Priming Aphytis: behavioral modification of host selection by exposure to a synthetic contact kairomone

The introduced parasitoid, Aphytis melinus DeBach (Hymenoptera: Aphelinidae), utilizes a kairomone, O-caffeoyltyrosine, to recognize California red scale, Aonidiella aurantii (Maskell) (Homoptera: Diaspididae). Wasps used in augmentative release programs for California red scale on California citrus are reared on oleander scale, Aspidiotus nerii Bouché (Homoptera: Diaspididae), themselves reared on squash. The goals of this study were: 1) to determine if long-term rearing on oleander scale had caused A. melinus to develop a preference for oleander scale, and 2) to determine if the preference of oleander-reared A. melinus for California red scale might be enhanced by exposing them to synthetic O-caffeoyltyrosine prior to release. Wasps that were removed from their hosts as pupae and allowed to emerge as adults isolated from their hosts retained a strong preference for California red scale regardless of rearing host. This preference was reduced if wasps were allowed to emerge from oleander scale, thus acquiring early adult experience with oleander scale. The preference for California red scale was restored, however, by exposing wasps reared on oleander scale to synthetic O-caffeoyltyrosine prior to bioassay. Exposure of A. melinus reared in commercial insectaries to O-caffeoyltyrosine prior to release may be a means to improve the effectiveness of such wasps in augmentative release programs to control California red scale.     

Environmental variation in physical and chemical cues used by the parasitic wasp,Aphytis melinus, for host recognition

O-caffeoyltyrosine is a host recognition kairomone forAphytis melinus DeBach (Aphelinidae) found in the covers of its host, California red scale,Aonidiella aurantii (Maskell) (Diaspidae). This study tests the hypothesis that the concentration ofO-caffeoyltyrosine and scale cover size are reliable indicators of scale body size, an important component of host quality forA. melinus, over a range of scale rearing conditions. Both scale cover area andO-caffeoyltyrosine concentrations were only qualitatively related to scale body size during the third instar, the scale life stage most suitable forA. melinus. Scale cover area andO-caffeoyltyrosine concentrations were reduced, relative to scale body size, when scale were reared on bark and leaves compared to fruits. Scale cover area andO-caffeoyltyrosine concentration were also relatively reduced when scales were reared in mid-summer compared to spring and fall, and when reared on orange cultivars compared to lemon cultivars in the field. Finally, scale cover area andO-caffeoyltyrosine concentration were reduced when scale were reared at 52% compared to 100% humidity in the laboratory. Scales appear to be chemically conspicuous toA. melinus for a short period of the time in which they are physiologically susceptible. Scales that minimize their cover size and maximize the incorporation rate ofO-caffeoyltyrosine into covers may minimize their conspicuousness toA. melinus. Minimizing scale cover size, but not necessarily incorporation rates, may make scales more vulnerable to predators, however.     

Uncoupling physical and chemical cues: The independent roles of scale cover size and kairomone concentration on host selection byAphytis melinus DeBach (Hymenoptera: Aphelinidae)

The ectoparasitoidAphytis melinus initially selects its host, California red scale,Aonidiella aurantii (Maskell) (Homoptera: Diaspididae), using characteristics of the host’s cover before assessing the quality of the scale body beneath. Host suitability is known to increase with host size until the scale insect reaches maturity, after which it is no longer available for parasitism. The wasp uses a combination of scale cover size and a kairomone,O-caffeoyltyrosine, in the cover for initial assessment. Under natural conditions these two factors are frequently coupled. We quantified the relative importance of cover size and kairomone concentration independently by removing and selectively reapplying controlled doses of syntheticO-caffeoyltyrosine to scale covers of known size. In the absence of the kairomone, wasps did not discriminate among scale covers differing in size. Wasps showed a curvilinear response to kairomone dose for each scale cover age group. Wasps preferred low doses ofO-caffeoyltyrosine on young, small scale covers, and high doses on old, large scale covers. The ability of wasps to respond quantitatively to the kairomone may be used in the field to differentiate small second-instar from larger and more suitable third-instar scale insect larvae.     

