Effects of Intraspecific Competition and Host-Parasitoid Developmental Timing on Foraging Behaviour of a Parasitoid Wasp

The evolutionarily stable strategy, animal contests, parasitoids, pest control and sociality

The evolutionarily stable strategy, ESS, concept was first used in biology to understand sex ratio bias and, shortly afterwards, to explore the logic of contests over essential and indivisible resources. ESS models formed the basis of much subsequent research on animal behaviour and placed game-theoretic thinking firmly within the behavioural ecology approach. Among behavioural ecologists studying parasitoids, it was those asking questions about the evolution of sex ratios who first made extensive use of the game-theoretic approach. A later growth of interest in parasitoid host defence and fighting behaviour made use of these tractable study species to explore contests and their connections to further aspects of life-history evolution plus some pest control applications. Our aims are to (i) introduce the topic of contests, which are engaged in by a very wide array of animal taxa, and the importance, both historical and conceptual, of the game-theoretic approach to their study, and (ii) review recent studies of parasitoid contests, including those that have considered the context of social evolution and the performance of parasitoids as agents of biological control. We consider that game-theoretic models are eminently testable and applicable and will likely endure as valuable tools in studies of parasitoid biology. This article is part of the theme issue 'Half a century of evolutionary games: a synthesis of theory, application and future directions'.

Wolbachia impairs post-eclosion host preference in a parasitoid wasp

Host preference behavior can result in adaptive advantages with important consequences for the fitness of individuals. Hopkin's host-selection principle (HHSP) suggests that organisms at higher trophic levels demonstrate a preference for the host species on which they developed during their own larval stage. Although investigated in many herbivorous and predatory insects, the HHSP has, to our knowledge, never been tested in the context of insects hosting selfish endosymbiotic passengers. Here, we investigated the effect of infection with the facultative bacterial symbiont Wolbachia on post-eclosion host preference in the parasitoid wasp Trichogramma brassicae (Hymenoptera: Trichogrammatidae). We compared host preference in Wolbachia-infected individuals and uninfected adult female parasitoids after rearing them on two different Lepidopteran hosts, namely the flour moth Ephestia kuehniella Zeller (Lepidoptera: Pyralidae) or the grain moth Sitotroga cerealella (Lepidoptera: Gelechiidae) in choice and no choice experimental design (n = 120 wasps per each choice/no choice experiments). We showed that in T. brassicae, Wolbachia affects the post-eclosion host preference of female wasps. Wolbachia-infected wasps did not show any host preference and more frequently switched hosts in the laboratory, while uninfected wasps significantly preferred to lay eggs on the host species they developed on. Additionally, Wolbachia significantly improved the emergence rate of infected wasps when reared on new hosts. Altogether, our results revealed that the wasp's infection with Wolbachia may lead to impairment of post-eclosion host preference and facilitates growing up on different host species. The impairment of host preference by Wolbachia may allow T. brassicae to shift between hosts, a behavior that might have important evolutionary consequences for the wasp and its symbiont.

Mechanism and consequences for avoidance of superparasitism in the solitary parasitoid Cotesia vestalis

A parasitoid’s decision to reject or accept a potential host is fundamental to its fitness. Superparasitism, in which more than one egg of a given parasitoid species can deposit in a single host, is usually considered sub-optimal in systems where the host is able to support the development of only a single parasitoid. It follows that selection pressure may drive the capacity for parasitoids to recognize parasitized hosts, especially if there is a fitness cost of superparasitism. Here, we used microsatellite studies of two distinct populations of Cotesia vestalis to demonstrate that an egg laid into a diamondback moth (Plutella xylostella) larva that was parasitized by a conspecific parasitoid 10 min, 2 or 6 h previously was as likely to develop and emerge successfully as was the first-laid egg. Consistent with this, a naive parasitoid encountering its first host was equally likely to accept a healthy larva as one parasitized 10 min prior, though handling time of parasitized hosts was extended. For second and third host encounters, parasitized hosts were less readily accepted than healthy larvae. If 12 h elapsed between parasitism events, the second-laid egg was much less likely to develop. Discrimination between parasitized and healthy hosts was evident when females were allowed physical contact with hosts, and healthy hosts were rendered less acceptable by manual injection of parasitoid venom into their hemolymph. Collectively, these results show a limited capacity to discriminate parasitized from healthy larvae despite a viability cost associated with failing to avoid superparasitism.

Probing behaviors and their plasticity for the aphid Sitobion avenae on three alternative host plants

Insects may develop different behavioral phenotypes in response to heterogeneous environments (e.g., host plants), but the plasticity of their feeding behaviors has been rarely explored. In order to address the issue, clones of the English grain aphid, Sitobion avenae (Fabricius), were collected from wheat, and their probing behaviors were recorded on three plants. Our results demonstrated that S. avenae individuals on the alternative plants (i.e., barley and oat) tended to have higher frequency of non-probing (Np), increased duration of the pathway phase, increased phloem salivation, and decreased phloem ingestion (E2), compared to those on the source plant (i.e., wheat), showing the resistance of barley and oat to this aphid's feeding. This aphid showed apparently high extents of plasticity for all test probing behaviors on barley or oat. Positive selection for higher extents of plasticity in E2 duration was identified on barley and oat. The factor 'clone' alone explained 30.6% to 70.1% of the total variance for each behavioral plasticity, suggesting that the divergence of probing behavior plasticity in S. avenae had a genetic basis. This aphid's fitness correlated positively with the plasticity of Np frequency and E2 frequency. Some behaviors and their corresponding plasticities (e.g., the frequency of xylem ingestion and its plasticity) were found to be correlated characters, probably reflecting the limits for the evolution of higher extents of behavioral plasticity in this aphid. The differential probing behaviors and their plasticity in S. avenae can have significant implications for the adaptation and management of aphids on different plants.

