Pairwise interactions between functional groups improve biological control

• An approach combining advantages of substitutive and additive designs is introduced.
• Combining predator functional groups improved biological control.
• There was no evidence of disruptive interactions between predators.

Ecologists have long debated whether predators primarily disrupt one another’s prey capture through interspecific interference, or instead complement one another by occupying different feeding niches. Resolution of this debate has been difficult because different experimental designs are typically used to study interference versus complementarity. We adopted a somewhat atypical approach, surveying communities of predatory insects on 73 free-growing Brassica oleracea plants, and then re-constructing each community in field cages to measure its impact on aphid prey. The predator communities naturally varied in species composition, richness, and relative abundance; in our experiment we kept total predator density constant to avoid confounding effects of differing overall abundance. The predator communities’ impacts on aphids differed by >10-fold. Using a generalized linear model, we found that pairings of several predators in the community improved aphid suppression while no pairings disrupted it. Indeed, accounting for the presence of the beneficial pairings provided more power than species richness to explain predators’ impacts on aphids. Altogether, our results suggest generally complementary or neutral, rather than disruptive, multi-predator effects in this community. Our approach may be useful for determining the frequency of complementary species-pairings in many other systems.

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