Increased parasitization of California red scale in the field after exposing its parasitoid, Aphytis melinus, to a synthetic kairomone

The introduced parasitoid, Aphytis melinus DeBach (Hymenoptera: Aphelinidae), utilizes a kairomone, O-caffeoyltyrosine, to recognize California red scale, Aonidiella aurantii (Maskell) (Homoptera: Diaspididae). Wasps used in augmentative release programs for California red scale on California citrus are reared on oleander scale, Aspidiotus nerii Bouché (Homoptera: Diaspididae), themselves reared on squash. Previous laboratory studies demonstrated that wasps reared on oleander scale but exposed to O-caffeoyltyrosine before bioassay more readily accepted California red scale covers for probing than wasps reared similarly but not exposed to O-caffeoyltyrosine. The goal of this study was to determine if wasps reared on oleander scale but exposed to O-caffeoyltyrosine actually parasitized a greater number of field-reared California red scale than unconditioned wasps. In order to distinguish between progeny of conditioned and unconditioned wasps in the field, we utilized two A.melinus strains that carried different electrophoretically distinguishable alleles. In half of the releases, Strain A was conditioned to O-caffeoyltyrosine and Strain B served as the control, while this pattern of conditioning was reversed for the remaining releases. Although there were differences in the rates of population growth of the two strains, whichever strain was the conditioned strain left a greater than expected number of progeny compared to the control strain in all comparisons. Exposure of A. melinus reared in commercial insectaries to O-caffeoyltyrosine prior to release may be a means to improve the effectiveness of such wasps in augmentative release programs to control California red scale.     

Host recognition and selection by Aphytis species: Response to California red, oleander, and cactus scale cover extracts

One or more chemicals associated with the host's cover appear to play a major role in host recognition by Aphytis species. Aphytis melinus DeBach, A. lingnanensis Compère, A. coheni DeBach and Comperiella bifasciata Howard, all parasitoids of diaspidid scale-insects, were found to respond to water extracts of California red scale covers, Aonidiella aurantii (Mask.); A. melinus and A. lingnanensis responded to water extracts of cactus scale covers, Diaspis echinocacti (Bouché); and A. chilensis Howard to water extracts of oleander scale covers, Aspidiotus nerii Bouché. A. lingnanensis responded to water extracts of cactus scale covers only if it had been reared on this scale. A. melinus also responded to ethanol extracts of California red scale covers while A. lingnanensis did not. A. melinus failed to respond to water extracts of California red scale covers after a single ovipositional experience on California red scale. Such kairomones might prove useful in screening natural enemies as potential biological control agents for specific hosts as well as in elucidating the mechanisms by which parasitoids of scale-insects differentiate among potential hosts.

---

Résumé Aphytis lingnanensis, A. melinus, A. coheni et Comperiella bifasciata réagissent tous à des extraits aqueux d'Aonidiella aurantii, tandis qu'A. chiliensis réagit aux extraits aqueux d'Aspidiotus nérii. La réaction de A. coheni aux extraits aqueux d'A. aurantii confirme les écrits de Quednau et Hübsche (1964). Nous n'avons pas été capables d'observer une réaction de A. chrysomphali aux extraits aqueux d'A. aurantii. Comme nous n'avons pas essayé de traiter les boucliers avec d'autres solvants, nous ne pouvons pas affirmer l'absence de réponse de cet aphélinide aux kairomones de cette cochenille.Que A. melinus et A. lingnanensis aient été élevés sur Aspidiotus nerii ou sur Aonidiella aurantii, ils ne réagissent pas aux extraits aqueux d'A. nerii; qui est accepté comme hôte par ces deux aphélinides au laboratoire. Ces résultats suggèrent que d'autres éléments, comme la forme ou la texture, jouent aussi un rôle important dans la détermination des hôtes. A. melinus et A. lingnanensis réagissent tous deux aux extraits aqueux de boucliers de Diaspis echinocacti; cependant, après élevage sur D. echinocacti, seul A. lingnanensis réagit à l'extrait. La réaction de A. melinus à l'extrait de D. echinocacti suggère que cet aphélinide est préadapté chimiosensoriellement à déceler cette cochenille, dont la distribution est néotropicale tandis que celle de A. melinus orientale. A. melinus a été élevé à partir de D. echinocacti récoltés dans la nature. La réaction de A. lingnanensis aux extraits de D. echinocacti uniquement lorsqu'il a élévé dessus, suggère qu'un conditionnement préimaginal peut aussi jouer un rôle dans la détermination de l'hôte.

