Host selection by Ibalia leucospoides based on temporal variations of volatiles from the hosts’ fungal symbiont

Insect parasitoids locate hosts via reliable and predictable cues such as volatile emissions from hosts and/or host plants. For insects that depend on mutualistic organisms, such as many wood‐boring insects, symbiont‐derived semiochemicals may represent a source of such cues to be exploited by natural enemies. Ultimately, exploitation of these signals may increase fitness by optimizing foraging efficiency. Female parasitoids of Ibalia leucospoides use volatiles from the fungal symbiont Amylostereum areolatum of their host Sirex noctlio to find concealed host eggs and young larvae within the xylem. We hypothesize that the temporal pattern of fungal emissions may indicate not only the presence of host larvae but also be used as a cue that indicates host suitability and age. Such information would allow female parasitoids to discern more efficiently between hosts within ovipositor reach from those already buried too deep into the xylem and out of reach. In this context, we assessed the behaviour of I. leucospoides females to volatiles of A. areolatum in a ‘Y’‐tube olfactometer at regular intervals over 30 days. We concurrently examined the fungal volatiles by headspace sampling through solid‐phase microextraction (SPME) followed by gas chromatography mass spectrometry (GC‐MS). We observed that females were attracted to volatiles produced by two‐week‐old fungal cultures, a period that matches when older larvae are still within ovipositor reach. Four chemical compounds were detected: ethanol, acetone, acetaldehyde and the sesquiterpene 2,2,8‐trimethyltricyclo[6.2.2.01,6]dodec‐5‐ene, with each compounds’ relative abundance changing over time. Results are discussed in the context of parasitoids fitness. Future studies involving electrophysiology, different collection techniques and further behavioural assays will help in identifying the compounds that convey temporal information to female parasitoids and have the potential for being used in integrated pest management programmes.     

Nutritional ecology of the interaction between larvae of the gregarious ectoparasitoid, Muscidifurax raptorellus (Hymenoptera: Pteromalidae), and their pupal host, Musca domestica (Diptera: Muscidae)

In this study we examined the relationship between clutch size and parasitoid development of Muscidifurax raptorellus (Hymenoptera: Pteromalidae), a gregarious idiobiont attacking pupae of the housefly, Musca domestica (Diptera: Muscidae). Host quality was controlled in the experiments by presenting female parasitoids with hosts of similar size and age. This is the first study to monitor the development of a gregarious idiobiont parasitoid throughout the course of parasitism. Most female wasps laid clutches of one to four eggs per host, although some hosts contained eight or more parasitoid larvae. In both sexes, parasitoids completed development more rapidly, but emerging adult wasp size decreased as parasitoid load increased. Furthermore, the size variability of eclosing parasitoid siblings of the same sex increased with clutch size. Irrespective of clutch size, parasitoids began feeding and growing rapidly soon after eclosion from the egg and this continued until pupation. However, parasitoids in hosts containing five or more parasitoid larvae pupated one day earlier than hosts containing one to four larvae. The results are discussed in relation to adaptive patterns of host utilization by gregarious idiobiont and koinobiont parasitoids.     

Visual and Chemical Cues Used in Host Location and Acceptance by a Dipteran Parasitoid

Locating potential hosts for egg laying is a critical challenge in the life history of many insects. Female insects in several orders have evolved mechanisms to find hosts by using olfactory and visual signals derived from their hosts. We describe visual and chemical cues used by the dipteran parasitoid Apocephalus paraponerae (Diptera: Phoridae) in the location and acceptance of its host ant Paraponera clavata (Hymenoptera: Formicidae: Ponerinae). Our results show that A. paraponerae uses the visual cue of ant body size when locating hosts at short range and that these flies lay more eggs in ants that retain their surface chemicals than in ants with these chemicals removed. We compare the cues used by A. paraponerae with cues used by tephritid fruit flies in location and acceptance of their hosts, and we suggest further avenues for the study of host location, acceptance, and host discrimination of A. paraponerae and other parasitoids of ants.     

