Plant quantity affects development and reproduction of a gregarious butterfly more than plant quality

Abstract

AbstractThe larvae of insect herbivores feed on plants that may vary nutritionally (qualitatively and/or quantitatively) over the course of insect development. Plant quality may change in response to interactions with the biotic environment that in turn may affect development and biomass of the insects feeding on these plants. However, the larvae of many gregariously feeding herbivores feed on comparatively small plants with limited biomass and may also experience variation in the quantity of plant food available. Pieris brassicae L. (Lepidoptera: Pieridae) is a gregarious butterfly species laying clutches of 10–150 eggs that are often laid on small brassicaceous food plants, including the plant used in this study, Brassica nigra L. (Brassicaceae). A single B. nigra plant provides insufficient resources for the development of an entire brood of P. brassicae. In this study, we investigated the effect of both plant quality and quantity on the performance of P. brassicae when feeding on B. nigra plants. When we compared the effects of changes in plant quality induced by (1) aphid infestation, (2) exposure to pathogenic and non‐pathogenic bacteria, and (3) inbreeding depression, which are all biotic stresses known to change plant quality, pupal mass and larval development time of P. brassicae were fairly similar. We then examined the effects of quantitative food constraints during immature development on pupal mass, which correlated strongly with adult size, longevity, and fecundity. Female pupal mass, longevity and fecundity were negatively correlated with the duration of starvation during larval development. No significant effect of male starvation was found on female reproduction and longevity. Thus, P. brassicae larvae were more affected by quantitative than by qualitative constraints in terms of pupal mass, which strongly correlated with female reproduction.