     

Disruption of parasitism of the California red scale (Homoptera: Diaspididae) by three ant species (Hymenoptera: Formicidae)

The relative influence of the southern fire ant, Solenopsis xyloni McCook, Argentine ant, Linepithema humile (Mayr), and native gray ant, Formica aerata (Francoeur), on parasitism of California red scale, Aonidiella aurantii Maskell, was studied in the laboratory for two parasitoids, Comperiella bifasciata Howard and Aphytis melinus DeBach. All three ant species reduced percentage parasitism by C. bifasciata and both percentage parasitism and host mutilation by A. melinus. Southern fire ant was the least disruptive and native gray ant the most disruptive. Southern fire ant removed 12% of scale from the lemons, presumably to feed on them, while the other ant species did not exhibit significant removal of scale compared to the controls. Percentage parasitism of California red scale exhibited by C. bifasciata was more than 2-fold the level exhibited by A. melinus. Percentage mutilation of California red scale, including probing and host feeding, was nearly 5-fold higher for A. melinus than C. bifasciata. Because A. melinus required a longer total host examination + oviposition period in the absence of ants than C. bifasciata and because oviposition occurs as the last act in a sequence of behaviors, disruption by the ants had a more significant negative effect on oviposition by A. melinus.     

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.     

Development of Habrolepis rouxi Compere (Hymenoptera: Encyrtidae) in two armoured scale hosts (Homoptera: Diaspididae) and parasite egg encapsulation by California red scale

Abstract. 1. California red scale Aonidiella aurantii (Maskell) and oleander scale Aspidiotus nerii Bouché (Homoptera: Diaspididae) are not completely suitable for the development of the encyrtid endoparasite Habrolepis rouxi Compere.
2. Mortality of parasite immatures in California red scale is caused mainly by egg encapsulation. Although oleander scale is unable to encapsulate H. rouxi , it is nutritionally unfavourable for the parasite's development.
3. Some immunity to the parasite is conferred to the oleander scale by the particular host plant used.
4. The parasite's rate of development is not influenced by the host species studied.
5. The mortality of parasite immatures in California red scale is lower than in oleander scale. The former host also is more acceptable and preferred by the parasite for ovipositon. These characteristics of the California red scale render it more suitable than oleander scale as host for H. rouxi.
6. Studies on the encapsulation of H. rouxi eggs by the California red scale indicate that (i) encapsulation does not prevent third instar host larvae from maturing and producing progeny; (ii) neither rearing temperature nor host age affect encapsulation frequency.     

Innate and Learned Cues: Scale Cover Selection by Aphytis melinus (Hymenoptera: Aphelinidae)

Aphytis melinus recognizes and accepts covers of its host, California red scale, Aonidiella aurantii (Maskell) (Homoptera: Diaspididae), before assessing the size or quality of the scale body beneath. We evaluated the role of a non-volatile kairomone, O-caffeoyltyrosine, and prior experience with hosts on the recognition (antennal drumming) and acceptance (ovipositor probing) of scale covers differing in age and, therefore, size. We tested several hypotheses concerning the role of experience with hosts on host recognition and acceptance. The first predicts that experience with a particular host size leads to increased selection of that host size in the future. The second predicts that the quality of the experience with hosts sets a threshold of quality for future acceptance. We manipulated the quality of wasp experience with hosts by exposing some wasps to high-quality hosts (large scale insects under large covers) and other wasps to low-quality hosts (small scale insects under small covers and small scale insects under large covers). Control (naive) wasps were held without experience with hosts. Wasps were then offered five size classes of covers directly after removal from the scale insects (unmanipulated) or after removing the kairomone from the covers (kairomone-free). For covers with natural levels of kairomone, no effect of experience with hosts on host recognition was observed. When the kairomone was removed, however, any experience with hosts increased recognition compared to naive wasps. Moreover, experienced wasps preferentially recognized covers of the size to which they had experience. Results for host acceptance differed from those for host recognition. Wasps given experience with large hosts accepted more covers with kairomone than wasps in other treatments, but the size preference did not vary among treatments. When the kairomone was removed, however, wasps given experience with large hosts under large covers preferred larger covers. Host recognition and acceptance are controlled by different behavioral mechanisms. Prior experience with hosts does not alter host recognition but does affect the rate of acceptance. The quality of the experience does not affect the acceptance of covers containing natural levels of kairomone but does affect host acceptance when the kairomone is removed. Experience with hosts alters the motivation of wasps to accept covers, and the direction of this effect is determined by the size (quality) of body the wasps were given during the experience.     

