Predators of aphids in cacao plantations in Brazil: Effectiveness in biological control and mechanisms of coexistence.

摘要

The need to stop biodiversity loss and to develop sustainable farming systems makes cacao agroforestry a balanced way to sustain biological diversity while providing ecological services to agroforestry systems. One of the ecological services provided by biodiversity in cacao plantations is insect pest control. This dissertation investigates whether different levels of shade tree diversity in cacao plantations increase the abundance and effectiveness of predators of the aphid Toxoptera aurantii (Boy.) (Hemiptera: Aphididae). Furthermore, it addresses processes in assemblages of natural enemies that determine the strengthening or disruption of top-down herbivore regulation, focusing on mechanisms of coexistence amongst predator species. Exclusion experiments showed that the predators of T. aurantii, mainly Syrphidae (Diptera), Coccinellidae (Coleoptera), Chrysopidae and Hemerobiidae (Neuroptera), are effective in reducing aphid populations in cacao farms. However, no difference was detected in predator effectiveness between different levels of shade tree diversity. A laboratory experiment found a Type II functional response of larvae of Ocyptamus antiphates (Walker) (Syrphidae: Diptera) in relation to the density of T. aurantii. The larva of O. antiphates consumed an average of 301.1 aphids, the same average maximum density of aphids occurring under field conditions when predators are absent. This explains the effectiveness of aphid control by this species. However, the presence of five additional predators in this tri-trophic system required understanding how all the species can coexist while competing for a scarce resource. Therefore, I conducted field studies to assess the spatial association of predators of T. aurantii and found that spatially independent distribution is one of the mechanisms of coexistence among the aphid predators. The role of anti-predator behavior of aphid-attending ants on the spatial distribution and coexistence of predators was also studied. Ants attacked the syrphid O. antiphates but never attacked a species of Coccinellidae. This ant behavior could be a parallel, spatially structured, mechanism of coexistence among predators, for it can benefit the non-attacked species by increasing the number of prey patches free of competitors or intraguild predators.