Parasitoid foraging behaviour in a competitive environment

In my thesis I investigated the foraging behaviour of the wasp Hyposoter horticola, an egg-larval parasitoid of the Glanville fritillary butterfly Melitaea cinxia, in the Åland islands in Finland. The particularity of this system is that the wasp is resource limited and faces strong intraspecific competition. ---------- I first focused on behaviour at an individual scale. In a series of experiments I tested how H. horticola s host searching behaviour was affected by developmental timing of both the parasitoid and the host, and direct intraspecific competition among foraging females. I found that the wasps visit host egg clusters before the hosts are susceptible to parasitism, presumably to cope with the limited time availability of the hosts. As the unparasitized hosts matured their value increased, competition became more frequent, and the wasps foraged more actively. Competition can also affect the parasitoid at earlier stages in its life. As larvae inside the hosts, the immature H. horticola suffered from competition due to superparasitism. Combining behavioural experiments in the laboratory and genetic analyses of sibship, I found that adult H. horticola deposit a chemical marking after oviposition that deters conspecifics from parasitizing a previously exploited host cluster. This protects parasitized host clusters from further exploitation. I found that the effectiveness of the deterrent persisted under natural conditions, where individual host egg clusters were each primarily parasitized by a single female H. horticola. Even when several females parasitized a cluster, the great majority of the offspring were full-siblings and the parasitism rate did not increase above the average 1/3 observed throughout the population. Considering that H. horticola is resource limited and faces intraspecific competition when foraging for hosts, it is surprising that only they parasitize a fraction of the hosts in each host egg cluster. After testing several physiological and evolutionary hypotheses for what might lead to this sub-maximal rate of host exploitation, I concluded that optimal foraging with avoidance of superparasitism was the most plausible explanation, as long as the search time between host clusters was low. ------ Then, I worked at a larger scale than individual behaviour. In the Åland islands, the butterfly host lives as a classic metapopulation with a high extinction rate of local populations. Due to strong competition, almost all the M. cinxia egg clusters in the population are found and parasitized by H. horticola. This suggests that the wasps must be good dispersers, which could influence the spatial genetic structure of the parasitoid population. I used DNA microsatellite markers and analysed H. horticola individuals sampled from over the entire population. My results indicate that, contrary to theory that higher trophic level species are more affected by habitat fragmentation than the species upon which they depend, the H. horticola population was less strongly genetically structured than the metapopulation of its butterfly host. It seems that H. horticola s dispersal ability allows it to compensate for the fragmented distribution of its host and not suffer from the metapopulation dynamics of the host local populations. Overall, the results of my thesis show that interactions between H. horticola and its host M. cinxia are strongly affected by competition among the adult female wasps. Intraspecific competition has an important role from an evolutionary perspective. Hyposoter horticola s deterrent marking behaviour has evolved in response to competition and the risk of superparasitism faced by immature offspring. Avoidance of superparasitism to limit competition is also the fundamental mechanism that controls H. horticola s optimal foraging strategy. And intraspecific competition modifies individual female host searching behaviour, increasing their foraging activity. -------- Interactions within a multitrophic system are complex and predictions concerning host-parasitoid interactions are difficult to generalise. However, as in this system, competition is factor that should receive more attention in empirical and theoretical studies of host-parasitoid interactions.Ei saatavill

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

In a context where hosts are distributed in patches and susceptible to parasitism for a limited time, female parasitoids foraging for hosts might experience intraspecific competition. We investigated the effects of host and parasitoid developmental stage and intraspecific competition among foraging females on host-searching behaviour in the parasitoid wasp Hyposoter horticola. We found that H. horticola females have a pre-reproductive adult stage during which their eggs are not mature yet and they forage very little for hosts. The wasps foraged for hosts more once they were mature. Behavioural experiments showed that wasps’ foraging activity also increased as host eggs aged and became susceptible to parasitism, and as competition among foraging wasps increased.Peer reviewe

