Interference among insect parasitoids: a multi-patch experiment

1. Interference among insect parasitoids leads to a reduction in the overal l search rate (the population equivalent of searching efficiency) with incr easing parasitoid population density. When this reduction is due to behavio ural responses of individuals to increased intraspecific competition, inter ference can serve as a stepping stone fron individual behaviour to populati on phenomena. 2. Interference can take different forms: (1) a direct within-patch reducti on of searching efficiency with parasitoid density (direct mutual interfere nce): (2) a decrease in overall search rate with increasing parasitoid dens ity 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. mo re or longer travelling at higher parasitoid densities; and (3b) a decrease in overall search rate due to a change in the distribution of parasitoid e ffort over patches with Increasing parasitoid density. These last two forms arise from behavioural responses to increased parasitoid density and are f orms of indirect mutual interference. 3. We present an expression for the overall search rate in a patchy environ ment where individual parasitoids travel between patches. We use this to sh ow 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 interfe rence in a multipatch experiment. In these experiments, different numbers o f parasitoids were introduced in an arena where the distribution of hosts o ver patches was either aggregated or uniform. We show that both pseudo-inte rference and indirect mutual interference play a role, and that they have a n 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 unif orm distribution of parasitoid effort over patches with increasing parasito id 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 spe nt 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 int erference to also include behavioural responses in sex allocation, clutch s ize and host acceptance to parasitoid density as forms of interference.