What Attracts Moths (and Other Insects) to a Flame?

At night in the Costa Rican cloud forest, a small team of international scientists switched on a light and waited. Soon, insects big and small descended out of the darkness. Moths with spots like unblinking eyes on each wing. Shiny armored beetles. Flies. Once, even a praying mantis. Each did the same hypnotic, dizzying dance around the bulb as if attached to it with invisible string.

Excitement spread through the group of researchers, even thoughthis phenomenon was not new to them. The difference is they now have cutting-edge technology and high-speed cameras — capable of capturing the fast, frenzied orbits — to map the hard-to-track movements of hundreds of insects and tease out secrets surrounding why they act so strange around light at night.

A good grasp of gravity is mandatory for all animals.

National Geographic funding helped the team travel to Costa Rica — a country rich with diverse insect life — with their cameras to find out.

In total, they collected more than 477 videos spanning more than 11 insect orders, and then used computer tools to reconstruct the points along 3D flight paths. Together with the motion capture data, the researchers concluded all the species did, in fact, flip upside down when exposed to light, just like the large yellow underwing in the lab.

“This has been a prehistorical question. In the earliest writings, people were noticing this around fire,” Theobald said. “It turns out all our speculations about why it happens have been wrong, so this is definitely the coolest project I’ve been part of.”

While the study confirms light is disruptive to insects, it also suggests light direction matters. The worst is an upward facing or just a bare bulb. Shrouding or shielding may be key to offsetting negative impacts to insects.

The team is also thinking about light color, like if cool versus warm tones have different impacts. And, of course, the still unexplained mystery surrounding attraction to light — and how it happens in the first place over great distances.

“I’d been told before you can’t ask why questions like this one, that there was no point,” Sondhi said. “But in being persistent and finding the right people, we came up with an answer none of us really thought of, but that’s so important to increasing awareness about how light impacts insect populations and informing changes that can help them out.”

Reference: Fabian ST, Sondhi Y, Allen PE, Theobald JC, Lin HT. Why flying insects gather at artificial light. Nat Commun. 2024;15(1):689. doi: 10.1038/s41467-024-44785-3

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