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 Aland 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.Peer reviewe

The Puzzle of Partial Resource Use by a Parasitoid Wasp

When there is conspicuous underexploitation of a limited resource, it is worth asking, what mechanisms allow presumably valuable resources to be left unused? Evolutionary biologists have generated a wide variety of hypotheses to explain this, ranging from interdemic group selection to selfishly prudent individual restraint. We consider a situation in which, despite high intraspecific competition, individuals leave most of a key resource unexploited. The parasitic wasp that does this finds virtually all host egg clusters in a landscape but parasitizes only about a third of the eggs in each and then leaves a deterrent mark around the cluster. We first test—and reject—a series of system-specific simple constraints that might limit full host exploitation, such as asynchronous maturation of host eggs. We then consider classical hypotheses for the evolution of restraint. Prudent predation and bet-hedging fail as explanations because the wasp lives as a large, well-mixed population. Additionally, we find no individual benefits to the parasitoid of developing in a sparsely parasitized host nest.However, an optimal foragingmodel, including empirically measured costs of superparasitism and hyperparasitism, can explain through individual selection both the consistently low rate of parasitism and deterrent marking.Peer reviewe

Identification of 24 new microsatellite loci in the sweat bee Lasioglossum malachurum (Hymenoptera: Halictidae)

OBJECTIVE: The objective here is to identify highly polymorphic microsatellite loci for the Palaearctic sweat bee Lasioglossum malachurum. Sweat bees (Hymenoptera: Halictidae) are widespread pollinators that exhibit an unusually large range of social behaviours from non-social, where each female nests alone, to eusocial, where a single queen reproduces while the other members of the colony help to rear her offspring. They thus represent excellent models for understanding social evolution. RESULTS: 24 new microsatellite loci were successfully optimized. When amplified across 23-40 unrelated females, the number of alleles per locus ranged from 3 to 17 and the observed heterozygosities 0.45 to 0.95. Only one locus showed evidence of significant deviation from Hardy-Weinberg equilibrium. No evidence of linkage disequilibrium was found. These 24 loci will enable researchers to gain greater understanding of colony relationships within this species, an important model for the study of eusociality. Furthermore, 22 of the same loci were also successfully amplified in L. calceatum, suggesting that these loci may be useful for investigating the ecology and evolution of sweat bees in general

Data from: Strong dispersal in a parasitoid wasp overwhelms habitat fragmentation and host population dynamics

The population dynamics of a parasite depend on species traits, host dynamics, and the environment. Those dynamics are reflected in the genetic structure of the population. Habitat fragmentation has a greater impact on parasites than on their hosts because resource distribution is increasingly fragmented for species at higher trophic levels. This could lead to either more or less genetic structure than the host, depending on the relative dispersal rates of species. We examined the spatial genetic structure of the parasitoid wasp Hyposoter horticola, and how it was influenced by dispersal, host population dynamics, and habitat fragmentation. The host, the Glanville fritillary butterfly, lives as a metapopulation in a fragmented landscape in the Åland islands, Finland. We collected wasps throughout the 50 by 70 km archipelago and determined the genetic diversity, spatial population structure, and genetic differentiation using 14 neutral DNA microsatellite loci. We compared genetic structure of the wasp with that of the host butterfly using published genetic data collected over the shared landscape. Using maternity assignment, we also identified full-siblings among the sampled parasitoids to estimate the dispersal range of individual females. We found that, because the parasitoid is dispersive, it has low genetic structure, is not very sensitive to habitat fragmentation, and has less spatial genetic structure than its butterfly host. The wasp is sensitive to regional rather than local host dynamics, and there is a geographic mosaic landscape for antagonistic coevolution of host resistance and parasite virulence

Data from: 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

habitat patches geographic coordinates

habitat patches geographic coordinate

genotype data

Hyposoter horticola DNA microsatellite genotype dat