Egg allocation in the tachinid parasitoid Nemorilla pyste in response to different densities of the host Choristoneura rosaceana (Tortricidae)

Nemorilla pyste is an important parasitoid of Choristoneura rosaceana (obliquebanded leafroller) in Washington tree fruit. Female N. pyste typically oviposit a single egg on late instar C. rosaceana larvae. However, parasitism success increases with higher numbers of eggs per host, and 2, or occasionally 3 or 4 smaller parasitoids can complete development from a single host. In this experiment we examine the reproductive response of N. pyste females to the provision of 6, 18, or 30 C. rosaceana larvae per female per day in laboratory cages. Daily egg production, the number of eggs per host, and progeny production were monitored across adult female life at each host density. Preliminary results indicate that host density affected the number of hosts attacked and the number of eggs produced. Females given 18 or 30 hosts attacked more hosts and produced more eggs than females given 6 hosts. Despite these differences, fertile females produced essentially the same number of progeny in each treatment group. However, the size of the progeny reflected the number of eggs allocated to hosts. Relative to the other groups, females given 30 hosts produced the fewest number of eggs per host, and also had the lowest frequency of progeny that shared hosts. As a result, parasitoid offspring were larger in this group. Because parasitoid size generally reflects quality (fitness), these results suggest N. pyste females can compensate for low host availability by allocating higher numbers of eggs to hosts, but do so at the expense of offspring quality.