Host instar, body size and fitness in the koinobiotic parasitoid Aphidius nigripes

In aphidiine parasitoids, resources for growth and adult body size increase with host instar used by ovipositing females, but the fitness consequences of body size on fitness are poorly documented. We compared the fitness of male and female A. nigripesadults that varied in size as a consequence of developing in different instars of their host Macrosiphum euphorbiae. When reproductive fitness was measured without considering time, female wasps from small and large hosts performed similarly, contributing 125–175 foundresses plus 100–180 sons to the next generation. However, when expressed as the innate capacity for increase (r _m), female fitness correlated with host-induced variation of wasp size, indicating that micropopulations initiated by large wasps would increase faster. In a wind-tunnel, a sex pheromone plume from large female wasps induced more males to fly upwind when released at a distance of 50 cm downwind than small females, indicating that large females were sexually more attractive. With respect to male body size effects on fitness, large individuals performed similar to small ones, whether fitness was measured by lifetime mating frequency, fertile inseminations, or proportion of daughters among progeny born from their mates. When young naive males of unequal size were directly competing for mating with a virgin female, small and large males had equal mating success, and large individuals were no more successful than small ones at displacing a competitor already positioned on a receptive female. In a wind-tunnel test where males were scored on their ability to reach a female pheromone source, small and large males were equally affected by wind speed but reached the source located 50 cm downwind in equal proportions, suggesting similar capacity for finding mates by flying upwind. Our results indicate that despite host resources not being fixed at the time of attack for the koinobiont A. nigripes, fitness consequences of resource limitation by the mother may be perceived to be greater for daughters than sons, which would explain male-biased sex ratio in early-instar hosts.     

Host examination walk and oviposition site selection ofTrichogramma minutum: Studies on spherical hosts

The host examination behavior of the parasitoid wasp Trichogramma minutumon spherical Manduca sextahosts is described. The mean walking speed during host examination was 0.64 ± 0.03 mm s ^–1 and is independent of wasp size. The paths taken by the wasps are not evenly distributed over the host surface; the majority of time is spent below 45 ° latitude. The distribution of oviposition sites is also nonrandom with respect to latitude and is not influenced by phototaxic and geotaxic responses. The initial transit made by the wasps over the host surface is a straight path which frequently passes across the highest point on the host. The length and duration of the initial transit are independent of wasp body length. However, stride length and stepping rates are proportional to wasp body length, and small wasps take more steps to complete their initial transit. The roles of the examination walk in host recognition and host volume measurement by Trichogrammaare also discussed.     

Sex ratio manipulation by the parasitoid waspSpalangia cameroni in response to host age: A test of the host-size model

Summary A sex ratio response to host resources as measured by external host dimensions has been demonstrated in many parasitoid wasps, includingSpalangia cameroni. The responses generally are in the direction predicted by sex ratio theory, specifically the host-size models. Here I show that femaleS. cameroni also respond to differences in resource availability not associated with changes in external host dimensions, and this response is in the direction predicted by host-size models. When given old and young hosts simultaneously, femaleS. cameroni oviposit a greater proportion of sons in old than in young host pupae, at least for 0-day old versus 3-day old hosts. Old hosts weigh less than young hosts but are not significantly different in external width. Thus it appears that the offspring sex ratio response may result from mothers detecting physical or chemical changes within the host which are associated with host age. No evidence is found that the manipulation in response to host age has been selected for via an effect of host age on wasp size; there was no significant effect of host age on either male of female wasp size. A second prediction of the host-size models is also supported by this study: when each female is presented with only a single host age, rather than two host ages simultaneously, host age has no effect on offspring sex ratio.     

