The role of chemical cues in host finding and acceptance by Callosobruchus chinensis

We studied the response of female Callosobruchus chinensis to chemical cues emitted by cowpea seeds at different stages of bruchid infestation (uninfested, egg carrying, L1-, and L4-infested). Olfactory attractiveness was determined in Y-tube olfactometer assays by testing individual seed categories against either clean air or uninfested seeds. Oviposition preferences between uninfested and infested seeds were determined in petri-dish choice-experiments. The olfactometer assays revealed that weevils discriminate between seeds containing different stages of developing bruchids on the basis of olfactory cues. While odors from uninfested and egg-carrying seeds acted as attractants, odors from L1- and L4-infested seeds failed to induce a positive response by the bruchids. When given a choice between uninfested and infested seeds in the olfactometer, weevils preferred uninfested seeds over L1- and L4-infested seeds, but failed to distinguish between uninfested and egg-carrying seeds. In the oviposition experiment as well, bruchids showed a distinct preference for uninfested seeds when offered in combination with L1- and L4-infested seeds. This experiment further showed a reduced acceptance of egg carrying seeds. Our results indicate that C. chinensis females use chemical information during both host searching and host acceptance. Volatiles from uninfested or egg carrying seeds act as attractants, while deterrence increases as development of bruchid immature stages progresses.     

Effect of host quality of Callosobruchus maculatus (Coleoptera: Bruchidae) on performance of the egg parasitoid Uscana lariophaga (Hymenoptera: Trichogrammatidae)

Development and reproductive success of the solitary egg parasitoid Uscana lariophaga Steffan were examined after development in eggs of the bruchid storage pest Callosobruchus maculatus Fabricius reared at either low or high densities on cowpea seeds and laid at day 1 and 4 of maternal life. Both bruchid larval competition and maternal age negatively affected egg size, but the latter more than the former. Uscana lariophaga reared in small hosts developed slower, were smaller and produced fewer eggs compared to parasitoids reared in large hosts. Fecundity of the parasitoid was heavily influenced by host egg size. This was reflected in the values for the intrinsic rate of increase of U. lariophaga, which differed for wasps that developed in host eggs laid by bruchid females of different age. Wasps allocated marginally more female offspring to larger hosts.     

Oviposition deterrents of Callosobruchus maculatus (Coleoptera: Bruchidae)

Abstract Many bruchids distribute their eggs uniformly among the host seeds available to them and tend to avoid adding eggs to seeds bearing any eggs if pristine seeds remain. Several authors have postulated that the adult cowpea seed beetles, Callosobruchus maculatus (F.) (Coleoptera: Bruchidae), produce pheromones inhibiting oviposition by females exposed to them. It is shown that both sexes produce compounds which can be washed from glass surfaces in a variety of organic solvents which do not deter oviposition as such, but seeds coated with such washings are avoided by ovipositing females when they are selecting seeds for egg-laying and uncoated seeds are available. Some components, at least, of the products persist for at least 30 days, longer than the normal development time of insects in the seeds. The compounds affect the distribution of eggs among the available seeds. At least three different biotypes of the species produce, recognize, and respond in the same way to each other's products. It is emphasized that the eggs themselves may not be the site of the product produced by females but that the physical presence of eggs cannot be overlooked in any discussion of the possible role of pheromones in oviposition and egg distribution by bruchids.     

Host discrimination modulates brood guarding behaviour and the adaptive superparasitism in the parasitoid wasp Trissolcus semistriatus (Hymenoptera: Scelionidae)

Because hosts utilized by parasitoids are vulnerable to further oviposition by conspecifics, host guarding benefits female wasps. The present study aims to test whether female adults regulate brood guarding behaviour by host discrimination in a solitary parasitoid Trissolcus semistriatus by presenting an intact or parasitized host egg mass to a female adult. Virgin females without oviposition experience have host discrimination ability, which enables them to adjust the number of eggs laid in the hosts. Mating experience increases superparasitism by female adults, whereas mated females achieve a higher discrimination ability as a result of oviposition experience and show a lower superparasitism rate. As expected, females exhibit brood guard after parasitizing an intact host egg mass, whereas those females visiting a previously parasitized host egg mass, do not. Because the survival of eggs in superparasitized hosts is relatively low, regulating brood guarding behaviour by host discrimination is adaptive for female wasps.     

