Give predators a complement: Conserving natural enemy biodiversity to improve biocontrol

Natural enemy biodiversity reflects both the number of species attacking pests (species richness) and their relative abundances (species evenness). Recent experimental work suggests that greater enemy biodiversity might lead to stronger pest suppression when natural enemies occupy different, complementary feeding niches. Complementarity can arise from natural enemy species attacking different pest species or stages located in different places or at different times, and/or when enemies use different hunting strategies. However, these benefits can be weakened when predators in diverse predator communities kill one another, which is more likely in simple foraging environments including few prey species. Here, I review our growing understanding of natural enemy biodiversity in agroecosystems while suggesting a range of approaches that conservation biological control (CBC) practitioners might deploy to specifically encourage complementarily and dampen interference. For example, mixing different crop species or allowing crop residue to remain in fields increases prey diversity and the complexity of the foraging environment, both of which are likely to increase partitioning of diet- and spatial-niches among natural enemy species, making intraguild predation less likely. Habitat management alongside fields likewise might be designed to include elements attractive to different natural enemy taxa that fill different feeding niches, explicitly engineering fields to bring together complementary natural enemies. For example, refuge strips could include both bunch grasses that provide refuge to predatory beetles and spiders that forage on the ground, and flowers attractive to generalist predators and parasitoids that forage in plant foliage. Caution is warranted however, as a general enrichment of prey communities might build trophic complexity that dampens overall herbivore suppression. Altogether, the approach suggested here is consistent with a growing realization that CBC efforts that simultaneously encourage multiple ecosystem services will be most likely to be adopted by growers. Furthermore, CBC projects provide an opportunity to examine whether predator biodiversity effects demonstrated in cages or other experimental arenas lead to real-world benefits for biological control, advancing our understanding of biodiversity-ecosystem services relationships more generally. Perhaps the clearest challenges remain in extending what is known about enemy biodiversity effects, and their enhancement through CBC, to species-rich tropical agroecosystems.