Interaction Between Chrysocharis flacilla and Diglyphus isaea (Hymenoptera: Eulophidae), Two Parasitoids of Liriomyza Leafminers

Agromyzid Liriomyza leafminer flies are a major threat to horticultural production in East Africa with low natural control reported. The endoparasitoid Chrysocharis flacilla (Walker; Hymenoptera: Eulophidae) was introduced from Peru into quarantine facilities at ICIPE in Kenya for a leafminer classical biological control program. Interaction assays with one of the dominant local parasitoids, Diglyphus isaea (Walker; Hymenoptera: Eulophidae), using Liriomyza huidobrensis (Blanchard; Diptera: Agromyzidae) was assessed through sole, simultaneous and sequential releases. C. flacilla resulted to superior host parasitism rates over D. isaea. When used separately, specific parasitism rates of D. isaea and C. flacilla were 26.33 ± 2.07% and 60.27 ± 2.53% respectively but, when simultaneously used, the total parasitism rose to 72.96 ± 4.12%. Presence of C. flacilla after D. isaea reduced significantly parasitism rate of D. isaea. Both parasitoids caused separately and simultaneously additionally significant nonreproductive host mortalities of between 48.33 ± 3.75% and 69.33 ± 3.92 for D. isaea and C. flacilla respectively. Sex ratios of C. flacilla and D. isaea F1 progenies were female biased and were not affected by interspecific interactions. Implications of these results for subsequent combined use of C. flacilla and D. isaea against Liriomyza leafminers in East Africa are discussed.

Spatial and temporal genetic structure at the fourth trophic level in a fragmented landscape

A fragmented habitat becomes increasingly fragmented for species at higher trophic levels, such as parasitoids. To persist, these species are expected to possess life-history traits, such as high dispersal, that facilitate their ability to use resources that become scarce in fragmented landscapes. If a specialized parasitoid disperses widely to take advantage of a sparse host, then the parasitoid population should have lower genetic structure than the host. We investigated the temporal and spatial genetic structure of a hyperparasitoid (fourth trophic level) in a fragmented landscape over 50 × 70 km, using microsatellite markers, and compared it with the known structures of its host parasitoid, and the butterfly host which lives as a classic metapopulation. We found that population genetic structure decreases with increasing trophic level. The hyperparasitoid has fewer genetic clusters (K = 4), than its host parasitoid (K = 15), which in turn is less structured than the host butterfly (K = 27). The genetic structure of the hyperparasitoid also shows temporal variation, with genetic differentiation increasing due to reduction of the population size, which reduces the effective population size. Overall, our study confirms the idea that specialized species must be dispersive to use a fragmented host resource, but that this adaptation has limits.

Wolbachia Infection in a Natural Parasitoid Wasp Population

The maternally transmitted bacterium Wolbachia pipientis is well known for spreading and persisting in insect populations through manipulation of the fitness of its host. Here, we identify three new Wolbachia pipientis strains, wHho, wHho2 and wHho3, infecting Hyposoter horticola, a specialist wasp parasitoid of the Glanville fritillary butterfly. The wHho strain (ST435) infects about 50% of the individuals in the Åland islands in Finland, with a different infection rate in the two mitochondrial (COI) haplotypes of the wasp. The vertical transmission rate of Wolbachia is imperfect, and lower in the haplotype with lower infection rate, suggesting a fitness trade-off. We found no association of the wHho infection with fecundity, longevity or dispersal ability of the parasitoid host. However, preliminary results convey spatial associations between Wolbachia infection, host mitochondrial haplotype and parasitism of H. horticola by its hyperparasitoid, Mesochorus cf. stigmaticus. We discuss the possibility that Wolbachia infection protects H. horticola against hyperparasitism.

Behavioural and genetic approaches to evaluate the effectiveness of deterrent marking by a parasitoid wasp

Some parasitoids deposit chemical signals after oviposition as an indication that the host has already been parasitized. This marking can deter subsequent conspecifics or one’s self from laying eggs in previously exploited hosts, thus reducing the risk of superparasitism. We investigated the egg laying behaviour of the parasitoid waspHyposoter horticola. In a laboratory experiment, we tested whether oviposition, post-oviposition marking, or both together deter subsequent oviposition by conspecifics. We then tested the effectiveness of the deterring mark under natural conditions using maternity assignment based on 14 polymorphic DNA microsatellite markers. The behavioural experiment showed that patch marking deters conspecifics from probing the host eggs, and oviposition deters those that probe from laying eggs in previously parasitized host clusters. These results were confirmed by the maternity assignment showing that under natural conditions, host egg clusters are primarily parasitized by a singleH. horticolafemale.