Variation between laboratory populations in the performance of the parasitoid Encarsia formosa on two host species, Bemisia tabaci and Trialeurodes vaporariorum

We tested the hypothesis that populations of the parthenogenetic parasitic wasp Encarsia formosa Gahan (Hymenoptera: Aphelinidae) differed in their ability to use two different host species, Bemisia tabaci Gennadius (Homoptera: Aleyrodidae) and Trialeurodes vaporariorum Westwood (Homoptera: Aleyrodidae). Of the three wasp populations tested, two populations had been reared for many generations on B. tabaci and one population had been reared for many years on T. vaporariorum. Performance was measured by the number of whitefly nymphs that were successfully parasitized by individual wasps, and performance on either host was measured in separate experiments. There was variation between wasp populations in their performance on the host B. tabaci, with one wasp population reared for many years on this host performing considerably better than the other two populations. There were no significant differences between populations in their use of the preferred host, T. vaporariorum. The experiments were conducted in such a way that we could distinguish heritable differences between populations from environmentally-induced conditioning differences due to the immediate host from which an individual wasp enclosed. In either experiment there were no significant effects of conditioning, although there was a trend within each population for wasps conditioned on T. vaporariorum to have higher performance than those conditioned on B. tabaci. Thirdly, we conducted a selection experiment, initiated with wasps from a single population historically reared on T. vaporariorum, to measure the effect of laboratory rearing on different hosts for 17 generations. We did not see any difference in the performance of wasps on B. tabaci after this period of rearing on either of the two hosts. In summary, populations of E. formosa do differ in their relative performance on B. tabaci. The one population that was tested further did not show any response to selection by rearing, but the ability to respond to selection on performance may not be equal for all populations. The possibility that wasp populations have differential performance on particular hosts may affect the use of this species as a biological control agent.     

Variation between laboratory populations of Encarsia formosa in their parasitization behavior on the host Bemisia tabaci

We tested the hypothesis that two populations of the parasitoid Encarsia formosa Gahan (Hymenoptera: Aphelinidae) differed in their behavioral interactions with the whitefly host Bemisia tabaci Gennadius (Homoptera: Aleyrodidae). The two wasp populations were studied because previous work suggested large differences between the populations in performance on this host. In this study the populations differed behaviorally in both the number of hosts encountered and their reactions to hosts once encountered. The population reared for many years on Trialeurodes vaporariorum Westwood (Homoptera: Aleyrodidae) that previously performed more poorly on B. tabaci had a higher host encounter rate but rejected hosts more frequently. The population reared for a number of years on B. tabaci encountered fewer hosts but accepted a higher percentage of hosts for oviposition. The number of parasitized hosts did not differ between the two populations, however. These data demonstrate that there are heritable differences between these two populations of asexual wasps in host-associated behavioral traits. These behavioral differences in host acceptance do not explain performance differences seen in the earlier study, however, possibly due to different conditions between the two experiments.     

Survey of brown soft scale Coccus hesperidum L. parasitoids in southern California citrus

The parasitoid complex of brown soft scale, Coccus hesperidum L., a multivoltine soft scale, was determined in southern California citrus over the period February 2004–March 2006. The survey was conducted by placing brown soft scale-infested yucca leaves in the canopy of citrus trees and subsequently rearing individually isolated parasitized scales in the laboratory. A total of 14 species parasitized brown soft scale in the field, the most abundant ones belonging to the genus Metaphycus Mercet (75%). The most abundant parasitoid species was Metaphycus angustifrons Compere (38% parasitism), and this is a new record of establishment for this species in California. Coccophagus species accounted for only 11% parasitism. There were important spatio-temporal differences across the parasitoid complex survey locations. We also found that the five most abundant encyrtid parasitoid species showed preferences for scales of different sizes. Our results have implications for biological control of citricola scale, Coccus pseudomagnoliarum (Kuwana), an important pest of citrus in the San Joaquin Valley of central California. Notably, this species is nearly absent in southern California. Brown soft scale is considered to be an alternate host for parasitoids of citricola scale, a univoltine soft scale, at times when the latter species is unavailable for parasitism.     