Influence of egg load and oviposition time interval on the host discrimination and offspring survival of Anagyrus pseudococci (Hymenoptera: encyrtidae), a solitary endoparasitoid of citrus mealybug, ++Planococcus citri (Hemiptera: pseudococcidae)

Oviposition and host discrimination behaviour of unmated Anagyrus pseudococci (Girault), an endoparasitoid of the citrus mealybug Planococcus citri (Risso), were investigated in the laboratory. Female parasitoids were able to discriminate between parasitized hosts and healthy ones. The mean number of ovipositions was significantly higher in unparasitized than in parasitized hosts. Conspecific-superparasitism occurred more often than self-superparasitism. Changes in consecutive ovipositions over three hours by A. pseudococci suggested that egg load influenced the discrimination behaviour of the parasitoids, with females which had low egg loads mostly avoiding oviposition in already parasitized hosts at time intervals ranging from 0 h to 96 h, and distributing their eggs in the high quality (unparasitized) hosts. The parasitized hosts were rejected more commonly through antennal perception of external markers than during ovipositor probing which could have encountered internal markers but this relationship changed with increasing time after oviposition. The parasitoid's oviposition rate in unparasitized and conspecific-parasitized hosts varied at the different oviposition time intervals when the females had fewer eggs in the ovaries. Percentage emergence of parasitized offspring was not significantly influenced by whether they developed in single or superparasitized mealybugs. The significance of host discrimination by A. pseudococci is discussed.     

Ovicide in the whitefly parasitoid, Encarsia formosa

The oviposition decisions made by insect parasitoids when encountering hosts of variable quality have been the subject of extensive theoretical and experimental investigation. For parasitoids that lay their eggs inside the host, the possible outcomes of encounters with parasitized hosts have been assumed to include only oviposition (superparasitism), rejection, or in some cases feeding on host haemolymph. We document another outcome in Encarsia formosa Gahan (Hymenoptera: Aphelinidae), a species that has been a model system for the study of oviposition behaviour. In E. formosa, females may kill eggs previously laid within the host by jabbing them with their ovipositor before ovipositing themselves. (1) Our observations indicated that jabbed eggs were indeed killed. (2) In experimental arenas in the laboratory, ovicide occurred in the majority of encounters with parasitized hosts and at highest frequency in encounters resulting in oviposition. (3) There was no significant difference in the handling time associated with oviposition+ovicide in parasitized hosts in comparison with oviposition alone, suggesting that there is no time cost to ovicide. (4) Ovicide did not appear to be incidental to normal probing within a host. Radial analysis of the direction of ovipositor movement with respect to the centre of the previously laid egg within the host showed that females engaged in ovicidal bouts probed most often in the direction of the egg. This is the first well-documented study of ovicide in an endoparasitoid. We suggest ovicide may be under-reported in other endoparasitoid species due to the difficulty of observing it. Copyright 2000 The Association for the Study of Animal Behaviour.     

Egg performance on an egg-carrying bug. Experiments in the field

Selection of oviposition sites has direct influence on female fitness. Differences in offspring survival among sites should favour females to select oviposition sites with the highest survival. Golden egg bug females ( Phyllomorpha laciniata; Coreidae, Heteroptera) use conspecifics as oviposition substrates. Most eggs are laid on the back of the bug but they can be found on all body parts. Females never carry their own eggs, and males commonly carry eggs received without copulation with the donor. We examined differences in egg survival relative to paternity to the egg, host sex and body size and attachment of an egg on a host's body in the field. We also studied which bugs received eggs. Egg loss was surprisingly common: 30–80% of recaptured individuals lost eggs during a six-day period. Unexpectedly, host's paternity to the eggs did not affect egg survival. Also, egg loss did not differ among hosts in respect to other parameters studied. Males received more eggs than females, but survival of the eggs was not better on a male's back than on a female's back. Since egg loss is common in the golden egg bug, we suggest that female oviposition strategy to lay eggs on conspecifics is not perfect. This is due to active behaviour of potential hosts (for example, rejected oviposition attempts) or lack of suitable oviposition substrates (conspecifics) in the field. Since eggs do not survive unless carried, females may be acting opportunistically, doing the best job possible by laying eggs on available conspecifics.     