Resource utilization and sex allocation in response to host size in two ectoparasitoid wasps on subcortical beetles

Patterns of host resource utilization and sex ratio manipulation in relation to host size were investigated for two solitary ectoparasitoid wasps,Atanycolus initiator andSpathius brevicaudis (Hymenoptera Braconidae). Both species parasitize subcortical beetles on the trunks of Japanese pine trees.A. initiator is on average 8 times larger in body weight and has an ovipositor that is 3.7 times longer than that ofS. brevicaudis. In both parasitoids, the size of emerging wasps was positively correlated with host size, but the host/wasp size regressions were linear for all three major host species inA. initiator, whereas inS. brevicaudis the regression was logarithmic for a relatively large host species. The sex ratios (proportion of males) of both parasitoids emerging from different host species decreased with increasing host size, but the overall sex ratio on each host species was male-biased inA. initiator, while female-biased inS. brevicaudis. How the proportion of host consumed changed in response to host size, differed between the two parasitoids for the same host species. In the field survey, the size and sex ratio of the emerging two parasitoids from a dead tree were closely related to host size. However, the spatial distribution of the two parasitoids depended on the bark thickness of the trunk. The data suggest that differences in the relative evaluation of host size and in ovipositor length may enable the coexistence of the two parasitoid wasps.     

Sex allocation of three solitary ectoparasitic wasp species on bean weevil larvae: Sex ratio change with host quality and local mate competition

Sex ratio manipulation by ovipositing females was surveyed in 3 solitary ectoparastic wasp species,Dinarmus basalis (Pteromalidae),Anisopteromalus calanrae (Pteromalidae), andHeterospilus prosopidis (Braconidae), that parasitize azuki bean weevil (Callosobruchus chinensis (L) (Coleoptera: Buruchidae)) larvae within azuki beans (Vigna angularis). Variables were local mate competition (LMC) and host quality (HQ). We used host age as a measure of host quality (from 9-to 16-day-old hosts), changed the number of ovipositing females to control the level of local mate competition (1 female and 10 females), and examined oviposition patterns of the wasps. The offspring sex ratios (proportion of females) of the 3 wasp species respond qualitatively same to HQ and LMC. The common qualitative tendency among the 3 species is an increase of sex ratios increase with host age. In the process of changing the sex ratio (9–13-day-old) 3 wasp species respond only to HQ. In the hosts that end development in size (14–16-day-old) wasps respond to LMC. The response of sex ratio change to LMC in the old host ageclasses are different among the 3 species. In the situation that there exists LMC (10 females) sex ratios are the same among the 3 wasps. However, the sex ratios in no LMC (single female) are heterogeneous among the 3 wasps.     

Host-size-dependent sex ratio in a parasitoid wasp

Charnov's host-size model explains parasitoid host-size-dependent sex ratio as an adaptive consequence when there is a differential effect of host size on the offspring fitness of parasitoid males versus females. This article tests the predictions and the assumptions of the host-size model. The parasitoid wasp Pimpla nipponica Uchida (Hymenoptera: Ichneumonidae) laid more female eggs in larger or fresher host pupae when choice among hosts of different sizes or ages was allowed. Then, whether an asymmetrical effect of host size and age on the fitness of females versus males existed in P. nipponica was examined. Larger or fresher host pupae yielded larger wasps. Larger females lived longer, whereas male size did not influence male longevity. Large males mated successfully with relatively large females but failed with small females, whereas small males could mate successfully either with small or with large females. Thus, small-male advantages were found, and this held true even under male–male competition. Ovariole and egg numbers at any one time did not differ among females of different sizes. Larger females attained higher oviposition success and spent less time and energy for oviposition in hosts. Larger females produced more eggs from a single host meal. Taken together, females gained more, and males lost more, by being large. Host size and age thus asymmetrically affected the fitness of offspring males versus females through the relationships between host size or hast age and wasp size, which means the basic assumption of the host-size model was satisfied. Therefore, sex ratio control by P. nipponica in response to host size and age is adaptive. Received: November 13, 1998 / Accepted: January 18, 1999     

Adaptiveness of sex ratio control by the pupal parasitoid Itoplectis naranyae (Hymenoptera: Ichneumonidae) in response to host size

