Costly copulation in the wild: mating increases the risk of parasitoid-mediated death in swarming locusts

Abstract

Increased vulnerability to predation while copulating is an often-touted cost of reproduction, but empirical data on such costs are rare. Here, I demonstrate a predation cost to mating in the wild, as levied by parasitoid digger wasps (Sphex cognatus) on a population of Australian plague locusts (Chortoicetes terminifera). In December 2010, brood-provisioning female S. cognatus were observed capturing, parasitizing, and burying locusts in southeastern Australia. Wasps caught 43 solo locusts, all of which were female, and 19 copulating pairs. This frequency of pair captures (30.6%) was over one order of magnitude greater than expected based on the relative availability of in copula locusts throughout the hunting environment (<3.0% of total locust counts). Only females in captured pairs were ever paralyzed, and although males remained active, they were trapped in coitus and buried alive in 18 cases. These data indicate that copulation magnified the instantaneous risk of wasp-mediated death by ∼10% (in absolute terms). Such an increase could pose an evolutionarily relevant cost to mating, especially if it is levied early in an individual's adult reproductive lifespan.