1. Insects commonly resist parasites using melanotic encapsulation. Many studies measuring immune response use the amount of melanin deposited on an artificial object that has been inserted into the animal as a proxy of the amount of resistance that the host is capable of mounting to natural parasites.
2. The relevance of this methodology to immune response in natural insect populations needs further study. Here, we examined two temperate damselfly species to elucidate the relationships among damselfly size, natural resistance to mites, and the immune response mounted by the same damselflies against nylon filaments.
3. The damselfly species that had high rates of melanotic encapsulation of mites in nature deposited more melanin on the nylon inserts than the species with low rates of natural resistance.
4. In females of this species, those that had resisted mites naturally melanised the nylon filament more aggressively than those that did not resist mites.
5. Our results show some support for the use of nylon filaments to assess natural patterns of immune response in these damselflies, but also suggest that caution should be used in interpreting the responses.