Adaptiveness of sex ratio control by the solitary parasitoid wasp Itoplectis naranyae (Hymenoptera: Ichneumonidae) in response to host size was studied, by examining whether differential effects of host size on the fitness of resulting wasps are to be found between males and females. The offspring sex ratio (male ratio) decreased with increasing host size. Larger hosts yielded larger wasps. Male larvae were less efficient in consuming larger hosts than female larvae. No significant interaction in development time was found between parasitoid sex and host size. Larger female wasps lived longer than smaller females, while longevity of male wasps did not increase with increasing wasp size. Smaller males were able to mate either with small or with large females, while larger males failed to mate with small females. Larger female wasps had a greater number of ovarioles and mature eggs at any one time than smaller females, although the number of eggs produced per host-feeding was not influenced by female wasps. Thus, the differential effect of host size on the fitness of males and females exists in I. naranyae. The basic assumption of the host-size model was therefore satisfied, demonstrating that sex ratio control by I. naranyae in response to host size is adaptive.     

Oviposition strategy of the parasitic waspDinarmus basalis (Hymenoptera,Pteromalidae)

Summary Host type choice and sex allocation were examined using the solitary parasitic waspDinarmus basalis (Pteromalidae, Hymenoptera) parasitizing larvae or pupae of the bean weevilCallosobruchus chinensis (Bruchidae, Coleoptera) within azuki beans (Vigna angularis). The wasps were offered two types of host; one was hard for the mother to lay eggs in, but was more beneficial for the offspring; the other was easy for the mother to lay eggs in, but was less beneficial for the offspring. The two types of host were one large host (17-day old host) in one bean and 6 small hosts (12-, or 13-day old hosts) in one bean. The same number of each host was presented at the same time to female wasps. The wasps accepted more 17-day old hosts than 12-day old hosts, and more 13-day old hosts than 17-day old hosts in each pair-wise choice experiment. The proportions of accepted host types were different from the proportions predicted by optimization models of random prey encounter with known or unknown prey densities. The wasps showed partial preference of host types. Incomplete information about prey densities, and about the costs and benefits of the two types of host may have generated the partial preference. Two predictions of host sizemodels, that (1) there should be a negative relationship between host size and offspring sex ratio (proportion of male offsprings), and (2) the sex ratio in each size host changes with the relative frequency of each size host utilized, were qualitatively supported.     

Host influences on sex ratio, longevity, and egg load of two Metaphycus species parasitic on soft scales: implications for insectary rearing

Metaphycus flavus (Howard) and M. stanleyi Compere (Hymenoptera: Encyrtidae) are currently being screened for use as augmentative biological control agents of citrus-infesting soft scales (Homoptera: Coccidae). Two factors were investigated, host quality-dependent sex allocation and local mate competition, which likely influence these parasitoid's sex allocation strategies and are therefore of interest for their mass-rearing. The results of these studies suggested that, under the mass-rearing protocol that is envisioned for these parasitoids, offspring sex ratios in both M. flavus and M. stanleyi are dominated by host quality (= size) influences, but not by interactions with other females. These results indicated that host size strongly influences offspring sex ratios and brood sizes; larger hosts led to more female offspring and larger broods. In contrast, increasing the number of parental females did not lead to fewer female offspring as expected under local mate competition. Additionally, within-brood sex ratios did not vary with brood size; this result is inconsistent with expected sex ratios due to local mate competition. Other results also indicated that host quality was a dominant influence on M. flavus' and M. stanleyi's sex ratios. Larger hosts led to a larger size in the emerging wasps, and larger wasps had greater egg loads and lived longer than smaller wasps. However, wasp longevity, and the influence of wasp size on longevity were mediated by a wasp's diet. Metaphycus flavus females lived the longest when they had access to hosts, honey, and water, followed by honey and water, and shortest when they had access to water alone; M. stanleyi females lived longest with honey and water, followed by hosts, honey, and water, and shortest with water alone. Greater wasp size led to greater longevity in females only when they had access to food (honey, or hosts and honey). Finally, other results suggested that both M. flavus and M. stanleyi are facultatively gregarious. Wasp size did not decrease with brood size as expected under superparasitism. Overall, the results of these studies suggested that holding newly emerged females of both M. flavus and M. stanleyi for several days in the presence of an appropriate food source before field release could enhance a female's performance as an augmentative biological control agent. It increases their initial life expectancy following release, and maximizes the females' egg load (both Metaphycus species) and resources for replacing oviposited eggs (M. flavus only).     

