From sodium-vapour to LEDs: how an outdoor lighting retrofit affects insects in Singapore

In the Anthropocene, the planet is warming and global biodiversity, including of insects, is being lost at an unprecedented rate. One largely urban sustainability solution, shifting to energy-efficient, light-emitting diode (LED) bulbs on outdoor lighting, has impacts on insects that are inadequately understood, especially in the tropics and brightly lit cities. Working in Singapore—a highly urbanised and light-polluted tropical city-state—we performed a field experiment to test the hypothesis that newly retrofitted LED lights are more attractive (i.e., detrimental) to insects compared to the high-pressure sodium (HPS) lamps they replace. Instead, our sticky traps, which we mounted on both light types, caught statistically equal numbers of arthropods (individuals and families). Traps on LED lamps also attracted proportionally fewer dipterans and more hemipterans and hymenopterans, but these effects were site-specific. Overall, we found no support for our hypothesis—our findings may reflect differential emissions of ultraviolet wavelengths by both light types and/or the possibility that urban insect assemblages with historic exposure to light pollution may not respond as expected to the retrofit. We caution against extrapolating findings from impact studies to untested contexts and highlight the need for (1) more studies in the rapidly urbanising tropics and (2) field tests of the effectiveness of measures to limit any negative ecological impacts of LED lighting, including other lethal and sublethal effects that we did not assess. We also discuss possible implications for ecosystem services and insect conservation.