Manipulation of oviposition choice of the parasitoid wasp, Encarsia pergandiella, by the endosymbiotic bacterium Cardinium

Reproductive manipulations of hosts by maternally inherited bacterial endosymbionts often result in an increase in the proportion of infected female hosts in the population. When this involves the conversion of incipient males to genetic or functional females, it presents unique difficulties for symbionts invading hosts with sex-specific reproductive behaviours, such as the autoparasitic Encarsia pergandiella. In sexual forms of this species, female eggs are laid in whitefly nymphs and male eggs are laid in conspecific or heterospecific parasitoids developing within the whitefly cuticle. Further, eggs laid in the 'wrong' host do not ordinarily complete development. This study explored the role of a bacterial symbiont, Cardinium, in manipulating oviposition behaviour in a thelytokous population of E. pergandiella. Oviposition choice was measured by the number and location of eggs deposited by both infected and uninfected adult waSPS in arenas containing equal numbers of hosts suitable for the development of male and female waSPS. Uninfected waSPS included antibiotic-treated female waSPS and (untreated) daughters of antibiotic-treated female waSPS. The choices of waSPS in the thelytokous population treatments were compared with those of a conspecific sexual population. We found that offspring of antibiotic-cured thelytokous waSPS reverted to the behaviour of unmated sexual waSPS, laying their few eggs almost exclusively in hosts appropriate for male eggs. Infected thelytokous waSPS distributed their eggs approximately evenly between host types, much like mated sexual female waSPS. The antibiotic-treated female waSPS exhibited choices intermediate to waSPS in the other two treatments. The change in the observed behaviour appears sufficient to allow invasion and persistence of Cardinium in sexual populations. Lastly, our results suggest a reduction in host discrimination as a possible mechanism by which Cardinium influences this change.     

Nocturnal activity and resource utilization in the aphid hyperparasitoid, Dendrocerus carpenteri

Abstract.

• 1 Females of the aphid hyperparasitoid Dendrocerus carpenteri (Curtis) search successfully for hosts during both day and night. Oviposition numbers per host patch did not differ significantly between day and night.
• 2 D.carpenteri females also displayed a nocturnal flight activity, showing that they are not only capable of searching on a given host plant but also of dispersing between host plants.
• 3 Nocturnal oviposition activity was mainly influenced by egg load. Females with a high egg load laid more eggs at night than females with a comparatively low egg load. Thus, D.carpenteri females may use nocturnal foraging to compensate for the lack of oviposition opportunities during day.
• 4 D.carpenteri females which foraged continuously for hosts both day and night (= for 24 h per day) benefitted from an 1.4-fold increase in lifetime reproductive success when compared to females which foraged only by day (= for 16 h per day).
• 5 The benefit of night foraging for this species is a significantly increased reproductive success.

     

Description and behavioural biology of two Ufens species (Hymenoptera: Trichogrammatidae), egg parasitoids of Homalodisca species (Hemiptera: Cicadellidae) in southern California

The morphology, distribution, and hosts of two egg parasitoids, Ufens principalis Owen sp. n. and U. ceratus Owen sp. n. (Hymenoptera: Trichogrammatidae), are described for the first time. These species are compared to U. niger (Ashmead), the only species of Ufens s. str. currently described from the Nearctic, and diagnostic differences are presented. The behavioural biology of U. principalis, and U. ceratus to a smaller extent, is also described for the first time. Ufens principalis exhibited a rapid and long-distance response in the form of directional flight toward freshly laid eggs of Homalodisca species, its primary hosts in southern California. Parasitism involved aggregations of female U. principalis on fresh Homalodisca egg masses, which remained attractive to U. principalis for a relatively short time. The level of oviposition by U. principalis females was low during most of the day and peaked before sunset in tandem with a peak in Homalodisca oviposition. Oviposition behaviour of U. principalis is described and the distribution of ovipositor probe durations showed that most probes were generally of very short duration. Mating of both Ufens species occurred on the egg mass, with males showing aggressive behaviour towards each other as they competed for emerging females. Ufens ceratus males displayed greater aggression towards other males than U. principalis males. By contrast, fights among U. principalis males involved more individuals and lasted longer than corresponding fights between U. ceratus males.     