The effect of host size on quality attributes of the egg parasitoid,Trichogramma pretiosum

In a study of the quality ofTrichogramma pretiosum Riley (Hymenoptera: Trichogrammidae), we compared female wasps emerging from natural hosts, parasitized in the laboratory or the field with those emerging from factitious hosts used for commercial mass production. Females from the natural hosts were larger, more fecund, and longer lived than those from the factitious hosts. Compared to small females, large female wasps are substantially more fecund when honey (carbohydrate) is available but marginally more fecund when honey is unavailable. The size of a femaleT. pretiosum depends on two factors: the size of the host egg from which it emerges even when the wasp was gregarious, and the number of conspecifics that emerge with it. The similarities in the size distribution of female wasps emerging from natural hosts, in conjunction with the mechanism by whichTrichogramma measure host size and allocate eggs accordingly, suggests the hypothesis that size related components of fitness in femaleT. pretiosum are under strong selection in the field.     

Separating the Effects of Experience, Size, Egg Load, and Genotype on Host Response in Trichogramma (Hymenoptera: Trichogrammatidae)

The effects of experience, egg load, and wasp size on the response of four strains of Trichogramma nr. brassicae (Hymenoptera: Trichogrammatidae) to three economically important hosts, Heliothis punctigera (Noctuidae), Phthorimaea opercullela (Gelechiidae), and Sitotroga cereallela (Gelechiidae) were investigated. Experience effects due to rearing host and oviposition were considered in all combinations of host species. Both these effects altered response levels to low-ranked hosts but not to highly preferred hosts. Size was correlated with host response in some strains; larger females took more encounters with a host before accepting it, while egg load was not correlated with number of host encounters. However, egg load, rearing host, and size all affected acceptance of the low-ranked host S. cereallela. Females were more likely to accept this host if they were reared on S. cereallela, had a small size, and had a high egg load. Effects were consistent across strains, although overall acceptance levels differed among strains.     

Influence of previous herbivory on behavior and development of Spodoptera exigua larvae on glanded and glandless cotton

The solitary endoparasitoid Aenasius vexans Kerrich (Hymenoptera: Encyrtidae) is used for augmentative releases against the cassava mealybug, Phenacoccus herreni Cox & Williams (Sternorrhyncha: Pseudococcidae), an important pest on cassava in South America. In light of the need for large numbers of high quality females, experiments were conducted on host stage suitability and sex allocation. In choice and no-choice experiments, individual female wasps were offered second and third instar, as well as adult, hosts. During the first five days after emergence, the wasps showed a steady increase in the number of hosts they successfully parasitised per day, but the respective secondary sex ratio for each instar remained constant. Parasitism was highest for third instar hosts in no-choice tests, while in choice tests parasitism was highest in both third instars and adults. The later the developmental stage of the host at oviposition, the faster the parasitoids developed and emerged, and for each host stage, the development time of males was shorter than for females. The sex ratio of the wasps emerging from hosts that were parasitised as second instars was strongly male-biased, while the apparently preferred later stages yielded significantly more females than males. Female and male A. vexans emerging from hosts parasitised at the third instar were significantly larger than for the other stages. This may explain the preference for the third instar as well as the female-biased sex ratio, as size is usually positively correlated with higher fitness, especially in females. The results suggest that third instar hosts are the most suitable for rearing high numbers of large females.     

Host recognition byPimpla instigator F. (Hymenoptera: Ichneumonidae): Preferences and learned responses

This study identifies some previously unreported tactile and visual cues used by the pupal parasitoid Pimpla instigatorF. (Ichneumonidae) to recognize potential hosts. Paper cylinders were presented to the wasps as simple models of lepidopteran pupae. Acceptance of these models was evaluated by determining the frequency with which the wasps punctured the cylinders with their ovipositors. The length of the cylinders did not influence acceptance of the models; however, both surface texture and structural modifications to the ends of the cylinder did affect the frequency of punctures. Smooth cylinders were punctured more often than roughened cylinders, and cylinders with closed ends were frequently punctured, whereas open-ended cylinders were consistently rejected. The wasps also discriminated between blue and yellow cylinders and could be trained to associate blue or yellow with the presence of hosts. Preferences were established during a single 90- min training period and persisted for at least 4 days following training.     