Egg laying rather than host quality or host feeding experience drives habitat estimation in the parasitic wasp Nasonia vitripennis

In variable environments, sampling information on habitat quality is essential for making adaptive foraging decisions. In insect parasitoids, females foraging for hosts have repeatedly been shown to employ behavioral strategies that are in line with predictions from optimal foraging models. Yet, which cues exactly are employed to sample information on habitat quality has rarely been investigated. Using the gregarious parasitoid Nasonia vitripennis (Walker; Hymenoptera: Pteromalidae), we provided females with different cues about hosts to elucidate, which of them would change a wasp's posterior behavior suggesting a change in information status. We employed posterior clutch size decisions on a host as proxy for a female's estimation of habitat quality. Taking into account changes in physiological state of the foraging parasitoid, we tested whether different host qualities encountered previously change the subsequent clutch size decision in females. Additionally, we investigated whether other kinds of positive experiences—such as ample time to investigate hosts, host feeding, or egg laying—would increase a wasp's estimated value of habitat quality. Contrary to our expectations, quality differences in previously encountered hosts did not affect clutch size decisions. However, we found that prior egg laying experience changes posterior egg allocation to a host, indicating a change in female information status. Host feeding and the time available for host inspection, though correlated with egg laying experience, did not seem to contribute to this change in information status.     

Learning and the role of host-specific volatiles during in-flight host-finding in the specialist parasitoid Microplitis croceipes

Abstract The effect of antennal contact with host-related stimuli on the subsequent flight response to host-specific volatiles was examined in the specialist parasitoid, Microplitis croceipes (Cresson) (Hymenoptera: Braconidae). In a choice test between two species of caterpillars feeding on cowpea, free-flying female M.croceipes lacking prior contact with host cues landed more often on sources of odour from its host, Heliothis tea (Boddie), than from non-host Spodoptera frugiperda (Smith). After female parasitoids contacted host cues with their antennae, more landings on sources containing hosts were observed only when caterpillars used in the pre-flight treatment and in the odour sources fed on the same plant species. The degree to which host-experienced wasps landed more often on hosts as compared with non-hosts varied with host-instar and plant species. The role of learning in host finding in specialist parasitoids is discussed.     

Optimum and maximum host sizes at parasitism for the endoparasitoid Hyposoter didymator (Hymenoptera: Ichneumonidae) differ greatly between two host species

Host size is considered a reliable indicator of host quality and an important determinant of parasitoid fitness. Koinobiont parasitoids attack hosts that continue feeding and growing during parasitism. In contrast with hemolymph-feeding koinobionts, tissue-feeding koinobionts face not only a minimum host size for successful development but also a maximum host size, because consumption of the entire host is often necessary for successful egression. Here we study interactions between a generalist tissue-feeding larval endoparasitoid, Hyposoter didymator Thunberg (Hymenoptera: Ichneumonidae) and two of its natural hosts, Spodoptera exigua Hübner and Chrysodeixis chalcites Esper (Lepidoptera: Noctuidae). Larvae of C. chalcites are up to three times larger than corresponding instars of S. exigua and also attain much higher terminal masses before pupation. We hypothesized that the range of host instars suitable for successful parasitism by H. didymator would be much more restricted in the large host C. chalcites than in the smaller S. exigua. To test this hypothesis, we monitored development of H. didymator in all instars of both host species and measured survival, larval development time, and adult body mass of the parasitioid. In contrast with our predictions, C. chalcites was qualitatively superior to S. exigua in terms of the survival of parasitized hosts, the proportion of parasitoids able to complete development, and adult parasitoid size. However, in both hosts, the proportion of mature parasitoid larvae that successfully developed into adults was low at the largest host sizes. Our results suggest that qualitative, as well as quantitative, factors are important in the success of tissue-feeding parasitoids.     

Interaction of vibrational and visual cues in parasitoid host location

Female parasitoids are guided by multisensory information during host finding. Individual cues are used in an interactive or a hierarchical manner according to the relative importance on the spatial scale of their effect. Unlike most studies that concentrate on single cues, the present paper investigates the interaction of two physical cues. The interaction of mechanosensory and visual cues was studied in the pupal parasitoid Pimpla turionellae (Hymenoptera: Ichneumonidae). This species uses, amongst other senses, vibrational sounding (echolocation in a solid substrate) to find its mainly endophytic hosts. Location and frequency of ovipositor insertions were scored on cylindrical plant stem models with single or combined cues. Single-cue experiments show that parasitoids use both visual and mechanosensory cues and achieve a similar precision of host location with either cue. The combination of vision and vibrational sounding increased the precision of host location by a factor of approximately two to three. We conclude that the two senses interact, resulting in an additive accuracy. Neither the visual nor the mechanosensory cue was favored when offered adjacent to each other on the same stem model. On the investigated spatial scale, both physical cues are used and seem to be equally important for host location in this species.     

