Parasitoid Strains Research Articles

Insecticide Resistance Development in the Filth Fly Pupal Parasitoid, Spalangia cameroni (Hymenoptera: Pteromalidae), Using Laboratory Selections.

Filth flies remain one of the most prevalent pest groups affecting the animal production industry. Spalangia spp. and Muscidifurax spp. are beneficial parasitic wasps that often are utilized to manage filth fly populations such as house flies, Musca domestica L. (Diptera: Muscidae), and stable flies, Stomoxys calcitrans (L.) (Diptera: Muscidae). These wasps search for filth fly pupae as hosts in areas potentially treated with insecticides, which may result in nontarget insecticide selection effects. However, research regarding resistance development in parasitic wasps such as S. cameroni Perkins (Hymenoptera: Pteromalidae) is limited. Therefore, a study was conducted to determine the potential of S. cameroni to develop resistance to the commonly used insecticide permethrin, as well as compare permethrin susceptibility among several S. cameroni strains. After 10 selected generations, susceptibility was significantly lower for the selected strain when compared with that of its unselected parent strain. A comparison of several parasitoid strains collected from different U.S. states indicated that permethrin susceptibility was not significantly different between a baseline strain and more recently established field strains. The potential implications of this previously unrecognized nontarget insecticide exposure effect on filth fly parasitoids are discussed.

Wolbachia

Ary Hoffman introduces Wolbachia.

Transgenerational phenotypic plasticity of diapause induction and related fitness cost in a commercial strain of the parasitoid Aphidius ervi Haliday.

Diapause is an adaptation that insects have evolved to synchronize their life cycle with that of seasonal climatic changes and resources availability. However, cues for its induction are not always clear and, in some cases, a maternal effect may be involved. At the population level, just a part of the individuals may exhibit diapause with important consequences in terms of winter survival. Moreover, clear indicators of diapause state are difficult to identify. Diapause induction was thus investigated in the aphid parasitoid species Aphidius ervi Haliday (Hymenoptera: Braconidae) developing in the aphid Sitobion avenae (Hemiptera: Aphididae) at four crossed photothermal regimes (16°C and 8°C, 16:8 h L:D and 8:16 h L:D), and during 2 successive generations. We analyzed the reliability of changes in mummy color to assess for the diapausing state compared to dissections, and we measured parasitoid morphological and physiological traits. We observed that the proportion of dark brown mummies increased after one generation under low photothermal regime compared to other regimes. No diapause was recorded at 16°C, 16:8 h L:D, while we observed 16.2% and 67.5% diapause incidence at 8°C, 8:16 h L:D, at 1st and 2nd generation, respectively. Diapause induction is thus increased by short day-length conditions and low temperatures as well as by maternal effects. All parasitoid life-history traits (weight, size, fat content, water content, egg-load, and longevity) were affected by the photothermal regime and/or the generation. These results raise new questions on the environmental thresholds needed to induce diapause and on survival and adaptation potential of commercially available parasitoid strains in different environments.

Trans-generational effects on diapause and life-history-traits of an aphid parasitoid

Transgenerational effects act on a wide range of insects’ life-history traits and can be involved in the control of developmental plasticity, such as diapause expression. Decrease in or total loss of winter diapause expression recently observed in some species could arise from inhibiting maternal effects. In this study, we explored transgenerational effects on diapause expression and traits in one commercial and one Canadian field strain of the aphid parasitoid Aphidius ervi. These strains were reared under short photoperiod (8:16 h LD) and low temperature (14 °C) conditions over two generations. Diapause levels, developmental times, physiological and morphological traits were measured. Diapause levels increased after one generation in the Canadian field but not in the commercial strain. For both strains, the second generation took longer to develop than the first one. Tibia length and wing surface decreased over generations while fat content increased. A crossed-generations experiment focusing on the industrial parasitoid strain showed that offspring from mothers reared at 14 °C took longer to develop, were heavier, taller with wider wings and with more fat reserves than those from mothers reared at 20 °C (8:16 h LD). No effect of the mother rearing conditions was shown on diapause expression. Additionally to direct plasticity of the offspring, results suggest transgenerational plasticity effects on diapause expression, development time, and on the values of life-history traits. We demonstrated that populations showing low diapause levels may recover higher levels through transgenerational plasticity in response to diapause-induction cues, provided that environmental conditions are reaching the induction-thresholds specific to each population. Transgenerational plasticity is thus important to consider when evaluating how insects adapt to changing environments.