Postemergence Experience Affects Sex Ratio Allocation in a Gregarious Insect Parasitoid

We tested the hypotheses that postemergence experience with plants (“early adult learning”) modifies sex ratio and clutch size allocations of Cotesia congregata (Say) (Hymenoptera: Braconidae), a gregarious larval endoparasitoid of Manduca sexta L. (Lepidoptera: Sphingidae). Emerging wasps were exposed for 2–3 h to (a) one of two host plants (tomato or tobacco) or no plant, and (b) one of two novel plants (arugula or parsley) or no plant. Each female was permitted a single oviposition in a host offered with one of the two plant species 24 h later. Hosts were reared on laboratory diet before and after parasitization. Wasps exposed to either host plant allocated proportionately more females to hosts offered with the plant species experienced at emergence than wasps with the alternate species, but clutch sizes did not differ. Irrespective of plant species, wasps exposed to novel plants allocated proportionately more females to hosts than wasps without plant experience, and larger clutches to hosts offered with parsley than with arugula. Differential responses to host and novel plants suggest inherent recognition of host foodplants by C. congregata. Results demonstrate a direct effect of learning on reproductive potential.     

SEX-RATIO MANIPULATION IN RESPONSE TO HOST SIZE BY THE PARASITOID WASP SPALANGIA CAMERONI: A LABORATORY STUDY

The prediction of Charnov et al.'s (1981) host-size model that there should be a negative relationship between host size and wasp sex ratio (proportion sons) was supported for Spalangia cameroni, a solitary parasitoid wasp. The relationship was shown to be a result of offspring sex manipulation by females in response to host size rather than a result of differential mortality of the sexes. A major assumption of the host-size model is that host size has a greater effect on the ultimate reproductive success of emerging female wasps than of males. This assumption was not supported. Host size had a positive effect on the size of both male and female S. cameroni. However, neither host size nor wasp size affected longevity, production of offspring by females, or ability of males to compete for mates. Host size may differentially affect the reproductive success of female and male wasps through effects on other aspects of reproductive success. Tests of the assumptions of offspring sex-ratio manipulation hypotheses are scarce but critical, not only for parasitoid wasps, but also for other organisms.     

Host range in solitary versus gregarious parasitoids: a laboratory experiment

Behavioural interactions among relatives may have consequences for many other traits. We tested the hypothesis that solitary parasitoids (displaying siblicidal behaviour in their larvae) have narrower host ranges than gregarious parasitoids (with tolerant larvae). In laboratory experiments, we compared parasitization success in two sister species of braconid wasp [Aphaereta genevensis (Fischer), solitary, and Aphaereta pallipes (Say), gregarious (Hymenoptera: Braconidae: Alysiini)] on eight Drosophila species or strains. Host species or strain was the most important factor affecting parasitization success, and some of this variation was accountable to host physiological defences. Although two hosts were more suitable for the solitary species, and one more suitable for the gregarious species, these differences were small, and there was no consistent difference across all hosts. Wasp body size was positively correlated with parasitization success in both wasp species. This may be because body size increases oviposition success, or the motivation to oviposit. In A. pallipes parasitization success peaked after 3–4 days, but later in A. genevensis. This is likely due to low life expectancy and high egg loads increasing oviposition tendency in young A. pallipes, and egg limitation decreasing oviposition tendency in old A. pallipes. These data suggest that interactions among wasp larvae do not greatly affect the size of the fundamental niche examined here. However, they show the potential for life history traits, which differ between the species as a likely consequence of larval interactions, to affect the extent of the realized niche.     

Sex ratio manipulation in response to host size in the parasitoid wasp Spalangia cameroni: is it adaptive?

