Insect Light Attraction: A New Explanation in the Spotlight
Written by James Feston, BCE, Director of Product Research and Development at Insects Limited, Inc.
One of the challenges we face here in the lab at Insects Limited is that we have far more questions than answers about product ideas and new research. This issue isn’t new to business where time and resources are limited. The pipeline for new ideas is long, and only certain projects get our attention at any given time. It seems that one of our favorite projects to circle around, revisit, and refine is light attraction in stored product pests. In fact, we’ve recently built a 1000-cubic-foot study chamber to test out and refine a variety of products. One of them is a light trap that has shown some promise with attracting webbing clothes moths, a species that typically doesn’t seem to respond strongly to conventional light traps. Recently, with some surprise and excitement, I came across some articles discussing a publication by Fabian et al. (2024), “Why flying insects gather at artificial light,” which advances our understanding of the fundamentals of insects' light attraction.
There are various versions of the phrase “like a moth to a flame,” but they all get at the same phenomenon: that moths and other flying insects seem to lose control around artificial light sources and sometimes, when the light source is a flame or a bug zapper, they meet a pretty dramatic end. So in an effort to understand our clothes moth subjects and their relationship with light, I dove into the article looking for anything helpful in a practical sense and was also curious to see what novel take the authors had on light attraction.
One of the authors’ first points was to lay out some of the standard issue explanations for the self-immolating behavior we have come to expect from moths. There are a few explanations, including:
The “Escape Mechanism” theory: Flying insects want to escape from inside a bush or vegetation and make a break towards the light. This theory would suggest that insects would fly directly towards artificial light sources which isn’t quite what we see.
The “Thermal Radiation” theory: Insects are attracted to the heat coming from a light source. Not a bad idea since we know insects like warmer temperatures. But insects seem to gather in huge quantities around street lamps even on the hottest of summer nights.
The “Celestial Compass” theory: This theory has been around since the late 1970s and is probably the most well-accepted to date. The idea is that flying insects use a fixed reference point like the light from the moon, and by maintaining a certain trajectory relative to that point, they are able to navigate. The system breaks down when artificial light point sources, like fire, are introduced, and the insect continues to recalculate its trajectory relative to the light source. Stuck in a positive feedback loop, it death spirals into the fire, and that’s that.
The newest explanation is not radically different from the celestial compass theory but does ditch the exciting bit about the death spiral. The research by Fabian et al. (2024) moves away from the idea that there is an everrecalculated flight angle relative to the light source in favor of what they call a “Dorsal Light Response” (DLR). Behind all this is a fair bit of geometry, statistics, and a fun amount of jargon typically reserved for human-engineered aviation, but I’d like to keep this article light, so to speak.
However, I would like to make at least one comparison to aviation. If you’ve ever seen the instrument panel on an airplane, there are usually a dizzying number of dials, knobs, and lights. Among them will be an attitude indicator or artificial horizon.
An attitude indicator, also called an artificial horizon, uses gyroscopes to help pilots determine pitch and roll when visibility is poor or nonexistent. It also helps when forces acting on the pilot’s body cause confusion and unreliable sensations of gravity. Moths and other flying insects perceive gravity but without an attitude indicator, they use light from the sky as a reference for which way is up.
The attitude indicator operates using an internal gyroscope and allows a pilot to see the roll (like when the plane banks into a turn) and pitch (like when the nose of the aircraft is pointed up or down) of the aircraft. This device is essential for safe and reliable air travel because visual cues are not always enough. The flight environment can give our bodies and brains inaccurate and misleading sensory cues which can lead to serious miscalculations. Insects have similar problems and no internal gyroscopes to fix them. This is where the DLR helps them out by partially filling the role of the attitude indicator. Of course, it all goes wrong when artificial light is introduced and their “instrumentation” fails to provide reliable information required for successful flight.
So how does the DLR work, and how does it function somewhat like the attitude indicator? The basic idea is that insects will try to keep a light source at their backs in a perpendicular fashion. Imagine those toy battery-powered airplanes that hang from the ceiling from a piece of fishing line. The airplane (or the moth) flies around and around while tilted at a slight angle away from the anchor point (the light source) on the ceiling. Imagine now that the light source was the sky. Since the moth is in no danger of ever reaching this point source, it can reasonably rely on the sky being up at all times. Just like a human pilot, the moth has other instrumentation/sensory input to help control flight, but it seems clear that the DLR is a powerful piece of sensory input that has strong effects on flight behavior.
The researchers in the Fabian study documented three flight behaviors associated with artificial light point sources: orbiting, stalling, and inverting.
Orbiting, Stalling, and Inverting are the 3 primary flight behaviors exhibited during the Dorsal Light Response. Imagine the insect orienting itself to keep the light source at its dorsal (back) side
The researchers observed these behaviors by altering the height of the light point source. A light placed high above an insect will cause them to climb rapidly (while trying to keep the light at their back), which will cause them to stall out. A light placed near the same height as flying insects will cause them to orbit the light like the toy airplane. A light placed below a flying insect results in the most dramatic behavior, causing them to invert themselves in an attempt to keep the light at their back and again stalling out or crashing. Insects caught in a cycle of these behaviors are considered “trapped” by the light. If the point source is a lethal one, like fire, it is often just a matter of time before the frenetic flight patterns associated with DLR result in accidental collision.
What does all this mean for our lab’s light trap study and light traps in general? What else does it explain about stored product insect flight behavior? One of the most important takeaways is that insects are not making a beeline to our light traps. They are not trying to reach the light source but are, in fact, trying to navigate (poorly) with it and likely spend a significant amount of time near the trap before going in. Perhaps that suggests that larger glue surfaces or satellite glue surfaces around a light point source would increase trap captures. Additionally, it would suggest that light traps work best when the number of competing artificial light sources is reduced.
I think this research also helps explain another phenomenon concerning flying insects in confined areas, including residential settings. Over the years, we have noticed floor traps can be, somewhat ironically, more effective than hanging traps for flying insects. This makes sense when considering that flying insects navigate poorly in homes, especially during evening hours, where there are conflicting light sources causing chaos in the insect navigational system. One would expect plenty of crashing, landing, and crawling during this period, as well as wasted energy leading to less flight overall.
So here we are again. We’ve got some interesting new answers but more questions than we had before. I am confident, however, that we will keep going around and around until we crash into some more good answers.
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