Role of egg-laying behavior, virulence and local adaptation in a parasitoid’s chances of reproducing in a new host

Understanding the ability of parasitoid insects to succeed in new host populations is a relevant question for biological control and adaptive mechanisms. Cotesia typhae is an African parasitoid specialized on the moth Sesamiae nonagrioides, also called the Mediterranean corn borer. Two Kenyan strains of C. typhae differ in their virulence against a new host population from France. We explored behavioral and physiological hypotheses about this differentiation. Cotesia genus belongs to a group of Hymenoptera in which females inject a domesticated virus in their host to overcome its resistance. Since viral particles are injected along with eggs and since the strain with the higher virulence injects more eggs, we hypothesized that virulence could be explained by the quantity of virus injected. To test this assumption, we measured the injected quantities of eggs and viral particles (estimated by viral DNA segments) of each parasitoid strain along several ovipositions, to vary these quantities. Unexpectedly, results showed that virulence against the French host was not correlated to the injected quantities of eggs or viral segments, indicating that virulence differentiation is explained by other causes. The virulence against the respective natural hosts of the two C. typhae strains was also measured, and results suggest that local adaptation to a more resistant natural host may explain the pre-adaptation of one strain to the new host population. We also identified a differentiation of oviposition strategy and subsequent offspring number between the parasitoid strains, which is important in a biocontrol perspective.

Comparative Time-Scale Gene Expression Analysis Highlights the Infection Processes of Two Amoebophrya Strains.

Understanding factors that generate, maintain, and constrain host-parasite associations is of major interest to biologists. Although little studied, many extremely virulent micro-eukaryotic parasites infecting microalgae have been reported in the marine plankton. This is the case for Amoebophrya, a diverse and highly widespread group of Syndiniales infecting and potentially controlling dinoflagellate populations. Here, we analyzed the time-scale gene expression of a complete infection cycle of two Amoebophrya strains infecting the same host (the dinoflagellate Scrippsiella acuminata), but diverging by their host range (one infecting a single host, the other infecting more than one species). Over two-thirds of genes showed two-fold differences in expression between at least two sampled stages of the Amoebophrya life cycle. Genes related to carbohydrate metabolism as well as signaling pathways involving proteases and transporters were overexpressed during the free-living stage of the parasitoid. Once inside the host, all genes related to transcription and translation pathways were actively expressed, suggesting the rapid and extensive protein translation needed following host-cell invasion. Finally, genes related to cellular division and components of the flagellum organization were overexpressed during the sporont stage. In order to gain a deeper understanding of the biological basis of the host-parasitoid interaction, we screened proteins involved in host-cell recognition, invasion, and protection against host-defense identified in model apicomplexan parasites. Very few of the genes encoding critical components of the parasitic lifestyle of apicomplexans could be unambiguously identified as highly expressed in Amoebophrya. Genes related to the oxidative stress response were identified as highly expressed in both parasitoid strains. Among them, the correlated expression of superoxide dismutase/ascorbate peroxidase in the specialist parasite was consistent with previous studies on Perkinsus marinus defense. However, this defense process could not be identified in the generalist Amoebophrya strain, suggesting the establishment of different strategies for parasite protection related to host specificity.

Parasitism of Diachasmimorpha longicaudata (Ashmead) on two fruit fly species

ABSTRACT: The aim of the present study was to investigate the parasitism of two fruit fly species by different Diachasmimorpha longicaudata strains. Females of two parasitoid strains were reared on Ceratitis capitata or Anastrepha fraterculus that were conditioned in adapted containers. Individual parasitoids were offered 20 larvae for 1 h, using no-choice (20 C. capitata or 20 A. fraterculus) or free-choice (10 of each species) assays. Parasitism was verified by counting scars, and the pupae were classified as non-parasitized (no scars), parasitized (one scar), or superparasitized (multiple scars). Parasitism intensity was compared by evaluating the proportions of non-parasitized, parasitized, and superparasitized larvae. Parasitism intensity and scar frequency were analyzed as functions of host species and parasitoid strain, using chi-square tests. In the non-choice tests, females of both parasitoid strains parasitized the larvae, regardless of host species. However, when the parasitoids had a choice, they preferred to superparasitize A. fraterculus larvae, which are larger than C. capitata larvae. In addition, the female parasitoids parasitized most of the host larvae, regardless of parasitoid strain or host species. However, the frequency of non-parasitized pupae was higher when the offered host species was different from the one in which the parasitoid was reared, and the frequency of superparasitized pupae was higher when the offered host species was the same as the one in which the parasitoid was reared.

