Parasites promote mating success: the case of a midge and a mite.

I tested the prediction that the hydracharinid mite Unionicola ypsilophora reduces mating success in the chironomid midge Paratrichocladius rufiventris. Males of the midge form mating swarms through which females fly to emerge after a few minutes with a mate. This mating system is believed to depend upon the male capturing a mate after aerial competition between males. Thus aerobatic ability is expected to determine success and a large ectoparasite should impair aerial performance. The proportion of infested males in swarms (ca. 4%) was less than that in mated pairs (ca. 15%). Infestation thus improved the expected mating success of the male midge. This finding is counterintuitive and may be a chance effect of no adaptive value to host or parasite. Alternatively, it may be an adaptive manipulation of the host by the parasite. This study provides evidence for the latter explanation. Copyright 1999 The Association for the Study of Animal Behaviour.