Functional diversity of predators and parasitoids does not explain aphid biocontrol efficiency

Abstract

Many studies demonstrate an important role of natural enemy biodiversity in the regulation of agricultural pests, but the role of different aspects of biodiversity in influencing this crucial ecosystem service remain controversial. We hypothesised that the functional diversity generated by combining divergent consumer groups (roaming coccinellid predators and parasitoid wasps) fosters complementarity, enhancing aphid biocontrol. We tested this using experimental mesocosms containing plants, aphids and natural enemies located in a greenhouse. We compared the aphid control efficiency (final aphid abundance) of low functional diversity treatments (two parasitoid species, or two predator species) with high functional diversity treatments (all four possible predator-parasitoid combinations). We also included all four enemies as single species treatments to allow calculation of the non-additive effects of combining natural enemies. Results showed that biocontrol (final aphid abundance) was driven by the species identity of natural enemies and positive non-additive effects in two treatments in which the most efficient predator species was combined with a parasitoid species and the other predator species, respectively. Functional diversity did not consistently influence biocontrol or non-additive effects. In conclusion, functional diversity, as defined by differences between roaming predator and parasitoid functional groups, failed to consistently explain biocontrol efficiency in our study. This calls for consideration of finer-scale functional traits and how they govern natural enemy interactions and cascading effects across ecosystems.