Spatial and temporal dispersal patterns of Diachasmimorpha longicaudata (Hymenoptera: Braconidae) reared on Ceratitis capitata and Anastrepha fraterculus (Diptera: Tephritidae)

The dispersal and parasitism abilities of Diachasmimorpha longicaudata, reared on the larval hosts Ceratitis capitata and Anastrepha fraterculus, were evaluated in a guava orchard. Eleven releases of approximately 3000 parasitoids were conducted in a 15-ha orchard and wasp dispersal was monitored from 6 to 272 m away from a central release point. At each monitoring point, larvae of C. capitata and A. fraterculus were offered as hosts for the parasitoids. In five releases, parasitoids were offered only one host species, and in six releases, they were given a choice. In total, 10,351 parasitoids were recovered from these hosts, all of which were D. longicaudata. Both parasitoid strains dispersed in all directions, but showed a slight bias towards the east. Both strains of the parasitoid survived and parasitized larvae of A. fraterculus and C. capitata for up to 15 days after release, with maximum activity during the first 7 days, but more individuals of the Anastrepha strain were recovered after 15 days. Although the highest rates of parasitism by both wasp strains occurred close to the release point, the Anastrepha strain parasitized more hosts at greater distances from the release point, whereas the Ceratitis strain parasitized more hosts close to release points. Releases of the Anastrepha strain of D. longicaudata are therefore recommended, as this strain yields high rates of parasitism further away from release points and appears to survive longer in the field.

Relationship between oviposition, virulence gene expression and parasitism success in Cotesia typhae nov. sp. parasitoid strains.

Studying mechanisms that drive host adaptation in parasitoids is crucial for the efficient use of parasitoids in biocontrol programs. Cotesia typhae nov. sp. (Fernández-Triana) (Hymenoptera: Braconidae) is a newly described parasitoid of the Mediterranean corn borer Sesamia nonagrioides (Lefebvre) (Lepidoptera: Noctuidae). Braconidae are known for their domesticated bracovirus, which is injected with eggs in the host larva to overcome its resistance. In this context, we compared reproductive success traits of four Kenyan strains of C. typhae on a French and a Kenyan populations of its host. Differences were found between the four strains and the two most contrasted ones were studied more thoroughly on the French host population. Parasitoid offspring size was correlated with parasitism success and the expression of bracovirus virulence genes (CrV1 and Cystatin) in the host larva after parasitism. Hybrids between these two parasitoid strains showed phenotype and gene expression profiles similar to the most successful parental strain, suggesting the involvement of dominant alleles in the reproductive traits. Ovary dissections revealed that the most successful strain injected more eggs in a single host larva than the less successful one, despite an equal initial ovocyte number in ovaries. It can be expected that the amount of viral particles increase with the number of eggs injected. The ability to bypass the resistance of the allopatric host may in consequence be related to the oviposition behaviour (eggs allocation). The influence of the number of injected eggs on parasitism success and on virulence gene expression was evaluated by oviposition interruption experiments.

Genetic structure and diversity of the Diachasmimorpha longicaudata species complex in Thailand: SSCP analysis of mitochondrial 16S rDNA and COI DNA sequences

Genetic variation among 20 populations of Diachasmimorpha longicaudata (Ashmead) (Hymenoptera: Braconidae) in Thailand was investigated using single strand conformation polymorphism (SSCP) analysis of mitochondrial DNA sequences. From a total 641 individual parasitoids, seven distinct haplotypes containing a total of 32 polymorphic sites were observed from cytochrome oxidase subunit I (COI) sequences along with five distinct haplotypes containing a total 16 polymorphic sites from 16S rDNA sequences. Values obtained through pairwise FST comparisons and analysis of molecular variance (AMOVA) indicated significant genetic differentiation among D. longicaudata populations in Thailand. Congruent relationships showing separation of these populations into three groups were obtained from Neighbor joining and Bayesian phylogenetic tree analyses along with the use of haplotype networks. This SSCP analysis of populations of the D. longicaudata species complex is the first report using molecular population genetic methods to analyze the structure of this parasitoid species in Thailand. This may provide useful information for release of parasitoid strains to maximize their benefit in biological control programs.

Tracing the history and ecological context of Wolbachia double infection in a specialist host (Urophora cardui)-parasitoid (Eurytoma serratulae) system.

