Integrating spatially explicit molecular and ecological methods to explore the significance of non-crop vegetation to predators of brassica pests

Abstract

There is mounting evidence that non-crop vegetation can promote natural enemies of crop pests but most studies use only one or a few approaches to explore key processes. Here we integrate field sampling, insect marking, insecticide disruption, and molecular gut content analysis to explore the potential value of non-crop habitats to predators of brassica pests in temperate Australia. Twelve monthly surveys of 13 farms established that an exotic ladybird (Hippodamia variegata) and a native lacewing (Micromus tasmaniae) were numerically dominant predatory arthropod species in brassica crops and present in adjacent perennial pasture, bushland and riparian vegetation. We applied dye to non-crop vegetation on three sites and subsequently sampled predators from adjacent brassica crops. Relatively large proportions of H. variegata and M. tasmaniae were marked, especially close to the non-crop vegetation though also extending 100m into the crop, indicating predator immigration into the crop. In a third study, predators were monitored in three brassica crops after the host farmers sprayed insecticide to control what they considered to be excessive pest densities. Within two days, H. variegata and M. tasmaniae adults were present in the crops and numbers increased significantly over 12days showing rapid crop recolonisation. Finally, molecular gut analysis indicated large proportions of both predator species sampled from non-crop (non-brassica) vegetation contained DNA of brassica-specialist herbivores suggesting predator movement from crop to non-crop vegetation, possibly to access nectar. Findings demonstrate H. variegata and M. tasmaniae are likely to be important predators of brassica pests in the region and expand our understanding of the significance of non-crop vegetation for coccinellids and lacewings.