Foraging egg parasitoids benefit from the presence of potential competitors

Competition is generally considered a negative interaction that lowers the fitness of individuals. Studies of competition and fitness traits in parasitoids have primarily focused on interference competition, with less attention paid to parasitoids under exploitative competition. Theoretical studies predict that interference competition will decrease the resource utilisation of parasitoids, while exploitative competition will not. In our study, we investigated the impact of intraspecific exploitative competition among parasitoids on fitness traits, including resource utilisation, the percentage of offspring that emerged from parasitised eggs, and the percentage of female offspring produced from parasitised eggs. We investigated whether these fitness traits were influenced by the presence of conspecific foraging females, as well as by parasitoid and resource densities. Since we were studying the effect of exploitative competition, we used Trichogramma chilonis, a non-aggressive egg parasitoid. Corcyra cephalonica, a stored grain pest, was used as the host. We found that T. chilonis resource utilisation increased in the presence of conspecifics. However, the density of those conspecifics and the resource density did not affect resource utilisation. The presence of conspecific foraging females did not affect the rate of offspring survival. However, at a lower resource density, offspring survival was higher than under resource-abundant conditions. We explored this counterintuitive pattern using a simple mathematical model, which shows that superparasitism, occurring at high parasitoid density, can lead to high emergence. This is because, given only some progeny survive, the chance of parasitoid emergence increases with the number of eggs laid in the host. The secondary sex ratio of the progeny was unaffected by the presence of conspecifics or by parasitoid or resource densities. We conclude that for T. chilonis, the presence of competitors has a positive effect. Its high performance under competitive conditions may contribute to its efficiency as a biological control agent.