Predation-controlled infection: Coexistence in amulti-exploiter system

Background/Question/Methods

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 Coccinelid, Azya orbigera, and an entomopathogenic fungus, Lecanicillium lecanii. Beetle adults and larvae were experimentally excluded and included using a mesh bag surrounding healthy populations of scale insects for 24 hours. After one week, proportion scales infected were counted and compared. To corroborate experimental results from this study, data from field surveys of a 12 x 7 ha plot conducted in June and July of 2009 were analyzed to look for patterns between L. lecanii, A. orbigera and C. viridis populations. Each coffee bush (n=428) was surveyed for the number of scale insects, average percentage of L. lecanii infection, and the number of A. orbigera adults and larvae.

Results/Conclusions

Experimental exclusion resulted in lower proportions of scales infected for leaves that were exposed to A. orbigera a week earlier. Field survey data from the previous year support experimental findings; lower proportions of scales were infected in July 2009 where A. orbigera were found in June 2009 (when all tested bushes had equal scale population sizes and no signs of L. lecanii infection). We attribute this decrease in infection to consumption of C. viridis by A. orbigera; removing susceptibles from the population. Scale consumption was greater and infection lower in adult treatments for the experiment, and larvae for the field survey. Adult A. orbigera may consume C. viridis at a greater rate than larvae, but migrate more often to different patches since they are capable of flight. We also extend the hypothesis that selection of larger, older C. viridis prey by A. orbigera is responsible for decreasing L. lecanii infection if as suspected, larger, older C. viridis are more susceptible to infection. Intraguild predation of L. lecanii through consumption of already infected C. viridis by A. orbigera may also decrease infection by removing infected individuals from the population.  In any case, the presence of the predator in this multi-exploiter system appears crucial for regulation of the second exploiter, and in effect, coexistence of the entire system.