Disentangling biotic and abiotic factors influencing host–parasitoid interactions of butterflies, tachinid flies, and nematodes

Studies of host–parasitoid dynamics have traditionally focused on parasitic insects, but nematodes occasionally show levels of parasitism comparable to those of flies and wasps. Because parasitic insects and nematodes differ greatly in phylogeny and ecology, the biotic and abiotic factors that affect parasitism rates in these taxa are expected to be quite different. We aimed to determine how the parasitism rates of tachinid flies and mermithid nematodes on the endangered butterfly Reverdin's Blue (Plebejus argyrognomon) were influenced by attendant ants and abiotic conditions. We found that parasitism by mermithid nematodes increased, while parasitism by tachinids decreased, with an increasing number of precipitation days. Ant species effective in defending against tachinid flies differed from those effective in defense against mermithid nematodes, which may be related to the degree of overlap in the daily activity patterns of parasitoids and ants. In addition, higher vegetation increased the parasitism rates of both tachinids and mermithids. This may reflect the creation of a more humid microclimate for mermithids and greater resource availability for tachinids in complex vegetation structures. The sum of the two parasitism rates showed a hump-shaped relationship with the number of precipitation days. Further clarification of the parasitism mechanisms in the two groups of parasitoids will be important for predicting how host populations will respond to future climate change.