Coexistence in Multi-Exploiter Systems

Abstract

The coexistence between members of a multi-exploiter system, whereby one victim (the host or prey) is attacked by two or more exploiters (predators, pathogens or parasites), was examined in the case of Coccus viridis; a sessile scale insect coffee pest that is attacked by a predatory Coccinellid, Azya orbigera, and an entomopathogenic fungus, Lecanicillium lecanii. Experimental inclusion of A. orbigera resulted in lower proportions of scales infected with L. lecanii; the trend further supported by field survey data from the previous year. Selective consumption by A. orbigera of late stage C. viridis instars that are more often infected by L. lecanii may act to remove the most susceptible individuals from the population thus lowering levels of infection in the presence of beetles. Lefkovitch stage-based matrix models provide evidence that L. lecanii is an important natural enemy of C. viridis, and that exploitation of C. viridis life stages 1-3 currently limits population growth. Thus, for systems where C. viridis is a major problem, promoting a variety of natural enemies that attack different instars can effectively control the pest if combined with infection from L. lecanii. In addition, theoretical analysis of a model describing a generalist pathogen and a specialist predator that share a common victim resource, hints to the importance of non-linear indirect ecological interactions such as intraguild predation in promoting coexistence between all components of the multi-exploiter system. Such self-limiting processes may be integral to how biological control through biocomplexity can maintain pests at below threshold levels, while also limiting the spread of the control agents themselves.