Drosophila melanogaster larvae defend themselves against parasitoid attack via the process of encapsulation. However, flies that successfully defend themselves have reduced fitness as adults. Adults which carry an encapsulated parasitoid egg are smaller and females produce significantly fewer eggs than controls. Capsule‐bearing males allowed repeated copulations with females do not show a reduction in their number of offspring, but those allowed to copulate only once did. No differences were found in time to first oviposition in females, or in time to first copulation in males. We interpret the results as arising from a trade‐off between investing resources in factors promoting fecundity and mating success, and in defence against parasitism. The outcome of this investment decision influences the strength of selection for defence against parasitism.

We designed an experiment to test whether two species of aphid feeding on different species of host plant influence each others population dynamics via shared parasitoids (apparent competition) or other indirect processes. Pea aphid (Acyrthosiphon pisum) colonies declined faster towards mid-summer when there were nearby colonies of nettle aphid (Microlophium carnosum), though the significance of the difference (P = 0.06) was just short of the traditional 0.05 level. Observations suggested that parasitoids were not responsible for this difference, however, and that it was highly likely to be caused by aphid-specific predators

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