beetlesCallosobruchus maculatus andBruchidius atrolineatus, on which parasitoid communities can develop. One of these parasitoids is the solitary ectoparasitoidEupelmus vuilleti (Eupelmidae). The storage conditions ofV. unguiculata andV. radiata favor the overlapping of all host stages during several months. These conditions suggest that female parasitoids would vary the sex ratio of their offspring according to the different sizes or developmental stages of hosts. The sex ratio ofE. vuilleti was strongly related to the developmental stage of the hostC. maculatus. Under our experimental conditions, where superparasitism is rare, the proportion of daughters varied between 5 and 25% on the third larval stage but reached 70 to 90% on the pupae. The increase in the proportion of daughters was also observed in the absence of superparasitism. In this case, there was an absolute coincidence between the sex ratio of eggs laid and that of emerged adults. Manipulation of the sex of the egg by the females seems to take place at the time of the egg's fertilization. The relation between host weight and egg sex showed that the male eggs are preferentially laid on lighter host larvae and the female eggs on heavier ones.     

Beneficial arthropod behavior mediated by airborne semiochemicals. VIII. Learning of host-related odors induced by a brief contact experience with host by-products inCotesia marginiventris (Cresson), a generalist larval parasitoid

Females of Cotesia marginiventris(Cresson), a generalist larval parasitoid, were observed to respond to host related odors in a four-arm olfactometer. The females were significantly more responsive to the odors after a brief contact experience with host-damaged leaves contaminated with host by products. During the experience, actual encounters with hosts were not required to improve subsequent responses to host-related odors. The response to odors of the plant-host complex with which parasitoids had experience was significantly higher than the response to odors of an alternative plant-host complex. This suggests that the experience effect is due, at least partly, to associative learning. We suspect that females of this generalist parasitoid, as was recently found for those of a specialist, recognize specific semiochemicals when they contact frass of suitable host larvae. The parasitoids, subsequently, associate the surrounding odors with the possible presence of hosts, and use these odors as cues in their search for more hosts. This could be an important component in the host-searching behavior of many parasitoids.Mention of a proprietary product does not constitute an endorsement or the recommendation for its use by the USDA.     

Variation in flight response of the specialist parasitoidMacrocentrus grandii goidanich to odours from food plants of its European corn borer host

The specialist parasitoidMacrocentrus grandii Goidanich (Hymenoptera: Braconidae) appears to parasitize its polyphagous host, European corn borer (Ostrinia nubilalis (Hübner)) (Lepidoptera: Pyralidae), in only certain habitats. To determine whether it differed in its olfactory response to host-habitat odours, volatiles from four plants were isolated using Tenax. Wind tunnel bioassays of the extracts revealed that, besides corn which was tested in an earlier study, olfactory stimuli for attraction ofM. grandii females were present in potato and snap bean but not in pepper or soybean. To further characterize the response to pepper and soybean, these extracts were bioassayed in combination with an attractive extract. The results indicated that pepper volatiles evoked a neutral response inM. grandii while response to soybean volatiles appeared to be neutral or slightly negative. The innate response to soybean volatiles was altered to one of attraction after oviposition experience on soybean. Seven days after oviposition, experienced females continued to respond positively to soybean volatiles. Components of soybean volatiles responsible for the change in flight behaviour resulting from oviposition experience were eluted by nonpolar and slightly polar solvents. These results support the idea that plant odour may be a factor determining the range of plants on whichM. grandii parasitizes its host. The study indicates the occurrence of associative learning of plant-related volatiles during oviposition inM. grandii, and suggests the involvement of diverse plant compounds in the learning process.     

Adaptation to a novel host modifies host discrimination by the seed beetle Callosobruchus maculatus

Females of the seed beetle Callosobruchus maculatus avoid adding eggs to seeds that already bear eggs. Geographical variation in this behaviour has been thought to depend on differences in host size. In populations that attack small-seeded legumes, only one or two larvae can develop within a seed, and females are especially adept at detecting and rejecting occupied (egg-laden) seeds. We performed a mass-selection experiment in which replicate lines of a population associated with a small host (mung bean, Vigna radiata) were either maintained on this host or were transferred to a larger host (cowpea, Vigna unguiculata) that can support several larvae per seed. After more than 40 generations, we estimated the strength of host discrimination by presenting females a choice of egg-free and egg-laden seeds, and by quantifying how uniformly females spread their eggs among egg-free seeds. Compared to females maintained on mung bean, females from cowpea lines were more likely to accept occupied seeds in choice tests. They also distributed their eggs less uniformly, especially when cowpea served as the test host. Cowpea lines thus evolved to resemble populations that have long been associated with the larger host. A separate study showed that weaker host discrimination in the cowpea lines was accompanied by a decline in larval competitiveness, which may have further relaxed selection for avoidance of occupied hosts. Our results demonstrate that switching to a novel resource can produce rapid and predictable changes in a fitness-related insect behaviour. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour     