Effect of entomopoxvirus infection of the smaller tea tortrix, Adoxophyes sp. on the development of the endoparasitoid, Ascogaster reticulatus

Infection of Adoxophyes sp. (Lepidoptera: Tortricidae) larvae by an entomopoxvirus (AsEPV) adversely affected the development of the endoparasitoid, Ascogaster reticulatus Watanabe (Hymenoptera: Braconidae). Parasitoid larvae developing in AsEPV-infected hosts grew more slowly and spent more time in their hosts than did parasitoid larvae developing in noninfected hosts. Percentages of emergence of larval parasitoids that developed in AsEPV-infected hosts were significantly lower than those of parasitoids that developed in noninfected hosts. Parasitoid larvae in AsEPV-infected host perished when their hosts died of AsEPV infection. Significant numbers of parasitized and infected larvae exhibited apolysis to the final instar, whereas noninfected-parasitized larvae died in the penultimate instars due to emergence of parasitoids.     

Effects of pea aphid secondary endosymbionts on aphid resistance and development of the aphid parasitoid Aphidius ervi: a correlative study

In order to reduce parasite‐induced mortality, hosts may be involved in mutualistic interactions in which the partner contributes to resistance against the parasite. The pea aphid, Acyrthosiphon pisum Harris (Hemiptera: Aphididae), harbours secondary bacterial endosymbionts, some of which have been reported to confer resistance against aphid parasitoids. Although this resistance often results in death of the developing parasitoid larvae, some parasitoid individuals succeed in developing into adults. Whether these individuals suffer from fitness reduction compared to parasitoids developing in pea aphid clones without symbionts has not been tested so far. Using 30 pea aphid clones that differed in their endosymbiont complement, we studied the effects of these endosymbionts on aphid resistance against the parasitoid Aphidius ervi Haliday (Hymenoptera: Braconidae: Aphidiinae), host–parasitoid physiological interactions, and fitness of emerging adult parasitoids. The number of symbiont species in an aphid clone was positively correlated with a number of resistance measurements but there were also clear symbiont‐specific effects on the host–parasitoid interaction. As in previous studies, pea aphid clones infected with Hamiltonella defensa Moran et al. showed resistance against the parasitoid. In addition, pea aphid clones infected with Regiella insecticola Moran et al. and co‐infections of H. defensa–Spiroplasma, R. insecticola–Spiroplasma, and R. insecticola–H. defensa showed reduced levels of parasitism and mummification. Parasitoids emerging from symbiont‐infected aphid clones often had a longer developmental time and reduced mass. The number of teratocytes was generally lower when parasitoids oviposited in aphid clones with a symbiont complement. Interestingly, unparasitized aphids infected with Serratia symbiotica Moran et al. and R. insecticola had a higher fecundity than unparasitized aphids of uninfected pea aphid clones. We conclude that in addition to conferring resistance, pea aphid symbionts also negatively affect parasitoids that successfully hatch from aphid mummies. Because of the link between aphid resistance and the number of teratocytes, the mechanism underlying resistance by symbiont infection may involve interference with teratocyte development.     

Herbivore species identity rather than diversity of the non‐host community determines foraging behaviour of the parasitoid wasp Cotesia glomerata

Extensive research has been conducted to reveal how species diversity affects ecosystem functions and services. Yet, consequences of diversity loss for ecosystems as a whole as well as for single community members are still difficult to predict. Arthropod communities typically are species‐rich, and their species interactions, such as those between herbivores and their predators or parasitoids, may be particularly sensitive to changes in community composition. Parasitoids forage for herbivorous hosts by using herbivore‐induced plant volatiles (indirect cues) and cues produced by their host (direct cues). However, in addition to hosts, non‐suitable herbivores are present in a parasitoid's environment which may complicate the foraging process for the parasitoid. Therefore, ecosystem changes in the diversity of herbivores may affect the foraging efficiency of parasitoids. The effect of herbivore diversity may be mediated by either species numbers per se, by specific species traits, or by both. To investigate how diversity and identity of non‐host herbivores influence the behaviour of parasitoids, we created environments with different levels of non‐host diversity. On individual plants in these environments, we complemented host herbivores with 1–4 non‐host herbivore species. We subsequently studied the behaviour of the gregarious endoparasitoid Cotesia glomerata L. (Hymenoptera: Braconidae) while foraging for its gregarious host Pieris brassicae L. (Lepidoptera: Pieridae). Neither non‐host species diversity nor non‐host identity influenced the preference of the parasitoid for herbivore‐infested plants. However, after landing on the plant, non‐host species identity did affect parasitoid behaviour, whereas non‐host diversity did not. One of the non‐host species, Trichoplusia ni Hübner (Lepidoptera: Noctuidae), reduced the time the parasitoid spent on the plant as well as the number of hosts it parasitized. We conclude that non‐host herbivore species identity has a larger influence on C. glomerata foraging behaviour than non‐host species diversity. Our study shows the importance of species identity over species diversity in a multitrophic interaction of plants, herbivores, and parasitoids.     

