The influence of competition between foragers on clutch size decisions in an insect parasitoid with scramble larval competition

Abstract

The effect of competition between ovipositing females on their clutch size decisions is studied in animals that lay their eggs in discrete units of larval food (hosts). In such species the effect of competition depends on the form of the larval competition within such units. In insect parasitoids, there might either be contest (solitary parasitoids) or scramble competition (gregarious parasitoids) between larvae within a host. For gregarious parasitoids, a decreasing clutch size with increasing competition between foragers is predicted. This prediction is tested in experiments using the parasitoid Aphaereta minuta. Parasitoids were either kept alone or in groups of four before the experiment, in which they were introduced singly in a patch containing unparasitized hosts. Animals kept together laid on average clutches of 0.74 eggs smaller than females kept alone (average clutch is 5.3), thereby confirming the prediction. Clutch size decreased with encounter number, which might be due to the adjustment of the female's estimate of the encounter rate with hosts. Finally, the results are compared with those reported for solitary parasitoids (that have scramble larval competition), for which it is predicted that the clutch size will increase with increasing levels of competition between females.