Effect of differential host exposure on reproduction in the solitary ectoparasitoid Eupelmus vuilleti

We examined self-superparasitism in Eupelmus vuilleti (CRW.), a solitary ectoparasitoid of larvae of Callosobruchus maculatus (F.) which develop in cowpea seeds. The reproduction of isolated females was analysed in the presence of seeds containing 10 or 30 hosts for 24, 48, and 72 h. At the two host densities, female reproductive activity increased with host exposure time, but variations in offspring number per female were limited. The propensity of the females to self-superparasitise their hosts after long exposure times could explain these results. The offspring sex ratio was female biased but tended to 0.5 for longer exposure times. The absence of host renewal for a 3-day period was followed by an increase in self-superparasitism which reduced the parasitoid survival rate and by emergence of a lower number of females in the offspring. The offspring production was then analysed when females were provided with ten hosts a day or with 30 hosts every three days throughout their life span. The reduction of host renewal frequency did not affect male production but caused a reduction in the number of females emerging from the seeds. The importance of a high rate of self-superparasitism on the population biology of this parasitoid is discussed.     

Influence des inflorescences de la plante hôte (Vigna unguiculata) sur la levée de la diapause reproductrice de Bruchidius atrolineatus

Bruchidius atrolineatus (Pic) présente en zone sahélienne une diapause reproductrice durant la saison sèche et une partie de la saison des pluies. Les femelles diapausantes ne produisent pas de vitellogénine et le germarium des ovarioles est seul développé. Chez les mâles la spermatogenèse est très ralentie et les glandes annexes sont inactives. Lorsque les insectes diapausants sont placés en présence d'inflorescences de Vigna unguiculata Walp, leurs organes reproducteurs deviennent fonctionnels après un temps de latence de 15 à 20 jours. Il n'y a par contre aucune levée de la diapause chez des bruches placées en présence de gousses sèches de V. unguiculata dans une atmosphère saturée. Des informations sensorielles issues des pièces florales ou (et) des facteurs nutritionnels sans doute liés a la consommation de nectar semblent être à l'origine de cette levée de la diapause. Le pollen fort peu consommé n'a par contre aucun rôle. Cette régulation du cycle reproducteur de B. atrolineatus par les inflorescences de V. unguiculata permet l'émission des oeufs dès que les gousses commencent à se former à la fin de la saison des pluies.

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Summary Bruchidius atrolineatus (Pic) is a widely distributed bruchid in the Sahelian zone which shows a reproductive diapause during the dry season and part of the rainy season. Diapausing females do not produce vitellogenin and their ovaries are reduced to the germarium. Spermatogenesis is very much reduced and male accessory glands are inactive. When these insects were placed in the presence of inflorescences of Vigna unguiculata which were renewed daily, the reproductive diapause of both males and females was interrupted after 15–20 days. Vitellogenesis occurred in the females and spermatogenesis increased in the males whilst their accessory glands became functional. When diapausing bruchids, found in stores of on V. unguiculata seeds during the dry season, were placed near the host plant's inflorescences, diapause was also terminated. In all cases, diapause was not interrupted when the insects were offered dry pods of V. unguiculata in a water-saturated atmosphere. The pollen, which is hardly eaten by this bruchid, did not seem to stimulate termination of diapause. Sensory stimulations derived from the flowers or/and nutritional factors may be the cause of the development of the reproductive organs. After termination of the diapause the males showed normal sexual activity whereas female fecundity was rather low, at least in our experimental conditions. This type of reproductive regulation allows this sahelian bruchid to resume its sexual activity when the host plant's flowers appear in the field at the end of the rainy season. Then the beetles lay their eggs on the pods as soon as the pods are developed.

