Interference among insect parasitoids: a multi‐patch experiment

Abstract

1. Interference among insect parasitoids leads to a reduction in the overall search rate (the population equivalent of searching efficiency) with increasing parasitoid population density. When this reduction is due to behavioural responses of individuals to increased intraspecific competition, interference can serve as a stepping stone from individual behaviour to population phenomena. 2. Interference can take different forms: (1) a direct within‐patch reduction of searching efficiency with parasitoid density (direct mutual interference); (2) a decrease in overall search rate with increasing parasitoid density if parasitoids have a non‐uniform distribution over patches where this distribution remains unaltered with increasing density (pseudo‐interference); (3a) a decrease in the time spent on patches by each individual, i.e. more or longer travelling at higher parasitoid densities; and (3b) a decrease in overall search rate due to a change in the distribution of parasitoid effort over patches with increasing parasitoid density. These last two forms arise from behavioural responses to increased parasitoid density and are forms of indirect mutual interference. 3. We present an expression for the overall search rate in a patchy environment where individual parasitoids travel between patches. We use this to show how the different forms of interference affect the overall search rate, contrasting environments with aggregated and uniform host distributions. 4. Using the data of Jones (1986) we explore the different forms of interference in a multipatch experiment. In these experiments, different numbers of parasitoids were introduced in an arena where the distribution of hosts over patches was either aggregated or uniform. We show that both pseudo‐interference and indirect mutual interference play a role, and that they have an opposite effect for a uniform host distribution, but amplify one another for aggregated host distributions. 5. The indirect mutual interference arises from a shift towards a more uniform distribution of parasitoid effort over patches with increasing parasitoid densities. This shift is caused by a behavioural response to parasitoid density, and is likely due to changes in the parasitoids’ patch arrival and departure decisions. These decisions underlie the distribution of time spent on patches, thereby linking individual behaviour to a phenomenon at the population level. 6. Finally, we put forward a more general framework for indirect mutual interference to also include behavioural responses in sex allocation, clutch size and host acceptance to parasitoid density as forms of interference.