Egg Load Evolution in Parasitoids

Parasitic wasps lay their eggs in or on other insects. Allocation of resources to reproduction (eggs) and survival (life span) should maximize reproductive success, but stochasticity in the number of hosts encountered precludes an exact match of allocation with reproductive opportunity. We study optimal egg loads using a general model for pro-ovigenic parasitoids (which only mature eggs before adult life) and a dynamic programming model for synovigenic parasitoids (which can mature additional eggs during adult life). We find that stochasticity has a major effect on optimal egg loads and that the patchy distribution of hosts is the major source of stochasticity in reproductive opportunities. In a stochastic world, the further "relative habitat quality" (expected number of oviposition opportunities gained by reducing reproductive investment by one egg) is below unity (as we argue it generally is), the more the optimal egg load exceeds the expected number of hosts. In unusually rich environments, however, stochasticity decreases optimal egg loads. Although synovigenic parasitoids can adjust to the conditions they encounter, they too have to invest for an uncertain future. Because they can replenish their egg supply throughout life, the incidence of egg limitation is even lower than in pro-ovigenic parasitoids.