I tested the prediction that the hydracharinid mite Unionicola ypsilophora
reduces mating success in the chironomid midge Paratrichocladius rufiventri
s. Males of the midge form mating swarms through which females fly to emerg
e 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. Thu
s aerobatic ability is expected to determine success and a large ectoparasi
te should impair aerial performance. The proportion of infested males in sw
arms (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 co
unterintuitive and may be a chance effect of no adaptive value to host or p
arasite. Alternatively, it may be an adaptive manipulation of the host by t
he parasite. This study provides evidence for the latter explanation. (C) 1
999 The Association for the Study of Animal Behaviour.