Many species of parasitoid wasps produce a greater proportionof sons in small than in large hosts. As described by the host-sizemodel, natural selection is becoming a standard explanationfor the evolution of this phenomenon. We examined a criticalassumption of the host-size model, that host size has a morepositive effect on female than on male reproductive success.In laboratory experiments with the parasitoid wasp Spalangiacameroni, females that developed on larger hosts contained moreeggs at emergence. However, more eggs did not translate intomore offspring, at high or low density and regardless of whethera female had to burrow to reach hosts. The size of host on whicha female developed was also unrelated to her longevity, regardlessof the presence or absence of hosts. The size of host on whicha male developed had no effect on his sperm production or abilityto inseminate females, regardless of whether insemination abilitywas measured by the amount of sperm transferred to a female,by the proportion of a male's mates that produced any daughters,or by the proportion of daughters that a male's mates produced.Thus, despite data on multiple measures of fitness under a rangeof conditions, sex ratio manipulation in response to host sizein S. cameroni does not appear to be adaptive, and another explanationis needed.     

Mixed sex allocation strategies in a parasitoid wasp

The sex allocation strategy of the parasitoid Laelius pedatus (Hymenoptera: Bethylidae) on different-sized hosts was investigated. The wasp lays from one to five eggs, and clutch size increases with host size. On the smallest hosts, single male eggs are laid, while on slightly larger hosts single female eggs are laid. On still larger hosts, gregarious clutches are laid which nearly always consist of a single male and one or more female eggs. The sex ratio strategy of the wasp appears to be influenced by a combination of local mate competition and conditional sex expression based on host quality. Received: 6 June 1996 / Accepted: 13 October 1996     

繁育寄主对三种赤眼蜂个体大小及抱卵量的影响

【目的】繁育寄主及成蜂日龄均会影响赤眼蜂体内的抱卵量,为了解繁育寄主及育出赤眼蜂个体大小与赤眼蜂抱卵量的关系,我们在室内研究了米蛾 Corcyra cephalonica（Stainton）卵和亚洲玉米螟 Ostrinia furnacalis （Güenée）卵作为繁育寄主对3种赤眼蜂雌蜂大小、抱卵量的影响。【方法】在光周期14L:10D、温度25±1℃的室内条件下,以米蛾卵、亚洲玉米螟卵作为繁育寄主,解剖不同日龄（羽化后12-72 h）松毛虫赤眼蜂 Trichogramma dendrolimi Matsumura、玉米螟赤眼蜂Trichogramma ostriniae Pang et Chen以及稻螟赤眼蜂Trichogramma japonicum Ashmead,观察雌蜂抱卵量,测量成蜂的大小,分析雌蜂抱卵量与其日龄、个体大小以及繁育寄主的关系【结果】在羽化后48 h以内,3种蜂的抱卵量随雌蜂日龄的增长而增加。羽化后72 h,除米蛾卵繁育的玉米螟赤眼蜂及玉米螟卵繁育的松毛虫赤眼蜂的抱卵量比羽化后48 h时略高外,其他组合均出现下降。以亚洲玉米螟卵为繁育寄主时的玉米螟赤眼蜂的抱卵量显著高于对应日龄的以米蛾卵为繁育寄主时的抱卵量,而松毛虫赤眼蜂（除羽化后12和24 h外）和稻螟赤眼蜂则相反;以米蛾卵为繁育寄主时,相同日龄的赤眼蜂中,松毛虫赤眼蜂抱卵量最高,稻螟赤眼蜂的抱卵量最低;以亚洲玉米螟卵为繁育寄主时,玉米螟赤眼蜂抱卵量最高,稻螟赤眼蜂抱卵量最低。雌蜂抱卵量与雌蜂个体大小呈线性正相关,相关程度随雌蜂日龄增加而更加明显。【结论】后足胫节长度可以作为评价松毛虫赤眼蜂、玉米螟赤眼蜂和稻螟赤眼蜂质量的指标。赤眼蜂抱卵量受繁育寄主、赤眼蜂蜂种及雌蜂日龄的显著影响,寄主的适合性也会影响育出赤眼蜂的大小,筛选合适的繁育寄主有助于提高赤眼蜂的质量。     

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.