Susceptibility to parasitism: experiments with the damselfly Enallagma ebrium (Odonata: Coenagrionidae) and larval water mites, Arrenurus spp. (Acari: Arrenuridae)

We found that larval damselflies actively defended against colonization by larval water mites (Arrenurus spp.) and that developmental stage of damselflies was related to their likelihood of being colonized. Because larval mites have a phoretic stage on damselflies, it is also possible to measure mass and condition of hosts before onset of natural parastism; usually, such measures are not obtained in studies of parasite-host interactions. Although mass and condition of male damselflies at emergence was unrelated to mite numbers, we found that smaller and lighter females carried more mites into adulthood in one of two samples. We argue that differences in mass between female hosts may be both an immediate cause and an eventual consequence of different levels of parasitism. Our results are likely applicable to other parasite- host asscociations and may help to explain the ubiquity of non-random distributions of parasites on their hosts.