Impact of mating status on egg-laying and superparasitism behaviour in a parasitoid wasp

Most parasitoid female wasps can distinguish between unparasitized and parasitized hosts and use this information to optimize their progeny and sex allocation. In this study, we explored the impact of mating on oviposition behaviour (parasitism and self‐ and conspecific superparasitism) on both unparasitized and already parasitized hosts in the solitary parasitoid wasp Eupelmus vuilleti (Crw.) (Hymenoptera: Eupelmidae). Virgin and mated females had the same oviposition behaviour and laid eggs preferentially on unparasitized hosts. The sex ratio (as the proportion of females) of eggs laid by mated females in parasitism and conspecific superparasitism was 0.67 ± 0.04 and 0.57 ± 0.09, respectively. Likewise, females laid more eggs in conspecific superparasitism than self‐superparasitism under our experimental conditions. These experiments demonstrate that E. vuilleti females can (i) discriminate between unparasitized and parasitized hosts and adapt the number of eggs they lay accordingly, and (ii) probably discriminate self from conspecific superparasitized hosts. Finally, mating does not appear to influence the host discrimination capacity, the ovarian function, or the oviposition behaviour.     

Oviposition and offspring survival within eggs of European corn borer: discrimination of the host embryo by female Trichogramma nubilale

Oviposition preference and offspring survival relative to the location of the host embryo was tested in the egg parasitoid Trichogramma nubilale on European corn borer host eggs. Females preferentially oviposited near to the embryo on hosts about 24 h old. Survival of Trichogramma was lower when females oviposited far from the embryo, and a higher proportion of host embryos hatched. Females did not show preferences for drilling or oviposition within very young hosts relative to where the embryo would develop, and these eggs did not produce wasps or host larvae. Female behavior was apparently adaptive in day-old hosts, not to very young hosts.     

Alternative seed defence mechanisms in a palo verde (Fabaceae) hybrid zone: effects on bruchid beetle abundance

Abstract.

• 1 We asked three questions about the patterns of relative abundance of insect herbivores across host plant taxa at a palo verde hybrid zone. (1) What is the morphological structure of the hybrid zone and does this suggest a certain pattern of introgression? (2) How are putative parental seed defence mechanisms expressed in hybrid plants? (3) Do ovipositing females prefer host plant taxa on which their offspring have best survivorship?
• 2 Morphologically, hybrids were either intermediate or tended to resemble one parental species. Previous studies have suggested that unidirectional introgression results in loss of parental defence mechanisms against herbivores. Hybrid plants in general lacked seed coat resistance and early pod abscission which are known to act as plant defence mechanisms against bruchid beetles in the parental palo verde trees.
• 3 All other sources of bruchid mortality that we examined did not vary across parental and hybrid taxa, with the possible exception of egg parasitism which occurred at a lower frequency on one parental palo verde species.
• 4 Thus, survivorship of bruchid offspring should be greater on hybrid palo verdes.
• 5 Patterns of egg densities suggested that females may prefer hybrid hosts in some years but not others. An oviposition choice experiment conducted in the field, however, showed bruchids have no preference for hybrids over one of the parental species.
• 6 These results suggest that some insect herbivores may have higher densities on hybrid host plants because they are less resistant.

