There are several examples of sex-biased parasitism of invertebrate hosts. Sex biases in parasitism could be explained by differences between males and females either in exposure to or susceptibility to parasites. Our results show that for the common spreadwing damselfly, Lestes disjunctus, there was a female bias in mean intensity of parasitism by larval Arrenurus pollictus mites for newly emerged individuals sampled over emergence periods in both 2002 and 2003. This bias could not be explained by host body size and timing of emergence, factors thought to influence exposure of host larvae to larval mites. We suggest a novel explanation for sex-biased parasitism based on differences in developmental trajectories of larval male and female hosts, which should influence frequency of contact by larval mites. This explanation may help explain female-biased parasitism in other lestid damselflies, which should be exaggerated for early emerging species with compressed emergence periods. Further work is needed to test this novel explanation and determine whether it is applicable to other invertebrate host–parasite associations where parasites first come into contact with immature stages of hosts.