There’s been no shortage of proposed explanations for the behavior – insects might mistake lights for the Moon, or flowers, or gaps in foliage they can zip through. Maybe they’re blinded by the glare and get lost. They could just like the light and warmth as a refuge in the cold of night. Or possibly they’re attracted by the other insects there, to mate and/or feed. Or it could be a mix of the above.
But short of asking the exhausted moths on our porch, how do we know which story is right? To find out once and for all, researchers at Florida International University (FIU) and the Imperial College London used high-speed, 3D cameras to untangle the flight paths of insects around an artificial light, both in the lab and in the jungles of Costa Rica.
It turns out, insects aren’t “attracted” to artificial light, per se. Your porch light is just messing with their navigation systems, honed over tens of millions of years of evolution. Basically, because insects are performing aerial maneuvers that would make the most experienced fighter jet pilots sick, they can’t rely on their usual sense of gravity to keep track of which way is up and which is down. So, they learned to keep the sky – nature’s brightest nightlight – at their backs to help them stay upright.
The problem, of course, is that the sky is no longer the brightest light. That means bugs will fly past a streetlight or other bulb and instinctively reorient themselves to put the light at their backs. As they zoom past, they keep adjusting their flight path to keep it at the same angle, which leads them to just loop-the-loop around the bulb. Eventually they get vertigo, and start crashing into the light in confusion, where the impact, heat or just plain exhaustion takes its toll.
The scientists on the new study first identified this explanation in lab tests, by attaching motion capture markers to moths and dragonflies to reconstruct how they flew around a light, including their rolling and rotating motions.
“On one of the very first experiments, I let a large yellow underwing moth take off from my hand and fly directly over UV bulb and it immediately flipped upside down,” said Sam Fabian, an author of the study. “But we didn’t know then if the behavior we saw and measured in the lab would also be seen in the wild.”
To find out, they headed to Costa Rica, home to one of the most diverse arrays of insects in the world, and set up a light in the jungle, surrounded by high-speed camera equipment to capture their movements in a similar way. Soon the area was buzzing with a range of insects, including moths, flies, dragonflies, beetles, even a praying mantis.
The researchers captured 477 videos of 10 different types of insect as they interacted with the light. And sure enough, all species kept the light at their backs, flipping upside down when they flew over the top of the light source.
“This has been a prehistorical question,” said Jamie Theobold, an author of the study. “In the earliest writings, people were noticing this around fire. 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.”
Next, the researchers plan to investigate if cool or warm tone lights have different effects on insects, and explore ways we could potentially minimize the disruption.