Coccinellid interactions mediated by vegetation heterogeneity

Environmental heterogeneity can have profound effects on agroecosystem function and it is important for improving ecosystem services such as biological control. Promoting system diversity via non-crop plants is one method for increasing habitat heterogeneity within farmscapes. Non-crop plants provide access to refuges and alternative food resources provide multiple benefits to enhance populations of arthropod predators. In this study, we examined the effects of small-scale spatial structure on life-stage specific interactions between the native coccinellid, Hippodamia convergens Guerin-Meneville, and the exoticHarmonia axyridis (Pallas) (both Coleoptera: Coccinellidae), which overlap in spatial distribution in many crop systems. Squash [Cucurbita pepo L. (Cucurbitaceae)] and non-crop mugwort [Artemisia vulgaris L. (Asteraceae)] plants with and without aphids were used as a model of spatial heterogeneity inmicro- and mesocosmexperiments. In response to factorial treatment combinations, we evaluated oviposition behavior, egg predation, larval survival, and larval predator-prey and predator-predator interactions. Adult H. convergens displayed higher foraging activity on aphids when exposed to complex habitats containing a non-crop plant. In the presence of the exotic coccinellid, H. convergens preferred to deposit eggs on the non-crop plant. Furthermore, a combination of spatial heterogeneity and prey availability reduced larval intraguild predation and cannibalism, and improved reproductive output of H. convergens by reducing intra-and interspecific egg predation. Our results provide evidence that life-stage-specific intraguild interactions are mediated by access to non-crop plants. Thus, the introduction or maintenance of non-crop plants has the potential to enhance coexistence of multiple natural enemies and improve top-down control of pests.