The behaviour of hoverfly larvae (Diptera, Syrphidae) lessens the effects of floral subsidy in agricultural landscapes

20 Modern agricultural landscapes favour crop pests: herbivores benefit from resource 21 concentration and/or the absence of natural enemies in large areas of intensively farmed fields 22 interspersed by small fragments of natural or non-crop habitats. 23 Conservation biological control (CBC) aims at increasing the functional diversity of 24 agricultural landscapes to make them more hospitable to natural enemies, and less to 25 herbivores. Although natural enemies readily respond to this management, very few studies 26 assess if they succeed in effectively protecting crops. 27 We set up a field experiment to study if an ecological infrastructure varying in size and 28 consisting of the provision of floral resources at the centre of lettuce plots would influence the 29 number of eggs laid by hoverflies, and ultimately the control of lettuce aphids. We found that 30 the hoverfly females lay more eggs in the plots with the larger flower resource compared to 31 the control. However, this response had no impact on the abundance of aphids on the lettuces. 32 We designed two laboratory experiments to understand this absence of response. We found 33 mutual interference between hoverfly larvae, and suggest it may undermine the biological 34 control of aphids. 35 This mismatch between landscape management and the response of hoverflies indicates CBC 36 should take into account insect behaviour, particularly their response to conspecific density, 37 additionally to landscape ecology. 38 39