Prey suppression improved despite intraguild predation: the roles of intraspecific regulation and resource partitioning

Appropriately high population levels of natural enemies are crucial for the success of many biological control programs. It is therefore important to understand the factors affecting population growth of predators. Cannibalism, a common behavior among arthropods, is a density-dependent regulation that can hinder population growth. The density-dependence mechanism is often associated with increasing encounter rates among individuals with increasing density, such that population-level cannibalism rates also increase. However it is also possible that the cannibalistic behavior of the individuals themselves escalate with increasing density, which would also lead to increasing population-level cannibalism rates. We investigated the effects of conspecific densities on cannibalism rates of Geocoris pallens Stål (Hemiptera: Geocoridae). Using Petri dishes sectioned into two halves with a mesh, we measured the cannibalism rates of focal G. pallens females that were exposed to different densities of conspecific individuals in the neighboring half of the Petri dish. When alone, G. pallens female cannibalism rates on own eggs were low (2-3%) but escalated by 10-15 fold (40%) in the presence of one or more conspecific females. The focal G. pallens female cannibalism rates also escalated similarly to the presence of a conspecific female, conspecific male or heterospecific female. We discuss the implications of this escalated cannibalistic behavior for biological control and the possible mechanisms responsible.