Phenotypic plasticity of pre-adult egg maturation in a parasitoid: Effects of host-starvation and brood size.

Abstract

Larvae of parasitoid wasps develop on a single arthropod host, and often face resource limitation that induces a tradeoff between egg maturation and somatic growth. Part of the variation in the growth-reproduction allocation was shown to be heritable, but how the larval developmental environment affects this allocation is not well-known. Detection of life history tradeoffs is often facilitated under stress conditions. We therefore exposed developing female larvae of the polyembryonic parasitoid Copidosoma koehleri (Hymenoptera: Encyrtidae) to laboratory manipulations aimed to restrict host resources (either host-starvation or high larval density). We compared the females’ body sizes and egg loads shortly after adult emergence (<24 h) to those of closely related control females, which developed at a lower larval density within non-starved hosts. Host-starvation reduced the females’ body sizes but not their initial egg loads. Females that experienced high larval density produced more eggs but were similar in body size to the low-density controls. Thus, the relative allocation to reproduction increased in response to both manipulations of host condition. Developmental duration and longevity were similar in all treatments. The negative correlation between body size and reproductive allocation, observed in the host-starvation treatment, is compatible with previous evidence from other parasitoids. In the high larval density treatment, however, reproductive allocation increased while body size was maintained, suggesting that the higher density increased rather than limited host resources per developing parasitoid female. The additional host resources that were diverted into egg production possibly resulted from increased feeding and body mass gain by hosts parasitized by large broods of wasps. Our results demonstrate phenotypic plasticity in resource allocation between growth and reproduction in a developing parasitoid. This plasticity may contribute to an adaptive balance between longevity and mobility vs. fecundity during the adult stage.