The natural enemies’ hypothesis predicts that more floristically diverse agroecosystems have higher abundance of natural enemies and thus lower pest populations than less diverse ones. According to this theoretical framework agroforestry systems could benefit of ecological services of pest predators, if the increase in shade vegetation diversity increases natural enemies diversity. The aphid Toxoptera aurantii (Boy.) (Hemiptera: Aphididae) is worldwide distributed in cacao plantations, although it is not considered a serious pest. This raises the questions: are natural enemies responsible for the no pest status of this aphid in cacao? Does shade system play a role in the effectiveness of natural enemies? To test the hypothesis that shade system influences the abundance and effectiveness of natural enemies in cacao plantations we carried out a cage-exclusion vs. no-cage experiment under two types of shade system, intensified simple shade vs. traditional diverse shade, in the State of Bahia, Brazil. This experimental setup was replicated in four sites with an intensified and a traditional shade farm paired within each site. The abundance of aphids and predators of the families Coccinellidae, Syrphidae, Chrysopidae and Hemerobiidae was daily recorded on aphid colonies for two weeks.
The overall abundance of predators was generally lower than 1 individual/colony. The maximum daily mean number of predators in aphid colonies was 1.1 individuals/colony. In spite of low densities, predators were clearly able to suppress the population growth of T. aurantii. Aphid population growth was significantly lower in colonies without predator exclusion. However, there was no difference in the abundance of aphids in the different shade systems. Though a top-down control is very evident in this predator-prey system, no evidence was found that the natural enemies’ hypothesis explains it. The predators are as effective in the intensified simple shade system as in the traditional diverse shade system. Syrphids were the most frequent predators and their feeding strategy based on early consumption of prey stock represents a higher competitive ability that, in our view, explains the regulation of T. aurantii populations in cacao plantations. We conclude that the ability of predators to suppress T. aurantii can be attributed to three factors: the synchronization of early predator and prey colonization, the ability of predators to increase prey consumption at higher prey densities (functional response) and the complementary action of the different species of hoverflies, lacewings and ladybird beetles.