Theoretical models predict that parasites reduce reproductive success of their hosts, but very few empirical studies have given support to this. Using the damselfly Coenagrion hastulatum (Charpentier, 1825), we tested how immune response, wing length, and the number of both endo-and ectoparasites affect egg production of host damselflies. The study was conducted with four different populations in Southwest Finland. We found a negative association between endoparasitic gregarines and the number

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... f host eggs. Furthermore, immune response increased with the number of water mites, but decreased with the number of eggs. Contrary to previous studies with other damselfly species, the number of ectoparasitic water mites did not affect the number of eggs. Moreover, wing length, used as an indicator of individual size, was not associated with egg numbers. The negative effect of gregarine parasites on egg numbers is likely to affect the composition of host populations, i.e. damselflies that show higher resistance to these endoparasites will have more of their offspring represented in subsequent generations. In future, more experimental research on the varying effects of different parasite species on the number of eggs is needed.