When trying to evade bats and their echolocation calls, moths will make a few last-ditch maneuvers to save their lives. One of these is known as the powerdive. But with all the energy-saving LED street lights these days, their evasive tactics are being thwarted, making them easy pickings for hungry bats at night. The findings were published in Royal Society Open Science this week.
To study changes in moth anti-predator flight responses, a University of Bristol team led by Andrew Wakefield conducted a series of experiments under LED-lit and unlit conditions at four sites around Bristol in the U.K. They fitted a commercially available street light with a panel of 24 LEDs mounted on top of a 4-meter-high tripod, and then directed light away from woodland areas into open grassy fields. Then they played recordings of insectivorous Nyctalus noctula bats – specifically, the “buzz” sequences made during their feeding calls. These were recorded a few months earlier at a lake in Somerset using an ultrasound bat detector.
After analyzing video recordings of 94 moths on 16 nights frame-by-frame, the team discovered that fewer moths performed their powerdive flight maneuvers in response to bat calls under LED street lights: In the dark, 60% of the moths made either a rapid dive or a spiraling flight toward the ground when they heard the bat calls, but only 24% of the moths attempted this evasive maneuver when they were lit by LEDs.
The researchers aren’t sure why LEDs have this effect, though they suspect that it might have something to do with the spectrum of light that they emit. Some moth species, such as the small china-mark moth, switch off their bat-evading ultrasound responses in the daytime. If the lit-up insects are interpreting LED lighting as sunlight, they might be unprepared to deal with their nocturnal predators.
By reducing the anti-predator tactics of moths, LEDs are shifting the balance in favor of aerial bats – that is, until moth populations drop so dramatically that the bats face a food shortage.