The endosymbiotic bacterium Wolbachia is the most widespread bacteria in insects, yet the ecology of novel acquisitions in natural host populations is poorly understood. Using temporal data separated by 12 years, I tested the hypothesis that immigration of a parasitoid wasp led to transmission of its Wolbachia strain to its dipteran host, resulting in double‐strain infection, and I used geographic and community surveys to explore the history of transmission in fly and parasitoid. Double infection in the fly host was present before immigration of the parasitoid. Equal prevalence of double infection in males and females, constant prevalence before and after immigration in two regions, and increase in one region of immigration indicate little if no competition between strains. Double infection was present throughout the fly's distribution range, but proportions varied highly (0–0.71, mean = 0.26). Two fly‐specific MLST strains, observed in Eastern and Western Europe, respectively, differed at hcpA only. Flies with either fly‐strain could be double infected with the parasitoid's strain. The geographic distribution of double infection implies that it is older than the fly host's extent distribution range and that different proportions of double infection are caused by demographic fluctuations in the fly. The geographic data in combination with community surveys of infections and strains further suggest that the parasitoid strain was the fly's ancestral strain that was transmitted to the parasitoid, that is, the reverse transmission route as first hypothesized. Based on these findings together with a comparison of oviposition strategies of other hosts harboring related Wolbachia strains, I hypothesize that trans‐infection during an insect host's puparial metamorphosis might be important in promoting horizontal transmission among diverse holometabolic taxa.

Genetic and phenotypic diversity characterization of natural populations of the parasitoid Parvilucifera sinerae

Parasites exert important top-down control of their host populations. The hostpara- site system formed by Alexandrium minutum (Dinophyceae) and Parvilucifera sinerae (Perkinso- zoa) offers an opportunity to advance our knowledge of parasitism in planktonic communities. In this study, DNA extracted from 73 clonal strains of P. sinerae, from 10 different locations along the Atlantic and Mediterranean coasts, was used to genetically characterize this parasitoid at the spe- cies level. All strains showed identical sequences of the small and large subunits and internal tran- scribed spacer of the ribosomal RNA, as well as of the β-tubulin genes. However, the phenotypical characterization showed variability in terms of host invasion, zoospore success, maturation time, half-maximal infection, and infection rate. This characterization grouped the strains within 3 phe- notypic types distinguished by virulence traits. A particular virulence pattern could not be ascribed to host-cell bloom appearance or to the location or year of parasite-strain isolation; rather, some parasitoid strains from the same bloom significantly differed in their virulence traits. Identi- cal markers such as ITS and β-tubulin genes of P. sinerae strains from different geographic areas and from different years precludes their use in assessing intra-specific diversity and could indicate a recent dispersion of this species.

Evaluation of pesticide tolerant strain of Cotesia flavipes Cameron (Hymenoptera: Braconidae) on maize stem borer, Chilo partellus (Swinhoe)

Field evaluation of pesticide tolerant strain of Cotesia flavipes Cameron, a gregarious larval endoparasitoid of the maize stem borer, Chilo partellus (Swinhoe) was carried out and the economics of such evaluation was worked out. Releases of the pesticide tolerant parasitoid strain, prior to application of pesticide or after application resulted in significantly higher net returns and yield, as compared to the releases of susceptible parasitoid strain and untreated control. Four releases of the tolerant parasitoid at 2000/ha at 500 per release registered maximum number of parasitized cocoons (52.6 per larva), higher yields (4019 kg/ha) and net profits (Rs. 7807 kg/ha), followed by two releases of the tolerant strain with two sprays of insecticide (40.8, 3809 kg/ha and Rs. 6847/ha, respectively). The insecticidal treatment gave higher net profits and yields than all other treatments. However, the additional costs, ecological hazards and pesticide residues (likely to occur) do not outweigh the benefits of biological control. Key words: Cotesia flavipes, Chilo partellus, population, pesticide tolerance.

Is parasitoid virulence against multiple hosts adaptive or constrained by phylogeny? A study of Leptopilina spp. (Hymenoptera: Figitidae)/Drosophila (Diptera: Drosophilidae) interactions