     

Oviposition decisions in an endoparasitoid under self-superparasitism conditions

Superparasitism is a widespread phenomenon. Having accepted superparasitism, mated female parasitoids must decide on the sex of each egg they subsequently lay into the same host. Theory predicts that this decision is either based on host quality, when more male eggs are laid in hosts that are already parasitized because they are perceived to be of poorer quality; or more eggs are laid of the sex that is most likely to be a strong larval competitor, i.e. generally females.Anastatus disparis is a facultative endoparasitic egg parasitoid. We used ‘artificial’ hosts to explore outcomes of decision making by A. disparis during superparasitism under a manipulated absence of larval competition. When only one egg was laid it was always female. As the number of eggs laid increased, so more of them were male. This supports the theory that oviposition decisions are based on host quality; more male eggs were laid in hosts that were already parasitized and thus of poorer quality.In a second experiment, eggs were exposed to parasitoids for different periods of time. Half the eggs were dissected to determine the number of parasitoid eggs that had been laid. The remaining eggs were incubated and the number and sex of offspring that ultimately emerged, following larval competition, were recorded. Under superparasitism conditions fierce larval competition ensued; only one offspring survived and they were predominantly female.In conclusion, oviposition decisions by female A. disparis accepting self-superparasitism were made based on host quality.     

Effects of seed type and bruchid genotype on the performance and oviposition behavior of Zabrotes subfasciatus (Coleoptera: Bruchidae)

The effect of different bean varieties on the performance of the bruchid beetle Zabrotes subfasciatus Boheman (Coleoptera: Bruchidae), was determined by using wild and cultivated seeds of the genus Phaseolus. Results showed that the quality of the host plant affected the performance and the oviposition behavior of female beetles. Overall, bruchid performance was higher on cultivated seeds than on wild seeds. It was also found that the oviposition behavior and the performance of their offspring differed between females that originated from wild versus cultivated seeds. We also demonstrated the importance of a genetic component in bruchid performance: longevity, fecundity, larval development, adult size, and the sex ratio of the progeny varied according to their maternal lineage (‘familyrsquo;). For example, on the same host type, some females laid twice as many eggs as females from other ‘familiesrsquo;, and the sex ratio among the offspring correlated with that of their mother. Thus, the performance and behavior of Z. subfasciatus are not only affected by environmen tal factors such as the quality of the seeds on which they develop, but also have a genetic basis which can counterbalance a less suitable quality of the host plant. For a crop pest such as Z. subfasciatus, its ability to survive and adapt on host plants of differing quality may be an important attribute to consider for pest management.     

Females of the weevil Zabrotes subfasciatus manipulate the size and number of eggs according to the host seed availability

Abstract. In some insects, the finding of oviposition substrate triggers the uptake into oocytes of yolk proteins that are stored in the fat body during post‐embryonic development. The main host of the bean weevil Zabrotes subfasciatus (Coleoptera; Chrysomelidae; Bruchinae; Amblycerini), in which larval resources are the sole source for future egg maturation, is Phaseolus vulgaris. Despite not feeding as adults, females of this species are able to lay eggs after encountering host seeds but it is not known how females react to changes in the availability of bean seeds. In the present study, the behaviour of Z. subfasciatus facing two very different environments for oviposition is investigated, as well as how this influences offspring fitness. The results obtained show that females of Z. subfasciatus react to variations in the availability of seeds belonging to the same host species by adjusting egg size and number. Females on low bean seed density lay larger and fewer eggs than those on high bean seed density, demonstrating a trade‐off between these reproductive traits. Moreover, females can adjust egg size to changing levels of host availability during the first 4 days of their oviposition period. Although no difference in offspring weight is found, those from small eggs (low competition environment) result in larger adults. No response to selection on these traits after rearing beetles on the same host for 40 generations is observed. This unresponsiveness may indicate that beetle populations behave according to their reaction norm that already allows rapid adaptation to a varying amount of host‐seed availability and better exploitation of the environments of this widespread stored‐seed pest.     