Effects of two insect growth regulators and a neonicotinoid on various life stages of <Emphasis Type="Italic">Aphytis melinus</Emphasis> (Hymenoptera: Aphelinidae)

The effect of two insect growth regulators and a neonicotinoid insecticide were tested on immature stages and adults of the parasitoid Aphtyis melinus DeBach (Hymenoptera: Aphelinidae), a key natural enemy of California red scale, Aonidiella aurantii (Maskell) (Hemiptera: Diaspididae), in California. No significant effects of the insect growth regulators on survival or development to the adult stage were found when the parasitoid was treated at any of the egg, larval, or pupal stages. The broad-spectrum neonicotinoid acetamiprid also showed no significant effect on the development of A. melinus to the pupal stage, probably because immature stages of this ectoparasitoid are protected under the cover of its armored scale host. However, 48 h exposure of adults to acetamiprid residues following emergence resulted in high levels of wasp mortality. Aphytis melinus adults treated with either of the two insect growth regulators as larvae survived 48 h exposure to pesticide residues as adults and showed levels of fecundity comparable with control insects. We conclude that the two insect growth regulators are compatible with augmentative releases of A. melinus but that treatments of acetamiprid should be avoided in situations where biological control by this parasitoid is important.     

Comparative Development and Performance of Artificially Reared versus Host-RearedDiapetimorpha introita(Cresson) (Hymenoptera: Ichneumonidae) Wasps

Diapetimorpha introita(Cresson) (Hymenoptera: Ichneumonidae), a native ectoparasitoid ofSpodopteraspp. pupae, was reared in the laboratory on an artificial diet devoid of any insect host components. Diet-reared wasps demonstrated a propensity to search for and parasitize natural hosts in a field cage trial. Longevity of the diet-reared wasps was comparable with the longevity of wasps reared on host pupae. Survival rate ofD. introitawas 61.3% when reared on diet and 76.3% when reared on host pupae. Wasps reared on the artificial diet had longer developmental times, reduced fecundity, and reduced adult weights compared to wasps reared on host pupae. These studies suggest that future research efforts should focus on increasing fecundity and weight of diet-reared wasps and decreasing the mortality and developmental time of wasps reared on the artificial diet. The ability to rearD. introitaon an inexpensive, artificial diet significantly enhances the potential of mass rearing this parasitoid for inundative releases againstSpodopteraspp.     

Discovery of a CI-inducing Wolbachia and its associated fitness costs in the biological control agent Aphytis melinus DeBach (Hymenoptera: Aphelinidae)

Wolbachia is an endosymbiotic bacterium that infects a large percentage of arthropods and can affect the fitness of its host. Here we verified for the first time that the biological control agent Aphytis melinus DeBach is infected with a Wolbachia that causes complete cytoplasmic incompatibility, and conducted an insectary and field survey to determine the infection frequency. A. melinus appears to suffer fitness costs associated with infection based on measurements of longevity and fecundity. We also quantified the Wolbachia titers of A. melinus reared at different temperatures and found that, although not completely cured, increased temperature resulted in a significant reduction in the number of Wolbachia copies found in an individual wasp. Implications of our results for biological control are discussed.     

Effects of winter temperatures on the sex ratios ofAphytis melinus [Hym.: Aphelinidae] in the San Joaquin Valley of California

Laboratory bioassays of overwintering field populations of the California red scale parasitoidAphytis melinus DeBach showed a shift in sex ratio of F₁ progeny after prolonged exposure to winter temperatures in Tulare County, California. In 7 of 8 tests, short-term winter field exposure (<75 days) did not result in a significant male bias in sex ratios of F₁ progeny when compared to sex ratios observed in laboratory cultures maintained at 27°C. Long term exposure (109–139 days), however, resulted in significantly (P<0.05) higher male bias in F₁ sex ratios in 5 of 6 tests. These results from field populations ofA. melinus agree with conclusions of low temperature laboratory tests onAphytis species (lingnanensis, melinus) by other workers. Mention of a proprietary or commercial product in this paper does not constitute an endorsement of this product by the University of California.     

