Why do sperm-depleted parasitoid males continue to mate?

Insect males of several parasitoid species have limited amount of sperm at emergence and experience sperm depletion when mate acquisition rate is high, suggesting that sperm production and storage could be a limiting factor for male's fitness. Sperm-depleted males of the parasitoid wasp Trichogramma evanescens continue to mate, and the impact of this behavior on sperm storage by females has been studied. Virgin females T. evanescens stored 50.6 +/- 13.1 (Mean +/- SD) sperm when mating first with a virgin male. However, when these females mated first with a sperm-depleted male and then with a virgin male, they stored only 18.9 +/- 7.8 sperm, indicating that mating with a sperm-depleted male has a cost and limits sperm acquisition from fertile males. Following a mating with a sperm-depleted male, females had to mate with three virgin males to restore their sperm supply confirming that additional mating enabled the female to store a limited number of sperm. According to our results, by continuing to mate, sperm-depleted males do not prevent mates from copulating again but they decrease their ability to store other male's sperm. Continuing to mate could be a strategy to increase the relative fitness of sperm-depleted males.