Summary. Some insects can develop immune resistance to koinobiont parasitoids. Reciprocally, adaptation to host immunology is critical for parasitoid success. Phylogenetic inertia and correlations between virulence against different hosts can act as constraints preventing these adaptations. Insights on these constraints may be obtained from the analysis of patterns of variations in the interactions at the species or genus level. Multivariate phylogenetic comparative methods were applied to virulence traits of 13 parasitoid strains of Leptopilina spp. (Hymenoptera: Figitidae) on five host strains of the Drosophila melanogaster species subgroup (Diptera Drosophilidae). Independent contrasts of virulence were calculated and principal component analysis (PCA) was performed on the independent contrasts to estimate the dimensionality of the interactions. Most of the variation of virulence was associated with the first component of the PCA (62.2%). But a significant proportion was explained by the second and third components, suggesting specific interactions. Strain–strain reciprocal specificity was observed in several pairs of host–parasitoid species. Significant phylogenetic inertia was observed on parasitoid virulence, but only at the genus level and only against hosts of intermediate resistance (phylogenetic R2 between 0.62 and 0.85). Some parts of the interaction matrix exhibited specific interactions and others were fixed due to ancestral non-specific virulence (or avirulence). The results were interpreted viewing virulence as a threshold trait determined by underlying liability. When liability is far from the threshold, virulence is fixed. When liability is close to the threshold, virulence varies specifically and reciprocal adaptations can take place. These phylogenetic constraints may lead to a scenario of escape and radiation coevolution in the host–parasitoid system.

Assessing European Egg Parasitoids as a Mean of Controlling the Invasive South American Tomato Pinworm Tuta absoluta

The South American tomato pinworm (Tuta absoluta) has recently invaded Europe and is rapidly spreading in the Afro-Eurasian continent where it is becoming a major pest on tomato crops. Laboratory tests were undertaken to evaluate the potential of 29 European strains of Trichogramma parasitoids to control T. absoluta. In addition to the host itself, the host plant (tomato) was used during the laboratory tests in order to increase the chance of selecting the best parasitoid strains. Trichogramma females were placed with T. absoluta eggs on a tomato leaflet in tubes. We compared the parasitism of T. absoluta by the various Trichogramma species tested to the Trichogramma species currently commercially available for the pest control in Europe, i.e. Trichogramma achaeae. Thereafter, the more promising strains were tested on a larger scale, in mesocosm (i.e. cages in greenhouses) and in greenhouse compartments to evaluate efficiency of laboratory selected strains under cropping conditions. The most efficient strain from the laboratory screening trials did not perform as efficiently under the greenhouse conditions. We discuss differences in parasitism levels among species and strains and among the different scales tested in the experiments, as well as implications of these results for further screening for biocontrol agents.

Odor‐mediated patch choice in the parasitoid Venturia canescens: temporal decision dynamics

AbstractParasitoids foraging for hosts in a heterogeneous environment would greatly benefit if they could decide already from a distance in which areas search for resources would be most profitable and to avoid areas of low fitness returns. Interestingly, the temporal dynamics of the decision process in parasitoid patch choice have rarely been investigated. In a Y‐tube olfactometer, we tested whether thelytokous and arrhenotokous females of the parasitoid wasp Venturia canescens (Gravenhorst) (Hymenoptera: Ichneumonidae) respond to differences in cues indicating the quality of a host‐containing patch and choose more profitable patches. Special attention was given to the time it took females to make their choices (patch choice time) when differences in patch quality were either qualitative (absence vs. presence of hosts and kairomone) or quantitative (various concentrations of hosts and kairomone, and presence of competitors). We found that both thelytokous and arrhenotokous wasps only chose the higher‐quality patch based on odor cues when the difference was qualitative. When patches differed only with respect to the number of hosts, or the presence or absence of competing female parasitoids, no significant preference could be found in females of either strain of the parasitoid. In contrast, both the time until females reached the junction of the Y‐tube olfactometer (response time) and the time until females decided for either patch (decision time) varied with parasitoid strain and odor treatment. Thelytokous wasps were faster than arrhenotokous wasps in their response time and in their decision time. However, females of both strains responded faster with increasing number of total hosts releasing kairomone. Yet, decision time for patches did not significantly vary as a function of patch quality offered to Venturia wasps.

Evolution of a Polydnavirus Gene in Relation to Parasitoid-Host Species Immune Resistance

CrV1, a polydisperse DNA virus (polydnavirus or PDV) gene contributes to the suppression of host immunity in Cotesia genus parasitoids. Its molecular evolution was analyzed in relation to levels of resistance in the sympatric host species. Natural selection for nonsynonymous substitutions (positive Darwinian selection) was observed at specific amino acid sites among CrV1 variants; particularly, between parasitoid strains immune suppressive and nonimmune suppressive to the main resistant stem borer host, Busseola fusca. In Cotesia sesamiae, geographic distribution of CrV1 alleles in Kenya was correlated to the relative abundance of B. fusca. These results suggest that PDV genes evolve through natural selection and are genetically linked to factors of suppression of local host resistance. We discuss the forces driving the evolution of CrV1 and its use as a marker to understand parasitoid adaptation to host resistance in biological control.

