Moths actually aren’t drawn to light as previously thought

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Singer-songwriter Christine Kane talks about "magic little moments" in her song Loving Hands, in which she mentions watching "moths under the midnight lamp". We've all probably seen them fluttering round and round outdoor lamps at night. There's even a saying that says we are drawn to something "like a moth to a flame", as if this butterfly relative is mesmerized by the light and uncontrollably revolves around it. Are they blinded and stuck in orbit? Is there something other than the light itself that triggers this never-ending flight patter? Recent research has provided some surprising answers to this behavior.

Image from Pixabay

Moths aren't the only insects that are attracted to light. And perhaps it would be wise to define what it means to be "attracted to" the light. Superficially, it sounds like an insect with such an attraction would fly directly towards a source of light, but that's not what really happens. In any case, researcher G. A. Mazkhin-Porshnyakov wrote the following in a 1960 paper: "Why do insects fly to light, generally choosing lamps rich in ultraviolet radiation? As we shall see there is no straight answer to this simple question."

Professor Samuel Fabian and colleagues at the Imperial College London (original 2023 article referred to in the top link) mentioned many models that other researchers have come up with over the years to answer the question.

1. Some say the attraction is an  an escape mechanism, where insects fly to light like they might aim for a bright gap in foliage. They are making a run to the border, as it were.
2. Others think that since they use the moon as a compass for navigation, they might make a mistake and use artificial lights instead.
3. Some say it's not the light brightness but the thermal radiation that attracts them.
4. Another says noctural insects' eyes are adapted to dark conditions of the night and get confused by the bright light sources, so they fly erratically or crash, and get trapped near the light.

First, let's look at light itself. Whether it's bright or dim, it can also be visible or invisible depending on whose eye is taking in the radiation. Human eyes and insect eyes perceive different wavelengths of light. Humans can't see ultraviolet (UV) or infrared (IR) wavelengths.

Many insects, though, can see UV light. In a 2016 study at the University of Bristol, Andrew Wakefield and his team found the following bugs (with taxonomic orders in parentheses) were attracted to artificial lights:

• flies, midges, gnats, and mosquitoes (Diptera)
• butterflies and moths (Lepidoptera)
• wasps, bees, sawflies, and ants (Hymenoptera)
• bed bugs, shield bugs, aphids, cicadas, planthoppers, leafhoppers (Hemiptera)
• beetles and weevils (Coleoptera)
• caddisflies (Trichoptera)
• barklice, booklice, or barkflies (Psocoptera)
• earwigs (Dermaptera)
• mayflies (Ephemeroptera)
• lacewings (Neuroptera).

So, for the nocturnal insects, what type of light is available? Here are the wavelengths of light radiation for different light sources. You can see that the only artificial lights with UV wavelengths (below 380 nm) are the mercury vapor, xenon, and incandescent lamp. Depending on which type of light is used for city streets, stadiums, and parking facilities, you might find different numbers of insects attracted, more for the ones that have greater UV wavelengths.

If you're interested in a long list of light measurements and bug trap designs, check out the 1974 Technical Bulletin No. 1498 by Truman E. Hienton at the Agricultural Research Service of the USDA.

In a 1984 review "Astronavigation in insects", Swiss zoologist Rudiger Wehner noted, "Insects seem to be specially predisposed to use skylight cues for one kind of orientation or another, because their large-field compound eyes often view the entire celestial hemisphere". By skylight, he meant insects rely on the sun, moon, stars, and polarized light reflecting off moisture to understand a sense of what is up and how to orient themselves level to the ground. The polarized light to them may look like a band of dark blue across the sky as the sun moves, as shown in the CGI video clip below. Humans don't see it, but bees may use it as a compass.

Screenshot from "Insect vision part 4: What do insects see?" (YouTube)

Since the 1950s, other researchers have strapped insects down to monitor their head and body movements in response to various light stimuli. They could flap their wings, but because they were held in place, they could only move by rolling left or right. Researchers observed how the locust or dragonfly tried to reposition itself to remain horizontal. See one setup below. It was placed in a box facing a wind

Yellow bars held the locust in place from head to tail, while the wings were allowed to move

and change the body position laterally as the bars were attached to red wheels in front and rear.

(modified from Goodman, 1965)

tunnel (blue circle below) whose size could be adjusted. The rear box wall was open. In each of the four boxes around it (top, bottom, left, right) was an incandescent lamp that could be shone through holes (orange circles below) at varying intensity. Only the light from the right side is shown below.

From Goodman, 1965

In other experiments, they painted a black stripe horizontally around the box where the insect was mounted. This was to simulate a horizon which the researchers could rotate by hand. The result was that the locust or dragonfly would twist itself so that the source of light was always on its back, in essence simulating sunlight or moonlight from "above" it.

So, what did Samuel Fabian and his team learn about moths?

They figured that clamping or waxing bugs into place was good for starters, but to examine real-life behavior around lights, they wanted to record their movements as they flew untethered. So, they used motion-capture cameras to monitor three species of moths flying near light sources. Such cameras could display multiple positions of a moth in its flight, so individual movements could be followed at many brief time points. Their first data showed orbiting level to the ground, rapid climbing (stalling) on the sides of the light, and flipping over (inverting) when on top or bottom of the light. In all cases, they set their back facing the light.

From Nature, 2024

But some movements can't be recorded quantitatively with multiple images. Fabian and his colleagues tried a technique used in sports tracking or movie making next. They stuck three markers to the thorax of the moths and used cameras connected to software to follow the body movements precisely.

Motion capture analysis of baseball pitching (Qualisys)

Fabian's team allowed the moths to fly around a 2-m diameter cylindrical tent with the only light source being one of three lights in it: a UV LED bulb (395 nm), a UV-Blue Actinic tube (spectral peak 370 nm), and a cool-white LED bulb. Compared to flying in open conditions, these three settings caused moths to fly differently. They calculated velocity vectors and showed that moths were indeed not flying towards the light as folklore erroneously states. Data also showed their patterns were due to the UV light, not the other LED. In all cases, they again flew with their backs facing the light sources.

So, moths keep their distance from lights and do not head into them, as previously thought by casual observers. They simply get caught up in a stronger UV light source than what is in the night environment. As they fly in the relative darkness, when they detect a strong UV light source about 30 meters ahead (which is about their limit), they think it's the sun or moon and flip over to reposition themselves. They are not caught in the light, frozen like deer in headlights as they stare at the oncoming car.

Picture from S. Fabian

Watch them fly in slow motion.