H. C. J. Godfray

Wolbachia in two insect host–parasitoid communities

Wolbachia form a group of intracellular bacteria that alter reproduction in their arthropod hosts. Two major phylogenetic subdivisions (A and B) of Wolbachia occur. Using a polymerase chain reaction assay we surveyed for the A and B group Wolbachia in 82 insect species from two temperate host–parasitoid communities (food webs) and a general collection of Lepidoptera caught at a light trap. One host–parasitoid community was based around leaf‐mining Lepidoptera, and the other around Aphids. We found that: (i) 22.0% of insects sampled were infected with Wolbachia; and (ii) the prevalence and type (A or B) of Wolbachia infection differed significantly between communities and taxonomic groups. We obtained DNA sequences from the ftsZ gene for the group B Wolbachia found in six leaf‐mining species and one of their parasitoids, as well as four of the Lepidoptera caught by a light trap. Taken together, the results of our survey and phylogenetic analyses of the sequence data suggest that host–parasitoid transfer of Wolbachia is not the major route through which the species we have examined become infected. In addition, the Wolbachia strains observed in five leaf‐mining species from the same genus were not closely related, indicating that transfer between species has not occurred due to a shared feeding niche or cospeciation.

Effects of social group size on information transfer and task allocation

SummarySocial animals exchange information during social interaction. The rate of interaction and, hence, the rate of information exchange, typically changes with density and density may be affected by the size of the social group. We investigate models in which each individual may be engaged in one of several tasks. For example, the different tasks could represent alternative foraging locations exploited by an ant colony. An individuals decision about which task to pursue depends both on environmental stimuli and on interactions among individuals. We examine how group size affects the allocation of individuals among the various tasks. Analysis of the models shows the following. (1) Simple interactions among individuals with limited ability to process information can lead to group behaviour that closely approximates the predictions of evolutionary optimality models, (2) Because per capita rates of social interaction may increase with group size, larger groups may be more efficient than smaller ones at tracking a changing environment, (3) Group behaviour is determined both by each individuals interaction with environmental stimuli and by social exchange of information. To keep these processes in balance across a range of group sizes, organisms are predicted to regulate per capita rates of social interaction and (4) Stochastic models show, at least in some cases, that the results described here occur even in small groups of approximately ten individuals.

The relationship between parasitoid size and fitness in the field, a study of Achrysocharoides zwoelferi (Hymenoptera: Eulophidae)

A knowledge of the relationship between body size and fitness is very important in many models of parasitoid behaviourial ecology. We estimated this relationship for Achrysocharoides zwoelferi (Hymenoptera, Eulophidae), a parasitoid of leaf mining moths (Gracillariidae) on willow (Salix). The size distribution of female wasps emerging from their pupae was compared with the size distribution of wasps caught while searching for hosts. A parametric statistical approach was used to obtain a function relating size to fitness. Laboratory estimates of the influence of size on a component of fitness, longetivity, were obtained under two different sets of experimental conditions. In the field, fitness increased rapidly with size over the first two-thirds of the observed size range, but plateaued (or possibly declined) in the last third of the range. One laboratory experiment showed no effect of size on fitness while the other did show an effect although much weaker than that observed in the field. This suggests caution in using results from the laboratory in parameterising behaviourial ecological models. The results are compared with two other field estimates of the influence of size on fitness which show broadly similar patterns.

THE DYNAMICS OF INSECT-PATHOGEN INTERACTIONS IN STAGE-STRUCTURED POPULATIONS

Few insects are susceptible to pathogens at all stages of their life cycle. The population dynamic consequences of introducing age and stage structure are examined through the use of models of insect-pathogen interactions in which hosts are assumed to contract the disease by ingesting infectious particles in the environment. The models are phrased as delay-differential equations that assume constant demographic parameters within age or stage classes. The first and simplest model assumes that only the adult insect is susceptible to infection, while the second model assumes a juvenile susceptible stage. Two further variants of the second model are considered: one in which the disease does not have a fixed incubation period and one that assumes that a reservoir for pathogen particles exists in the environment. Both linear and nonlinear transmission processes are examined. The introduction of explicit time delays has little effect on model equilibria but strongly influences dynamics. Multigenerational host-pathogen cycles are predicted by all models, although the parameter space in which they occur is markedly model specific. A type of population dynamics previously observed in host-parasitoid models, but not in insect-pathogen models, was also found: cycles with a period of one host generation or of a fraction of a host generation. This dynamic behavior arises through the interaction of unequal time delays in the two populations. The absence of a fixed incubation period and the presence of a pathogen reservoir both tend to be stabilizing. The models developed in this article are most applicable to insect-pathogen interactions in tropical or subtropical environments, where populations are not synchronized by a severe winter.

