Effects of LED light on adult caddisflies at two rivers in Canterbury, New Zealand.

Abstract

Artificial light at night is a global phenomenon that has become a critical convenience for human activities. However, artificial light is increasingly recognised as having unintended effects on terrestrial and aquatic ecosystems. For example, artificial lights can attract adult insects from their flight path along rivers, result in reduced insect recruitment back into the river, and cause fatigue or death of insects from flying near lights. Light-emitting Diodes (LEDs) have become the predominant artificial light source globally due to their energy and economic efficiencies. In New Zealand, there has been a recent conversion of streetlights from high-pressure sodium (HPS) bulbs to LEDs. LEDs emit a broad spectrum of white light and higher colour temperatures of LED (e.g. 6500 K) produce a higher peak in intensity of light in the blue light wavelength range than lower colour temperature LEDs (e.g. 3000 K). Research on the response of freshwater adult insects to LEDs is limited. In my thesis, I have investigated the effects of LED light on adult caddisflies (Trichoptera) to varying colour temperatures of LED light. Caddisflies are expected to be strongly attracted to high colour temperature LEDs because they are understood to be particularly sensitive to lower wavelengths of lights (UV, blue, and green). The two main objectives were to investigate the effect of; (1) four different colour temperatures of LEDs (3000 Kelvin (K), 4000 K, 5650 K, 6500 K), and (2) LEDs placed at varying distances of 0 – 100 m from the water’s edge on the abundance, species richness, and sex ratios of adult caddisflies. These objectives were tested over 22 nights of field trials at two rivers; the Avon (urban) and the Hawdon (high country braided) Rivers in Canterbury, New Zealand. My study focused primarily on adult caddisflies, but also other co-occurring insects. At both locations, LEDs of all colour temperatures attracted insects from a range of orders, including a high abundance and diversity of caddisflies. The paired light trapping experiments with four different colour temperatures of LEDs found that over 1.6 times as many caddisflies were caught by the 6500 K compared to 3000 K LEDs, and also the 5650 K compared to 4000 K LEDs, respectively. This is likely caused by a higher peak in blue light intensity in the higher colour temperature 5650 K and 6500 K LEDs. There was a significant decline in caddisfly abundance when light traps were set out at distances of 0 to 100 m from the Hawdon River. The decline in abundance was greatest 0 – 30 m from

the river, with 65% of all caddisflies caught by 20 m from the water’s edge. Most caddisfly families and species showed a similar response in both the colour temperature and distance experiments. This is with the exception of the Polycentropodidae family and the leptercerid Hudsonema amabile which showed no significant difference in abundance between LED colour temperatures. In addition, the Polycentropodidae family and the hydrobiosid Psilochorema leptoharpax, showed no significant decline with distance. Caddisfly species richness did not significantly differ between any pair of colour temperatures but decreased significantly with distance. In total, 56% of caddisflies (excluding the Hydroptilidae family) caught were female and 44% were males. Neither LED colour temperature nor the distance from the river had a significant effect on sex ratios. These findings suggest that LED lighting placement near rivers where freshwater insects could be threatened needs consideration. To minimise attraction of caddisflies for the purpose of conservation, my results suggest that lower colour temperature LEDs (i.e. 3000 K) would be recommended for artificial lighting at night. Furthermore, LED light placement should be restricted within 0 – 10 m of a river and avoided between 10-30 m if feasible. Overall, my research has extended knowledge of the effects of LED lights on adult caddisflies near waterways in New Zealand, and indicates that ecologically-considered artificial lighting design is achievable by considering light colour temperature and the distances at which they are placed from waterways.