Where and Why do Females of the Parasitic Fly Istocheta aldrichi Lay Their Eggs on the Body of Adult Japanese Beetles?

Egg-laying decisions by female insects should balance aspects related to immature survival and foraging profitability of emerging larvae. Using field-captured individuals, we characterized the egg distribution of Istocheta aldrichi (Diptera: Tachinidae) on the body of its adult host, the Japanese beetle Popillia japonica (Coleoptera: Scarabaeidae). We determined whether the pattern varies as a function of host sex and the number of eggs laid on a given host. Out of the 5667 eggs observed, 95.9% were located on the host pronotum. Furthermore, eggs on the pronotum were mostly positioned in the center of this sclerite and oriented perpendicular to the host body axis. The proportion of eggs placed on other parts of the host body increased with the number of eggs per host (superparasitism). Successful development of I. aldrichi into pupae was maximum for eggs laid on the host pronotum than for eggs laid on other parts of the host. Oviposition decisions in I. aldrichi are likely shaped by trade-offs between vulnerability to grooming by the host and foraging profitability of neonate larvae (i.e., their capacity to penetrate the host cuticle), but also by the mating and defensive behaviors of the Japanese beetle.