Varying the spatial arrangement of synthetic herbivore-induced plant volatiles and companion plants to improve conservation biological control.
Abstract
Conservation biological control aims to control pests by promoting wild populations of natural enemies. One challenge is to attract and retain efficient natural enemies in crop fields, which often are a suboptimal environment. Towards this goal, the attract-and-reward strategy relies on combining attractive synthetically produced herbivore-induced plant volatiles (HIPVs) with companion plants (non-crop plants which provide alternative resources to the targeted natural enemies). Although severely overlooked, the spatial arrangement of HIPV dispensers and rewards inside crop fields may strongly influence the foraging behaviour and persistence of natural enemies and thus the success of this pest management strategy. We tested the impact of two contrasting spatial arrangements of HIPV dispensers and rewards, alternatively inside and around a block of target apple trees, on the efficacy of the biological control of Aphis citricola populations by the common predatory ladybird Propylea japonica in apple orchards in northern China. We used synthetic methyl salicylate (MeSA) as an attractant and the companion plant Calendula officinalis as a reward. To better understand how the spatial arrangement of MeSA dispensers and companion plants affected the attraction and foraging behaviour of adult ladybirds, we conducted indoor experiments in a flight mill, an olfactometer and a wind-tunnel. Blocks of target trees treated with MeSA dispensers inside and companion plants around provided the most efficient pest control in orchards, compared with the opposite spatial arrangement. The synthetic MeSA dispenser and the companion plant synergistically attracted ladybirds in the olfactometer and enhanced their flight activity in the flight mill. In the wind-tunnel, MeSA served as a spatial cue for ladybirds to find nearby prey, while companion plants were sought in the absence of prey. Synthesis and applications. This study aims to further improve aphid control in apple orchards through a careful spatial arrangement of herbivore-induced plant volatiles dispensers (HIPVs) and rewards (companion plants) in optimized attract-and-reward strategies. Without such assessment, these strategies may be hazardous even with well-identified targeted natural enemies. Associated lab experiments highlight that interactions between HIPVs and companion plants influence ladybird foraging pattern, and that their spatial arrangement can modulate the ability of such key predators to find their prey.