     

Hyperparasitisme facultatif de parasitoïdes en cours de développement par les femelles des ectoparasitoïdes Eupelmus vuilleti et Eupelmus orientalis Craw

In a community of three ectoparasitoids, Dinarmus basalis, Eupelmus vuilleti and E. orientalis, the host Callosobruchus maculatus parasitised 48 h before by D. basalis, is accepted by E. vuilleti females after they have eliminated the eggs and neonatal larvae of D. basalis. This ovicidal and larvicidal behaviour enables E. vuilleti to develop on C. maculatus instead of D. basalis. E. vuilleti females are able to parasitise the L5 larval stage and the pupa of D. basalis: their larvae therefore feed at the expense of the developing parasitoid. This trophic level is that of hyperparasitism. However, E. vuilleti females rarely practise hyperparasitism on their own L5 larvae and on those of E. orientalis. This behaviour reveals a high behavioural plasticity enabled by intra- and interspecific recognition of parasitoids used as hosts. Hyperparasitism activity in E. orientalis females is higher than that in E. vuilleti females since they hyperparasitise host parasitoids more frequently without preferential species choice. However, E. vuilleti seems to be free from competitive pressure with E. orientalis, as the former penetrates deeply into a grain store contaminated with C. maculatus in contrast to E. orientalis females, which remain on the surface from where they escaped.     

The Role of Chemical Cues in Host and Mate Location in the Pear Psylla Cacopsylla bidens (Homoptera: Psyllidae)

Chemical communication was shown to play a role in the pear psylla, Cacopsylla bidens. Electrophysiological (EAG) and behavioral responses were investigated in males and females pear psylla . Males were found to be attracted to females, and especially to those on host plants, but not to males, uninfested host plants, or plants infested with conspecific larvae. On the other hand, females were not attracted to males or females but displayed some attraction to host plants. Furthermore, females showed a preference for uninfested pear versus plants infested with conspecific larvae. The antennae of males gave highest electroantenographic response to volatiles from pears infested with females but not males, while females, responded also toward the volatiles of pear alone. These results indicate that females of C. bidens emit sex pheromones that are attractive to the males and suggest that, host volatiles may play a role in host selection by pear psylla females.     

Learning of host-infested plant volatiles in the larval parasitoid Cotesia kariyai

The ability to learn plant volatiles in Cotesia kariyai females was examined by wind tunnel bioassays. Searching experience on a host-infested corn plant increased subsequent flight responses of females to the infested plant. Females experiencing host by-products together with the volatiles extracted from infested leaves one time showed an increased response. However, such behavioral changes were not observed in females which experienced only the host by-products or the volatiles. Thus, the increased response is considered to be preference learning. Multiple experiences of C. kariyai with host by-products together with the volatiles did not increase their flight response to the volatiles. Furthermore, this learned response gradually decreased within 2 days. These behavioral modifications based on experience would be advantageous for C. kariyai to locate their polyphagous hosts efficiently.     

Oviposition behavior of Edovum puttleri, reared on two hosts, Leptinotarsa decemlineata and L. texana

Female Edovum puttleri Grissell [Hymenoptera: Eulophidae], reared from eggs of Leptinotarsa decemlineata (Say) or Leptinotarsa texana Schaeffer [Coleoptera: Chrysomelidae], were videotaped as they attacked egg masses of L. decemlineata containing 20 host eggs. We identified 15 components of ovipositional behavior. Parasitoids reared on L. texana attacked and oviposited in significantly more host eggs than did females reared on L. decemlineata. Ethometric analyses of behavioral transitions and a clustering analysis of 34 behavioral parameters showed that females reared on L. texana attacked the host egg mass in a different manner than those reared from L. decemlineata. It was concluded that differences were associated with the host species upon which they were reared. Contrary to previous reports, mortality of unparasitized hosts was caused by an ovipositor probe of short duration, which was not related to host-feeding.     

Host location in Oomyzus gallerucae (Hymenoptera: Eulophidae), an egg parasitoid of the elm leaf beetle Xanthogaleruca luteola (Coleoptera: Chrysomelidae)

Eggs of the elm leaf beetle Xanthogaleruca luteola are often heavily attacked by the chalcidoid wasp Oomyzus gallerucae. We studied the chemical signals mediating interactions between the egg parasitoid, its host, and the plant Ulmus campestris. Olfactometer bioassays with O. gallerucae showed that volatiles of the host-plant complex attract the parasitoid. In order to determine the source of attractive volatiles within this host-plant-complex, we tested separately the effect of odours of eggs, gravid elm leaf beetle females, faeces of the beetles and elm twigs (with undamaged leaves and leaves damaged either mechanically or by feeding of the beetles). Odours of faeces of the elm leaf beetle were attractive, whereas neither volatiles from eggs nor from gravid females acted as attractants. Volatiles from undamaged or damaged plants did not elicit a positive reaction in O. gallerucae, whereas volatiles from feeding-damaged plants onto which host eggs had been deposited were attractive. This latter result suggests that it is not feeding but deposition of host eggs onto elm leaves that induces the production of plant volatiles attractive to the egg parasitoid. Investigations of the search patterns of O. gallerucae within the habitat by laboratory bioassays revealed that the egg parasitoid encounters host eggs by chance. Contact kairomones from faeces were demonstrated to be important in microhabitat acceptance, while contact kairomones isolated from the host eggs are relevant for host recognition. Received: 12 February 1997 / Accepted: 29 April 1997     

