The colour temperature and placement of LED lighting near rivers can reduce attraction of flying adult caddisflies

Abstract

Artificial light at night (ALAN), which can confuse and distract insects, has been implicated as a contributor of their decline worldwide. Changes in artificial light technology has led to a widespread conversion of outdoor lighting to light‐emitting diode (LED) bulbs, which are more energy efficient than older high‐intensity discharge or fluorescent lamps. However, many outdoor LED lights emit a proportion of short‐wavelength blue light, to which insects are commonly sensitive. The potential to reduce flying insect attraction to outdoor lights by using LEDs that emit different amounts of blue light, or by placing lights further from insect sources has not been well‐studied, particularly for freshwater insects. We tested whether four blue‐white LEDs that varied in their emittance of blue light intensity (3000, 4000, 5650, 6500 Kelvin [K] colour temperature) differed in their attractiveness to flying insects, particularly adult caddisflies (Trichoptera) over distances up to 100 m from a river. LEDs were deployed at a lit urban river and a predominantly unimpacted braided river in Canterbury, New Zealand. We expected a greater abundance and species richness of adult caddisflies to be attracted to LEDs that emit more blue light (cooler white LEDs [6500, 5650 K]) than those that emit less (warmer colour temperatures [3000 K, 4000 K]). We also expected the abundance and species richness of adult caddisflies in light traps to decline rapidly with increasing distance from the river, regardless of colour temperature. Moths, flies and a total of 26 caddisfly species were collected from both rivers. Significantly more caddisflies were attracted to higher than lower colour temperature LEDs. For example, a more than 1.6‐fold number of caddisflies were caught by the bluer‐white 6500 K LEDs than the warmer‐white 3000 K LEDs. This trend was consistent across most caddisfly taxa and both rivers. The number of caddisflies attracted to LEDs significantly declined with increasing distance from the river edge for the two most abundant caddisfly families and the four most abundant species; 65% of all caddisflies were caught by 20 m from the river margin. In order to reduce the attraction of caddisflies to outdoor lights such as streetlights, our study highlights the importance of considering a low LED colour temperature (3000 K or 4000 K) or those that emit less blue light and the benefit of positioning lights further back from waterways, where possible. Our results indicate that plans for urban outdoor lighting should consider the ecological implications of different colour temperature LEDs and their spatial position as project design considerations.