Nutritional stress and behavioural immunity of damselflies

Increased mortality in the presence of stress may result from stress-reduced availability of energy for immune function, coupled with the presence of pathogens or parasites. We tested the hypothesis that stress reduces antiparasite responses of damselflies Ischnuraverticalis (Hagen) to their ectoparasitic mitesArrenuruspseudosuperior (Marshall). Numbers of colonizing mites did not differ between nutritionally stressed and unstressed damselflies. However, unstressed damselflies successfully removed more attached mites than nutritionally stressed host larvae. Furthermore, certain damselfly behaviours increased in the presence of nonfeeding mite larvae. Some of these behaviours were effective in defending against mites, but were reduced by nutritional stress. These results are sufficient to explain inverse relations found between damselfly condition and intensity of mite parasitism seen in nature, and are likely to be applicable to other host–ectoparasite associations.