Theoretical models predict that parasites reduce reproductive success of their hosts, but very few empirical studies

have given support to this. Using the spearhead bluet (Coenagrion hastulatum (Charpentier, 1825)) damselfly, we tested how

immune response, wing length, and the number of both endo- and ecto-parasites 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 number of 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.