Insects within bushes assemble and forage closer to artificial light at night

Abstract

Artificial light at night (ALAN) has been implicated in the global decline of insect populations. Causal mechanisms contributing to declines remain unclear, however. Here we examine causal factors which could link some declining terrestrial insect populations with ALAN. To do so, we defined the closest and furthest halves of individual bushes according to the nearest source of artificial light. We sampled leaves and invertebrates from both sides to determine herbivory and abundance. First, we observed that, within bushes, leaves were significantly more likely to display herbivory at closer distances to established streetlighting. This may be due to the phenomenon of positive phototaxis in insects (flight to light). Further, insects within bushes were significantly more abundant on the side closest to streetlighting at midnight, but not at midday, when lights are unilluminated. Consequently, we argue that ALAN creates bottom-up trophic effects, driven by insect light attraction behaviours even at the scale of single plants.