The role of herbivore‐ and plant‐related experiences in intraspecific host preference of a relatively specialized parasitoid

Parasitoids use odor cues from infested plants and herbivore hosts to locate their hosts. Specialist parasitoids of generalist herbivores are predicted to rely more on herbivorederived cues than plant-derived cues. Microplitis croceipes (Cresson)(Hymenoptera: Braconidae) is a relatively specialized larval endoparasitoid of Heliothis virescens (F.)(Lepidoptera: Noctuidae), which is a generalist herbivore on several crops including cotton and soybean. Using M. croceipes/H. virescens as a model system, we tested the following predictions about specialist parasitoids of generalist herbivores:(i) naive parasitoids will show innate responses to herbivore-emitted kairomones, regardless of host plant identity and (ii) herbivore-related experience will have a greater influence on intraspecific oviposition preference than plant-related experience. Inexperienced (naive) female M. croceipes did not discriminate between cotton-fed and soybean-fed H. virescens in oviposition choice tests, supporting our first prediction. Oviposition experience alone with either host group influenced subsequent oviposition preference while experience with infested plants alone did not elicit preference in M. croceipes, supporting our second prediction. Furthermore, associative learning of oviposition with host-damaged plants facilitated host location. I terestingly, naive parasitoids attacked more soybeathan cotton-fed host larvae in two-choice tests when a background of host-infested cotton odor was supplied, and vice versa. This suggests that plant volatiles may have created an olfactory contrast effect. We discussed ecological significance of the results and concluded that both plant- and herbivore-related experiences play important role in parasitoid host foraging.     

Learning in the Generalist Tachinid Parasitoid Exorista Mella Walker (Diptera: Tachinidae)

Many parasitoids have been shown to learn visual and/or olfactory cues associated with hosts. In contrast to the Hymenoptera, learning in dipteran parasitoids is relatively unstudied. This study explores the ability of a polyphagous tachinid, Exorista mella, to learn to associate visual and olfactory cues with hosts. In an experiment involving colored host models, flies trained on models of one color were subsequently attracted more strongly to models of the color that they had not experienced. The unsuitability of these models as hosts suggested that the flies may have engaged in avoidance learning. Flies demonstrated the ability to learn to associate colored disks with hosts. A separate experiment demonstrated that flies responded to volatile plant compounds but failed to find evidence for odor learning. Learning of host-associated cues by E. mella may allow this generalist parasitoid to take advantage of locally abundant host populations and maintain host-searching efficiency in an environment.     

Does the solitary parasitoid Microplitis pennatulae use a combinatorial approach to manipulate its host?

Host manipulation is a strategy used by some parasites to enhance their transmission. These parasites use a combination of neuropharmacological, psychoneuroimmunological, genomic/proteomic, or symbiont-mediated mechanisms to manipulate their hosts. Bodyguard manipulation occurs when parasitized hosts guard parasitoid pupae to protect them from their natural enemies. Bodyguard-manipulated hosts exhibit altered behaviours only after the egression of parasitoid prepupae. Behavioural changes in post-parasitoid egressed hosts could have resulted from their altered physiology. Previous studies have shown that gregarious manipulative parasitoids induce multiple physiological changes in their host, but the physiological changes induced by solitary manipulative parasitoids are unknown. Microplitis pennatulae Ranjith & Rajesh (Hymenoptera: Braconidae) is a larval parasitoid of Psalis pennatula Fabricius (Lepidoptera: Erebidae). After the egression of parasitoid prepupae, P. pennatula stops its routine activities and protects the parasitoid pupa from hyperparasitoids by body thrashes. In this study, we looked into the physiological changes induced by the solitary manipulative parasitoid, M. pennatulae, in its host, P. pennatula, during various stages of parasitization. We considered octopamine concentration and phenoloxidase (PO) activity as biomarkers of physiological change. We also examined whether M. pennatulae has a symbiotic virus and whether the wasp transfers it to the host during parasitization. We found that octopamine concentration was low in the pre-parasitoid egressed host, but it was elevated after the parasitoid egressed. Phenoloxidase activity was lower in the pre- and post-parasitoid egressed host than in the unparasitized host. We also detected symbiotic bracovirus (BV) in the wasp ovaries and isolated the BV virulence gene from the parasitised host. Our study suggests that solitary parasitoids also induce multiple physiological changes to influence the host behaviour to their advantage, as is the case with the gregarious parasitoids.     

