Companion planting enhances pest suppression despite reducing parasitoid emergence

Abstract

Engineering agricultural habitats to increase resources for natural enemies has been widely adopted for promoting conservation biological control of arthropod pests. In this context, companion planting (i.e., intercropping) has been a common habitat manipulation used for bolstering the abundance of natural enemies by means of providing them with alternative food and shelter. However, the mechanistic understanding of how companion plants interact with the target crop, as well as with the pest and natural enemy community to influence biological control, remains poorly understood. In this field study, we assessed how companion plants (i.e., non-flowering parsley) intercropped with collard greens could affect the natural infestation by aphids, natural enemy activity and parasitoid emergence. Aphid densities were significantly lower in companion plant treatments over the first four weeks of the experiment, which likely reflected increased predation by sheltered generalist predators. The results also suggest that companion planting can indirectly influence the time and strength of specialist natural enemies’ numerical response. Moreover, companion planting was associated with lower parasitoid emergence, which was likely a response to increased intraguild predation. It appears that by mediating a lower aphid density, companion plants may breakdown the dilution effect and consequently promote higher predation of mummies by generalist predators. Lastly, reducing aphid density through companion planting in the brassica system is appealing since Brazilian growers have a limited number of insecticides registered for aphids, which have been a detrimental pest that often turns the collard leaves unmarketable due to stunted growth.