Comparison of different European strains of Trichogramma aurosum (Hymenoptera: Trichogrammatidae) using fertility life tables

Life table parameters were assessed for seven strains of Trichogramma aurosum Sugonjaev and Sorokina (Hymenoptera: Trichogrammatidae) collected in different European countries, in order to compare their performance when reared on eggs of Ephestia kuehniella Zeller (Lepidoptera: Pyralidae) as a potential factitious host for mass-rearing. The average number of progeny per female, cumulative fertility and emergence rate did not differ significantly, whereas female longevity and sex ratio significantly differed between the seven parasitoid strains. The Danish strain survived the longest (6.05 days) and the Dutch strain survived the shortest (2.75 days). Progeny was always female-biased with varying proportions (57.7–96.7%). Survival rates started to decrease after 3 days for some of the strains studied. The mean cohort generation duration (T c) was 11.40, 10.15, 10.62, 10.63, 9.28, 9.70 and 11.30 days for the Austrian, Luxemburgian, Belgian, French, Dutch, Danish and German strains, respectively. Population doubling time (D t) was 4.50, 7.96, 3.56, 5.30, 5.23, 7.36 and 3.30 days, respectively. Daily intrinsic rate of increase (r m) and finite rate of increase (exp. r m) ranged between 0.087 and 0.210 and 1.091–1.233, respectively. The German strain might be a potential candidate for mass rearing and releases against the codling moth, Cydia pomonella (L.) (Lepidoptera: Tortricidae), due to its high net reproduction rate (R 0=10.65 female), a high intrinsic rate of natural increase (r m=0.210), a high finite rate of increase (exp. r m=1.23), and a short population doubling time (D t=3.3 days). The relevance of intra- and interstrain variability as well as the usefulness of fertility life tables for pre-introductory research is discussed.

Variation of success of Leptopilina boulardi in Drosophila yakuba: The mechanisms explored

Despite an increasing knowledge of insect immune defences and virulence strategies used by parasitoids to escape them, the mechanisms underlying variation of success between parasitoid strains are still poorly understood. We have investigated this point using two lines of the parasitoid wasp Leptopilina boulardi that differ in virulence towards Drosophila yakuba. By injecting oil drops in D. yakuba larvae parasitized by virulent IS y females and then dissecting the larvae at different times following injection, we demonstrate that the IS y line alters host encapsulation ability but only during the early parasitism period. This effect is mimicked by injecting venom gland extracts, indicating that venom proteins are likely involved in immunosuppression. By contrast, the IS m line, unsuccessful on D. yakuba, has no immunosuppressive effect. This variation in virulence may be explained by the striking difference we report in haemocytic profiles between IS m- and IS y-parasitized larvae. We discuss our results in the light of our knowledge of the strategies evolved by Leptopilina species to counteract the D. melanogaster immune system as well as the role of parasitoid venoms in intra-specific variation of parasitoid virulence.

Promoting selection of resistance to spinosad in the parasitoid Cotesia plutellae by integrating resistance of hosts to the insecticide into the selection process

Abstract Insecticide resistance in parasitoids is a trait of value for a better integration of biological and chemical controls in certain situations. However, practical use of this trait has been rare and may have been partly hindered by the very low rate of success in artificial selection of insecticide resistance in parasitoids. In this study, we explored the feasibility of promoting selection of resistance to spinosad in the parasitoid Cotesia plutellae by integrating resistance of hosts to the insecticide into the selection process. Five colonies of C. plutellae were established from the same field populations and reared in strains of its host, the diamondback moth, either susceptible or highly resistant to the insecticide. These colonies were exposed to selection by spinosad in larval, pupal or adult stage. Two colonies, which were reared in either susceptible or resistant hosts and exposed to selection at adult stage, developed only marginal levels of resistance after 22 and 28 selection cycles, respectively. A colony that was reared on resistant hosts and exposed to selection at the pupal stage failed to develop resistance after 12 selection cycles. In contrast, a colony that was reared on resistant hosts and exposed to selection at the larval stage inside the host larva developed 21.7-fold resistance after 22 selection cycles, a level of resistance that may enable the parasitoid to survive field application rate. These results demonstrate that exposure of parasitoids to selection pressure at the larval stage harbored by resistant hosts can effectively promote the response to selection by the parasitoids and help produce potentially useful resistant strains of parasitoids for field application.