Cardinium in Plagiomerus diaspidis (Hymenoptera: Encyrtidae)

The bacterial symbiont Cardinium (Bacteroidetes) was previously implicated in the thelytokous reproduction of the parasitoid Plagiomerus diaspidis Crawford (Hymenoptera: Encyrtidae). Horizontal transmission of the symbiont among the cactus scale Diaspis echinocacti Bouché (Homoptera: Diaspididae) and its hymenopteran parasitoids has been suggested. In this study, the bacteria associated with D. echinocacti, its parasitoids P. diaspidis and Aphytis sp. (Hymenoptera: Aphelinidae), and the hyperparasitoid Marietta leopardina Motschulsky (Hymenoptera: Aphelinidae) were characterized using molecular fingerprinting techniques, and the localization of Cardinium in P. diaspidis was studied using fluorescence in situ hybridizations (FISH). Cardinium was the only bacterium found in P. diaspidis, but it could not be detected in any of the other insects tested. The symbiont was specifically located in the reproductive tissues of its P. diaspidis host.     

Influence of visual cues on host‐searching and learning behaviour of the egg parasitoids Telenomus podisi and Trissolcus basalis

Insect parasitoids use a variety of chemical and physical cues when foraging for hosts and food. Parasitoids can learn cues that lead them to the hosts, thus contributing to better foraging. One of the cues that influence host‐searching behaviour could be colour. In this study, we investigated the ability of females of the parasitoid wasps Telenomus podisi Ashmead and Trissolcus basalis Wollaston (both Hymenoptera: Scelionidae) to respond to colours and to associate the presence of hosts – eggs of Euschistus heros (Fabricius) (Hemiptera: Pentatomidae) – with coloured substrates after training (associative learning). Two sets of experiments were conducted: in one the innate preference for substrate colours was examined, in the other associative learning of substrate colour and host presence was tested in multiple‐choice and dual‐choice experiments. In the associative learning experiments, Te. podisi and Tr. basalis were trained to respond to differently coloured substrates containing hosts in two sessions of 2 h each, with 1‐h intervals. In multiple‐choice experiments, the wasps displayed innate preference for yellow substrates over green, brown, black, or white ones. Even after being trained on substrates of different colours, both parasitoids continued to show preference for yellow substrates. The response to the colours of substrates of both parasitoids was related with the orientation to the plant foliage during the search for hosts.     

Does mother really know best? Oviposition preference reduces reproductive performance in the generalist parasitoid Aphidius ervi

The reproductive success of female parasitoids is dependent on their ability to accurately assess the suitability of a host for larval development. For generalist parasitoids, which utilize a broad range of species and instars as hosts, a set of assessment criteria determines whether a host is accepted or rejected. The suitability of a host, however, can only be imperfectly assessed by the female parasitoid, which can result in the selection of lesser quality hosts for oviposition. In this study we explored the disparity between host quality and host preference using the generalist koinobiotic parasitoid Aphidius ervi Haliday (Hymenoptera: Aphidiidae) and the host Aulacorthum solani (Harris) (Homoptera: Aphididae), the foxglove aphid. The second instar hosts produced the highest level of reproductive success, while third and fourth instars resulted in a substantially reduced reproductive performance. When given a choice of host instars, parasitoids preferred the older hosts for oviposition disregarding their reduced suitability for larval development. Results are discussed in context of mechanisms involved in A. ervi host selection and biases in the criteria used to assess hosts that may arise when parasitoids transfer host species between generations.     

