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Ben M. writes, “I was at the Cambridge ‘Moth Ball’ on July 23 and was impressed by just how many little critters were attracted by the lights shining on the sheets at Fresh Pond. I suspect that using such lights to attract bugs in early November would yield fewer critters. Would any show up?”
I did not know the answer to Ben’s question, so I asked entomologist Teá Montagna for her thoughts. She tells me that November is not the best time to look for moths in Massachusetts, as there are significantly fewer than in June or July.
But you can still find some. In fact, several genera of native moths emerge only in the fall or winter (Lithophane, Eupsilia, Xylena and Sunira, for example). Other native moths at this time of year are moths from the family Geometridae and the genus Operophtera. There is also an invasive Operophtera, which is difficult to distinguish from the native one. This invasive moth emerges in November but has not been a big problem lately.
Montagna also tells me that later in the season, bait (a mixture of bananas, brown sugar, molasses and beer) is better at attracting moths than light. She says that on a warm November night, she might get 20 moths at a light but 50 at a bait station. What explains this difference? When moths emerge at that time of year, their first priority is to feed, and feeding usually occurs around dusk when lights are less effective due to ambient light.
I also notice that Ben uses terminology that indicates moths are attracted to light. We have all heard the phrase “drawn like moths to flame,” which indicates an irresistible attraction to something, such as the powerful attraction of moths to bright light. Are moths really attracted to bright light?
Recent studies suggest no, not really.
People since ancient times have observed that moths at night fly around a fire or a light. They have posited several reasons: that moths are attracted to light or to heat, or that moths think light indicates a safe passageway through thick growth. All three of these ideas now seem unfounded.
Moths and most other flying insects evolved at a time when the greatest light source came from the sky above – at night, that would be the moon. Moths evolved to use this light source to orient themselves while flying. Moths (and many other insects) keep their backs to a light source. This behavior helps insects know which way is up and it keeps them level during night flights. Assuming this light source is the moon, for example, insects flying with their back toward the sky and their feet toward the ground to fly in a stable and predictable manner.
When moths fly near an artificial light bulb, they think the bulb is up. They turn their backs toward this light and fly in circles around the bulb in an attempt to stay level. Just the opposite happens: By keeping their back oriented toward the light, they fly in erratic circles. Sometimes their orbit stalls. Sometimes they even fall (as you might imagine when a moth is upside-down with its back to a bulb below).
To document this behavior, scientists used infrared lights and three-dimensional motion-capture cameras to study moth flight patterns. They learned that moths tilt their backs toward the brightest light source as they fly. When the light source is an artificial light bulb, for example, the moths flew in erratic orbits around it. Below the bulb, the moths flew normally. As they climbed with their back toward the bulb, they often stalled out beside it. They inverted themselves when they were above the bulb. Often they fell, then righted themselves once they were below the bulb again.
These scientists took hundreds of slow-motion videos of butterflies, moths, bees, dragonflies and more. They found that these insects were not affected by faraway lights. Their flight behavior changed, however, when they passed a nearby light. They oriented their backs toward the light, even if this behavior halted their forward progress. (The oleander hawkmoth and drosophila flies were the only studied insects that did not exhibit this behavior.) This behavior is called the dorsal-light response, meaning that insects keep their dorsal (top) side facing the brightest visual region of the sky.
In one experiment, the scientists shined a shrouded light onto a white sheet on the ground. The insects flying over the sheet inverted themselves and tumbled onto the sheet. Then the scientists moved the white sheet above the light to simulate the night sky. This time the insects were not disoriented. They flew with their backs toward the sheet above and continued normally without disruption to their flight. (This experiment confirmed that insects do not fly toward light. They use light to orient their bodies in flight.)
To make a long story short, moths are not drawn to a light. They fly with their back toward the light.
Another interesting observation is that artificial lights affect only insects passing close by. Insects far away are not drawn in to or disoriented by it. Presumably celestial sources of light are stronger than artificial light at longer distances.
Understanding the impact of artificial light on insects is important because light pollution has become more and more common. As anyone who has flown on a plane at night can attest, much of our planet is lit up at night. In fact, more than 23 percent of the planet’s land surface experiences artificial brightness at night. In the United States, 99 percent of people live where the skies are polluted by light.
What can we do to lessen the impact of lights on insect behavior? We can use as few lights as possible at home so as not to disorient insects. We can close our blinds or curtains at night; this prevents stray light from spilling out. We can use motion detector lights when possible, so lights are on only when needed. We can cover outdoor lights so they shine downward and not upward. This focused light will affect fewer insects.
Have you taken photos of our urban wild things? Send your images to Cambridge Day, and we may use them as part of a future feature. Include the photographer’s name and the general location where the photo was taken.
Jeanine Farley is an educational writer who has lived in the Boston area for more than 30 years. She enjoys taking photos of our urban wild things.
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