Effect of host deprivation on egg quality,egg load,and oviposition in a solitary parasitoid,Chetogena edwardsii (Diptera: Tachinidae)

Solitary parasitoids are limited to laying one egg per host because larvae compete within hosts. If host encounter rate is low, females should not increase the number of eggs/host in response. The tachinid fly, Chetogena edwardsii,was used to evaluate the effect of host deprivation on egg accumulation, oviposition behavior, and egg quality in a solitary parasitoid. Females deprived of hosts for 2– 7 days accumulate about 1 day's supply of eggs. Egg output of deprived females once hosts are restored does not differ from that of control females. Deprived females retain one egg in the uterus where it undergoes embryogenesis. Maggots emerging from retained eggs are more likely to survive in hosts molting in 40 h or less after receipt of an egg than are maggots emerging from eggs fertilized shortly before oviposition. Egg retention is a consequence of host deprivation that permits females to broaden the range of hosts they can exploit to include soon-to-molt hosts and possibly multiply parasitized hosts.     

Variance in number of eggs per patch: oviposition behaviour and population dispersion in a seed parasitic moth

ABSTRACT. 1. The relationship between oviposition behaviour and population dispersion was analysed for Grey a subalba Braun (Lep., Incurvariidae), a species in which females usually, but not invariably. lay eggs in only a few schizocarps (seed-pairs) on each umbellet they visit on Lomatium dissectum (Umbelliferae). During each of four years females exhibited a constant probability of leaving an umbellet after each egg was laid. Hence, some umbellets received more eggs than others and the behaviour matched a geometric distribution of eggs among umbellets.
2. The variance in number of schizocarps attacked per umbellet by individual females did not result from females choosing schizocarps of particular sizes or avoiding schizocarps that already contained eggs or larvae. A potential, but speculative, explanation of the variance is that females distribute their eggs among umbellets in a way that maximizes unpredictability on larval dispersion to a searching parasitoid, The pattern of oviposition by Greya females fits the truncated geometric distribution that is predicted if information, as indexed by the Shannon-Wiener entropy measure, is minimized. None the less, other measures of unpredictability or minimum information are possible, and the results highlight the problem of how to identify patterns of movement that may result from selection for unpredictability to enemies.
3. In systematic surveys of the L.dissectum population during three consecutive years, virtually all plants had between 10% and 65% of their seeds attacked. Hence, the separation of oviposition bouts between umbellets and the constant probability of leaving an umbellet after each egg was laid resulted in a broad distribution of Greya attack among L.dissectum plants.     

Effects of Asobara japonica venom on larval survival of host and nonhost Drosophila species

Abstract Ovipositing Asobara japonica females inject venom (containing paralysis‐inducing factors) immediately after the insertion of their ovipositors into Drosophila larvae, and lay eggs a little later. Interruption of their oviposition behaviour before egg laying causes high larval mortality in host Drosophila species, whereas normal oviposition does not. This suggests that venom of this parasitoid is toxic to larvae of these host species but its toxicity is suppressed by factor(s) provided by parasitoid females at the time of laying egg or by parasitoid embryos developing in the hosts. On the other hand, venom does not show toxicity to larvae of nonhost Drosophia species. Possible functions of venom are discussed.     

Host selection and egg allocation behaviour by Aphytis melinus and A.lingnanensis: comparison of two facultatively gregarious parasitoids

Abstract. 1. The ovipositional and egg allocation behaviour of individual females of Aphytis melinus DeBach and A.lingnunensis Compere were compared.
2. Both Aphytis species exhibit the same behavioural sequence during oviposition.
3. Aphytis melinus laid most of its female eggs on the dorsum of a scale-insect beneath its cover, and most of its male eggs under the scale-insect's body. Aphytis lingnanensis also oviposited both dorsally and ventrally on scale-insect hosts, but female and male progeny arose with equal frequency from eggs laid in both locations.
4. Both A.melinus and A. lingnanensis are facultatively gregarious parasitoids. The degree of gregariousness depends on host size, i.e. the larger the host, the more the Iikelihood that several eggs will be deposited at each visit by the parasitoid.
5. When two eggs were laid during the same host visit, both A.melinus and A.lingnanensis laid one female and one male egg more often than would be expected under an assumption of random allocation of sexes.
6. Because A.melinus successfulIy utilize smaller hosts than A.lingnanensis to produce progeny, these parasitoids should not be considered ecological homo-logues, as suggested by DeBach & Sundby (1963).