Pre‐Patch Experience Affects the Egg Distribution Pattern in a Polyembryonic Parasitoid of Moth Egg Batches

According to foraging theory, female parasitoids should alter their host choice in response to cues that indicate a limitation of resources. We tested whether females of the polyembryonic parasitoid Ageniaspis fuscicollis (Hymenoptera: Encyrtidae), which attack egg batches of small ermine moths (Lepidoptera: Yponomeutidae), would alter their host acceptance pattern in response to different pre‐patch experience. We kept females of the parasitoid prior to a patch visit under different conditions, which should indicate different levels of competition for hosts. With increased competition as pre‐patch experience, females laid more eggs per host egg and self‐superparasitized more often, and the resultant egg distributions showed a trend from more regular distributions to increasingly Poisson and aggregated distributions. Consequently, females with a pre‐patch experience that would indicate low competition for hosts had the most even egg distributions. We conclude that pre‐patch experience of competitors may lead to a significant change of mutual interference patterns in egg‐laying A. fuscicollis wasps.     

Use of learned visual cues during habitat location by Brachymeria intermedia

Like many other parasitoids, Brachymeria intermedia (Hymenoptera: Chalcididae) uses olfactory cues to find its host, but in addition it appears to use learned visual cues to focus its searching efforts at the macrohabitat level. In laboratory experiments, females were held in cages where Lymantria dispar pupae were hidden either in a vertical, tree-like structure or on the floor. After four days females had learned to search for hosts in the structure in which they previously had found pupae. Such training was reversible. During similar tests in a semi-natural situation, in which pupae were hung from a tree trunk or were hidden under leaf litter, females also tailored their subsequent searching to favour the macrohabitat where hosts previously had been found. This parasitoid most likely uses a combination of visual and olfactory cues during host searching.     

Innate Host Habitat Preference in the Parasitoid Diachasmimorpha longicaudata: Functional Significance and Modifications through Learning

Parasitoids searching for polyphagous herbivores can find their hosts in a variety of habitats. Under this scenario, chemical cues from the host habitat (not related to the host) represent poor indicators of host location. Hence, it is unlikely that naïve females show a strong response to host habitat cues, which would become important only if the parasitoids learn to associate such cues to the host presence. This concept does not consider that habitats can vary in profitability or host nutritional quality, which according to the optimal foraging theory and the preference-performance hypothesis (respectively) could shape the way in which parasitoids make use of chemical cues from the host habitat. We assessed innate preference in the fruit fly parasitoid Diachasmimorpha longicaudata among chemical cues from four host habitats (apple, fig, orange and peach) using a Y-tube olfactometer. Contrary to what was predicted, we found a hierarchic pattern of preference. The parasitism rate realized on these fruit species and the weight of the host correlates positively, to some extent, with the preference pattern, whereas preference did not correlate with survival and fecundity of the progeny. As expected for a parasitoid foraging for generalist hosts, habitat preference changed markedly depending on their previous experience and the abundance of hosts. These findings suggest that the pattern of preference for host habitats is attributable to differences in encounter rate and host quality. Host habitat preference seems to be, however, quite plastic and easily modified according to the information obtained during foraging.     

Volatile cues used by the parasitoid, Aphytis melinus, for host location: California red scale revisited

Aphytis melinus DeBach (Hymenoptera: Aphelinidae) is a biological control agent of diaspidid scale insects. The parasitoid has a narrow host range but its hosts are polyphagous. We determined the source of volatile cues the wasp uses to locate its few host species when those hosts occur on more than one host plant species. We addressed four questions in regard to the use of volatile cues in host location of California red scale, Aonidiella aurantii (Maskell) (Homoptera: Diaspididae): (1) Does A. melinus use volatile cues to assist in host location? (2) Are these cues innately recognized or learned? (3) Are cues produced by female California red scale, or from other sources? (4) Are the cues specific to the host or host plant? These questions were tested through the use of a Y-tube olfactometer. Female A. melinus used volatile cues to orient toward both infested and uninfested host plant material. Wasps learned these cues by associating odors from the host plant with host presence. They had no innate preferences for scale insect or host plant volatile stimuli. Contrary to previous studies, we found no evidence of orientation toward the female-produced sex pheromone of California red scale, nor to volatile cues from the attacked host stage. Wasps given experience with scale insects growing on lemon fruit subsequently oriented toward lemon and orange fruit and leaves. The scale species with which the wasp was given experience did not affect this preference. Wasps given experience with California red scale growing on squash did not orient toward infested lemon fruit. The host ranges of the parasitoid and its hosts are used to explain the adaptive value for the evolution of learned rather than fixed responses to cues used in foraging behavior.