Grouping increases visual detection risk by specialist parasitoids

The benefits of prey grouping may be offset by increased detectability. With a focus on visual detectability, I investigated the potential costs of 2 traits, mine size and group size (number of mines per leaf), of a leaf-mining species, Antispila nysaefoliella (Lepidoptera: Heliozelidae), on the risk of visual detection by parasitoids and the ability to evade attack and capture after detection. Through field experimentation using artificial leaves and mines coated with a nontoxic adhesive spray for trapping insects, I found that the visual cues from groups of mines caused a significant increase in the number of parasitoids captured on experimental leaves (with artificial mines) relative to control leaves (without artificial mines). However, mine size did not. The observational evidence not only supports these patterns but also shows that the per capita risk of parasitism declines with large groups. These results provide evidence of a trade-off between avoiding visual detection and escaping attack after detection. Copyright 2008, Oxford University Press.