Larval diet influence on oviposition behaviour inSpodoptera littoralis

Females ofSpodoptera littoralis Boisd. (Lepidoptera: Noctuidae) with different feeding experiences during their larval development were tested for their ovipositional response to methanol extracts of larval frass and semisynthetic diets. The effect of the following frass, diet and diet component extracts was tested: (a) frass fromS. littoralis orAgrotis segetum larvae fed on a potato-based diet; (b) frass fromS. littoralis larvae fed on a wheat germ-based diet; (c) potato and wheat germ-based diets; and (d) potatoes and wheat germ. Ovipositing females without prior experience of the potato diet were deterred by extracts of: (1) larval frass from either species fed on potato diet; (2) the potato-based diet; (3) potato. Also females with experience of the potato diet during only a part of their larval development were deterred from oviposition by frass of larvae reared on the potato diet and by the diet itself. However, for females reared on the potato diet for their entire larval development, oviposition was no longer deterred by either of the three extracts listed above. Extracts of: (1) frass from larvae of either species reared on wheat germ diet: (2) the wheat germ diet; or (3) wheat germ did not significantly affect oviposition. Females with ablated antennae were still deterred by frass extracts from larvae fed on potato diet, when they had been reared on the wheat germ diet. In feeding experiments, larvae of larval stage one and of larval stage three-four reared on either of the two diets preferred to feed on the wheat germ diet. However, the preference was significantly stronger for larvae with no prior contact with the potato diet. The effect of larval experience on the loss of oviposition-deterring activity by extracts of larval frass, diets and diet components is discussed in view of induction and selection.     

Effect of host diet and preflight experience on the flight responses of Microplitis croceipes (Cresson)

Abstract. Female Microplitis croceipes without previous exposure to host diet, feeding damage or faeces, and with no oviposition experience (i.e. inexperienced) exhibited flight tunnel responses towards extracts of faeces from several diet–host species combinations, excluding artificial diet. Inexperienced females exhibited the following preference order towards faeces from Heliothis zea larvae fed selected diets: cotton > wild geranium > cowpea > artificial diet, suggesting and adaptation to 'preferred' host plants.
Females having a preflight experience with a plant-host complex (PHC) prior to testing exhibited a higher percentage of oriented flights in the wind tunnel than did inexperienced females. Oviposition Per se had little effect on the flight response of females. The increased flight response after an experience is apparently due to a combination of sensitization and associative learning. In a choice situation, a single experience was insufficient to establish a preference for the PHC experienced; however, repeated experience resulted in preferences, and the degree of preference tended to increase with the number of experiences.     

Olfactory response of three parasitoid species (Hymenoptera: Braconidae) to volatiles of guavas infested or not with fruit fly larvae (Diptera: Tephritidae)

The olfactory responses of the native parasitoids Doryctobracon areolatus (Szépligeti) and Asobara anastrephae (Muesebeck) and of the exotic parasitoid Diachasmimorpha longicaudata (Ashmead) to guava (Psidium guajava L.) infested or not with fruit fly larvae were evaluated. D. areolatus and D. longicaudata females responded to the odors of uninfested rotting guavas, although D. areolatus was also attracted to fruits at the initial maturation (turning) stage. The females of these species recognized the volatiles of guavas containing Ceratitis capitata (Wied.) larvae. However, in bioassays involving fruits with larvae of different instars, D. longicaudata females were not able to separate between fruits containing C. capitata larvae at the initial instars and larvae at the third instar. In the evaluations of volatiles released by guavas containing C. capitata and Anastrepha fraterculus (Wied.) larvae, the D. longicaudata females were oriented toward the volatiles of fruits containing both host species, but differed significantly from volatiles of guavas containing C. capitata larvae. The D. areolatus females also showed responses to both species, although with a preference for volatiles of fruits containing A. fraterculus larvae. The A. anastrephae females were oriented toward the odors of fruits infested with both fruit fly species. In the shade house, D. longicaudata females were oriented to volatiles of rotting fruits containing larvae or not, but could not significantly differentiate between hosts. D. areolatus females were not attracted toward fruits on the ground in the shade house, regardless of host, suggesting that this parasitoid does not forage on fallen fruits.     