Recent developments in sex ratio studies

The allocation of resources to male and female progeny is a major component of the reproductive strategies of all sexual plants and animals. Over the past 30 years there has been intensive theoretical and experimental investigation of how natural selection moulds the sex ratio. Here, we discuss recent exciting developments and new applications of sex allocation theory and highlight some unresolved issues.

Predators and mutualists influence the exclusion of aphid species from natural communities

Abstract We investigated why two species of aphids (Aphis jacobaeae and Brachycaudus cardui) were very rare in a study site despite their abundance in the surrounding area. The study site contained many common species of aphid and we tested the hypothesis that the community of aphid predators in the field excluded the missing species. Colonies of the two aphid species were artificially initiated in the experimental site and allocated to one of four treatments: control; ground predator exclusion; total predator exclusion, and provision of mutualist ants. Two measures of colony performance were analysed: longevity and cumulative aphid density. Colonies decline naturally in late summer but control colonies disappeared very quickly while colonies protected from all predators survived the longest. The performance of colonies protected from just ground predators was intermediate. We failed to persuade ants to tend A. jacobaeae. Colonies of B. cardui attended by ants performed better than controls and those with ground predators excluded, but not as well as those with all predators excluded. We conclude that the absence of the two species of aphid in the study site is influenced by the resident predator community, and by the availability of mutualists.

Host-feeding strategies of parasitoid wasps

SummaryThree models of the evolution of host-feeding behaviour in parasitoid wasps are developed. The first assumes that the wasp host feeds purely to obtain resources to mature eggs (limited resource model) while the second assumes that host feeding provides energy for maintenance (pro-ovigenic model). The third model assumes that host feeding provides resources for both maintenance and egg maturation (resource pool model). Two variants of the third model are examined: the first assumes that the risk of mortality is constant and state-independent, the second that resource-depleted individuals suffer a higher risk of mortality. The models are analysed using a combination of stochastic dynamic programming and analytical techniques. The models make different predictions about the relationships between the probability of host feeding and egg load and host density. The available experimental evidence best supports the resource pool model.

Local mate competition, variable fecundity and information use in a parasitoid

Experiments with sex allocation in parasitic wasps offer excellent opportunities for testing how the way in which organisms process information about their environment influences behaviour. If mating takes place in temporary patches, where only a small number of females produce offspring, then sex allocation theory predicts a female-biased sex ratio. When females lay different numbers of offspring in a patch, females that produce relatively fewer offspring should lay a less female-biased, or even male-biased, sex ratio. Recent theoretical models have predicted that the exact form of this relationship depends upon whether females know only their own clutch size (self knowledge) or also the clutch sizes laid by the other females on the patch (complete knowledge). We tested the predictions of these models by examining sex allocation when two females of the parasitoid wasp Nasonia vitripennis oviposited simultaneously on a patch. The offspring sex ratio (proportion of males) produced by a female was: (1) negatively correlated with the number of offspring that she laid; and (2) positively correlated with the body size of the other female on the patch. Larger females matured more eggs and laid more offspring in the experimental patch. This suggests that, as predicted by the complete knowledge model, the offspring sex ratio laid by a female became more female biased as she laid a greater proportion of the total offspring laid on the patch. Furthermore, females use the body size of other females to assess the clutch sizes that these will lay. Copyright 1998 The Association for the Study of Animal Behaviour

The recruitment of parasitoid species to two invading herbivores

1. Two small species of leaf-mining moth, Phyllonorycter leucographella and P. plantani, colonized the British Isles during the 1980s. The parasitoid assemblage on the invading species is described and compared with assemblages on native Phyllonorcyter species. 2. Sixteen species of parasitoid were recorded from each invading host. Against initial predictions, the size of the assemblages were indistinguishable from those on native species. 3. The equitability of the distribution of individuals among paragitoid species was relatively low for assemblages on the two introduced hosts in comparison with native hosts. 4. It was predicted that the parasitoids attacking novel hosts should have a wider host range than those attacking native hosts. This was not found to be the case. 5. Cluster analysis revealed the assemblages on introduced species to be relatively dissimilar to those on native hosts. There was no particular similarity with native hosts on related host plants. 6. The most important parasitoid on P. platani was a specialist that invaded the British Isles at the same time as its host. The most important species on P. leucographella attacked hosts that had ecological features in common with the invader

Relative movement patterns of a tephritid fly and its parasitoid wasps

The extent of within-patch dispersal by a tephritid fly and its four major parasitoids was examined over three field seasons. Hosts and parasitoids were marked using acrylic paint and observed as they oviposited into the flowerheads of marsh thistle, Cirsium palustre. The average recapture rate pooled across all species was 22%. The four parasitoids showed consistently greater rates of movement than the host in all three years. In nearly all comparisons, male dispersal was less than female dispersal. There was no evidence that parasitoids moved longer distances after visiting low quality rather than high quality patches. In the one season it was studied, no correlations between movement and insect size were observed. The relevance of these observations to host-parasitoid population dynamics is discussed.