Ants provide biological control on tropical organic farms influenced by local and landscape factors

Abstract

Multiple predator species can provide effective biological control of pest insects if predators have complementary effects. This could be the case of predatory ants in species-rich assemblages, such as in the tropics. However, predator assemblages are structured by variation in local and landscape factors. Different spatial scales can affect species interaction and consequently the biological control provided. We investigated ant suppression of pest insects in organic vegetable crops, and the role of local and landscape factors on biological control. We used a sentinel prey assay to determine the effectiveness of ant biological control on 18 organic farms in the Brazilian Federal District. Ants and other predatory groups were sampled on farms to determine natural enemy assemblage composition. Ants were the group that most contributed to the predation rates, and the other predator groups complemented the predation rates exerted by ants. The predation of sentinel prey increased 3.07-fold when ants were present. These results varied with the farming management practices employed. The species richness and occurrence were positively related to the predation rates, indicating that ant species-rich assemblage may be more relevant than single species in providing biological control. The predation exerted by ants and other predators were benefited by the abundance of predators in the landscape. Tree cover and cropped area in the landscape reduced ant predation but increased the predation exerted by other predators, because most ant species we sampled usually occur in open habitats. In conclusion, species-rich assemblages play a central role in providing biological control, and biological control provided by ants and other predators depends on local habitat management and landscape composition and therefore is a scale dependent process.