Choosy egg parasitoids: Specificity of oviposition-induced pine volatiles exploited by an egg parasitoid of pine sawflies

Generalist parasitoids are well‐known to be able to cope with the high genotypic and phenotypic plasticity of plant volatiles by learning odours during their host encounters. In contrast, specialised parasitoids often respond innately to host‐specific cues. Previous studies have shown that females of the specialised egg parasitoid Chrysonotomyia ruforum Krausse (Hymenoptera: Eulophidae) are attracted to volatiles from Pinus sylvestris L. induced by the egg deposition of its host Diprion pini L. (Hymenoptera: Diprionidae), when they have previously experienced pine twigs with host eggs. In this study we investigated by olfactometer bioassays how specifically C. ruforum responded to oviposition‐induced plant volatiles. Furthermore, we studied whether parasitoids show an innate response to oviposition‐induced pine volatiles. Naïve parasitoids were not attracted to oviposition‐induced pine volatiles. The attractiveness of volatiles from pines carrying eggs was shown to be specific for the pine and herbivore species, respectively (species specificity). We also tested whether not only oviposition, but also larval feeding, induces attractive volatiles (developmental stage specificity). The feeding of D. pini larvae did not induce the emission of P. sylvestris volatiles attractive to the egg parasitoid. Our results show that a specialist egg parasitoid does not innately show a positive response to oviposition‐induced plant volatiles, but needs to learn them. Furthermore, the results show that C. ruforum as a specialist does not learn a wide range of volatiles as some generalists do, but instead learns only a very specific oviposition‐induced plant volatile pattern, i.e., a pattern induced by the most preferred host species laying eggs on the most preferred food plant.     

Comparative headspace analysis of cabbage plants damaged by two species ofPieris caterpillars: consequences for in-flight host location byCotesia parasitoids

Headspace composition, collected from intact cabbage plants and cabbage plants infested with eitherPieris brassicae L. orP. rapae L. (Lepidoptera: Pieridae) first instar larvae, was determined by GC-MS. Twenty-one volatiles were identified in the headspace of intact plants. Twenty-two volatiles were identified in the headspace of plants infested byP. brassicae larvae, 2 of which, Z-3-hexenyl butyrate and Z-3-hexenyl isovalerate, were not detected in the headspace of either intact orP. rapae damaged plants. In the headspace of the latter, 21 compounds were identified, all of which which were also produced by intact plants. No significant quantitative differences were found between headspace composition of the plants damaged by one or the other caterpillar species. Major differences between intact and caterpillar-damaged plants in contribution to the headspace profile were revealed for hexyl acetate, Z-3-hexenyl acetate, myrcene, sabinene and 1,8-cineole. The larval endoparasitoidCotesia glomerata L. was attracted by the volatiles emanating fromB. oleracea damaged byP. brassicae first instar larvae.C. rubecula L., a specialized larval endoparasitoids ofP. rapae, was attracted by the volatiles released from theB. oleracea-P. rapae plant-host complex. This shows that cabbage plants kept under the conditions of headspace collection produce attractive volatiles for both parasitoids.     

Foraging behaviour of the parasitoid Lysiphlebus testaceipes (Hymenoptera: Braconidae) in response to plant volatiles,with reference to biocontrol of aphids in peri-urban vegetable production systems

Foraging behaviour of Lysiphlebus testaceipes was studied in a Y-tube olfactometer. Naïve females responded to the Aphis gossypii/eggplant complex on which they had been reared, but did not show a preference for this aphid/plant complex over an uninfested eggplant, an eggplant that had been damaged by the aphid or an A. gossypii/okra complex. Females that had been allowed to forage on the A. gossypii/eggplant complex before being tested in the olfactometer showed a strong preference for an uninfested eggplant over an uninfested okra plant and for the A. gossypii/eggplant complex over an A. gossypii/okra complex, indicating that they had learned to associate eggplant-derived volatiles with host availability. When females that had been reared on the A. gossypii/eggplant complex were tested after foraging experience on the A. gossypii/okra complex, they did not show a preference for either complex, suggesting that they still retained a memory of the cues associated with the complex on which they had been reared. The efficiency of L. testaceipes as a biological control agent in peri-urban vegetable production, such as the Cuban organoponics system, potentially could be improved by producing and releasing females behaviourally primed to prefer specific host/plant complexes.