Mite parasitization on armored scale insects: host suitability

Suitability of three armored scale insects (Homoptera: Diaspididae), their stages, the host plants and their parts, for the parasitic miteHemisarcoptes coccophagus Meyer was investigated. The studied scales were chaff scales,Parlatoria pergandii Comstock andP. cinerea Hadden, infesting grapefruit and orange orchards, latania scale,Hemiberlesia lataniae (Signoret), and oleander scale,Aspidiotus nerii Bouche occurring on acacia (Acacia cyanophylla) branches and leaves. Host suitability was evaluated by mite survival, mean intensity (active mites/attacked scales) and fecundity. It was highest on latania scale, lowest on oleander scale and intermediate on chaff scales. Results of laboratory experiments were consistent with these field findings: while developing on oleander scale, mites suffered 3× higher mortality, and their generation time was twice as long as on latania scale; the full egg clutch on the latter was high whereas on oleander scale it was negligible. Our data suggest that ovipositing females of latania and oleander scales, but not of chaff scales, are more suitable hosts than young scale females and the latter more suitable than immature stages. No direct effects of acacia plant parts on host suitability were detected. Citrus species affected chaff scale suitability for the mite through their effect on host diaspidid size. The ability ofH. coccophagus to control pestiferous diaspidids may depend on the specific host scale, its age structure and the specific host plant. The implications of these findings for the biological control of armored scale insects are discussed.     

The effect of nitrogen fertilizer applied to Euphorbia pulcherrima on the parasitization of Bemisia argentifolii by the parasitoid Encarsia formosa

More wasps of Encarsia formosa Gahan (Hymenoptera: Aphelinidae) were found on fertilized poinsettias, Euphorbia pulcherrima (Willd.) (Euphorbiaceae), than on non-fertilized plants. Parasitization of Bemisia argentifolii Bellows & Perring (Homoptera: Aleyrodidae) by E. formosa was higher on plants treated with calcium nitrate than with ammonium nitrate or on control plants. In a no-choice test, host feeding by E. formosa was higher when hosts were on fertilized plants than when hosts were on control plants. The nitrogen content of whitefly pupae reared on plants treated with ammonium nitrate was higher than those on calcium nitrate-treated plants.Variability in the parasitization of B. argentifolii by E. formosa appears to be due to host plant-mediated differences in the whiteflies. E. formosa may be influenced by the nutritional suitability of the host, which influences whether wasps continue to oviposit, feed, or disperse.     

Host Preferences of Trichogramma pretiosum and the Influence of Prior Ovipositional Experience on the Parasitism of Plutella xylostella and Pseudoplusia includens Eggs

Trichogramma wasps are generalist egg parasitoids used in biological control efforts. In a multi host situation they may preferentially parasitize a non-target host species to the detriment of the control program. Plutella xylostella Linnaeus (Lepidoptera: Plutellidae) is a very serious pest of cabbage, but is only one in a number of species in the ‘cabbageworm’ complex. We investigated the host preferences of Trichogramma pretiosum Riley (Hymenoptera: Chalcidoidea) when offered the eggs of Plutella xylostella and the eggs of Pseudoplusia includens Hübner (Lepidoptera: Noctuidae), another species in the ‘cabbageworm’ complex. Trichogramma pretiosum reared on the eggs of the factitious host Ephestia kuehniella Zeller (Lepidoptera: Pyralidae) parasitized both Plutella xylostella and Pseudoplusia includens eggs under laboratory conditions. For both choice and no-choice experiments, T. pretiosum parasitized significantly more P. xylostella eggs than P. includens eggs. Prior ovipositional experience with one or other of the two host species had no effect on the subsequent parasitism levels of the two host species. The preference for P. xylostella eggs was also not affected by this prior ovipositional experience.     

The effect of host nutrition on growth and development of the parasitoid waspVenturia canescens

We investigated the effect of host (Plodia interpunctella; Lepidoptera: Pyralidae) nutritional status on development of the solitary endoparasitoid,Venturia canescens (Hymenoptera: Ichneumonidae). Parasitoids from 3rd (L3) instars reared on a deficient diet during early parasitism took longer to develop and suffered higher mortality than those reared from hosts fedad libitum although there was not a significant difference in the size of eclosing wasps from the two groups. L5 hosts reared at high density produced smaller parasitoids, which developed more rapidly than those reared from hosts from low density containers, although mortality was higher in the latter. In a separate experiment we starved groups of 10–20 hosts (parasitized as L3) daily beginning on the 4th day after parasitism, to determine the host developmental stage required for successful parasitoid development to eclosion. Parasitoid survivorship increased with length of host access to food, while the egg-to-adult parasitoid development time increased throughout the experiment. Parasitoid size decreased with increasing periods of host starvation. The successful emergence ofVenturia depends uponPlodia reaching the size normally attained in the mid-5th instar, or 50–70% of the mass of healthy late 5th instars. Our results show that when earlier instars are parasitized, host growth is essential for successful parasitoid development to eclosion. Furthermore, they suggest that, for many koinobionts, host suitability may be greatly influenced by feeding rate and food quality.