CLEMSON, South Carolina – In the middle of his workday, Clemson University entomologist Peter Adler stepped outside his office to take in the Aug. 21 total solar eclipse. Adler’s intention was to see whether creatures of the night – such as katydids, cicadas and moths – would make an appearance during the 2 minutes and 37 seconds of totality.

Adler noticed a change in butterfly behavior during the Aug. 21 eclipse.

Adler noticed a change in butterfly behavior during the Aug. 21 eclipse.
Image Credit: Carmony Corley

“I had nothing formal that I was going to test,” said Adler, a professor of plant and environmental sciences in the College of Agriculture, Forestry and Life Sciences. “What I was interested in doing was gaining some insight into what various insects would do during this very brief, but dramatic change in a typical day. I went out with some questions in my mind and informally watched and listened to the insects.”

Adler noted that, in the same location, a total solar eclipse will typically happen only once every several hundred years. Considering that insects can give rise to multiple generations of offspring in just one year, it was safe to say that none of the insects flying around Clemson on Aug. 21 had ever experienced a total solar eclipse. Neither had their parents, grandparents nor great grandparents – 500 times removed.

“It’s a rare event, so biologists want to know how are organisms going to respond to this? It’s so different,” Adler said. “It’s not like clouds passing over today and then maybe a week later. Insects are well-adapted to those occurrences and they’ve adjusted. But insights into what would happen during this event are limited. So I went out with a perspective in mind and I watched and listened.”

Pometto utilized a light trap at the Cherry Farm to collect her data.

Pometto utilized a light trap at the Cherry Farm to collect her data.
Image Credit: Suellen Pometto / Clemson University

As totality approached, a chorus of cicadas erupted and insects like diurnal gnats and butterflies disappeared.

“Butterflies are exclusively active during the day. Moths tend to become active at dusk and throughout the night,” Adler explained. “It’s kind of like a changing of the guards. You go out to a patch of flowers and during the day there are lots of butterflies. Then as dusk begins to fall, the butterflies just disappear. They go into the bushes and the trees and they rest until the next day when it’s light again. At dusk the moths come out.”

Every butterfly in the garden at the Cherry Farm Insectary – where Adler’s office is located – disappeared, giving way to the nocturnal moths. However, when totality ended and light reappeared, Adler noticed an interesting response.

“The biologist doesn’t like to give human characteristics to insects, but it was as though the moths were confused,” Adler said. “They’d only had a brief time, a couple of minutes on the flowers, then it was daylight and they began to seek darkness again. They moved into the shadows among the flowers and the plants to feed, and soon disappeared. Shortly after that, the butterflies came back out.”

Just like humans and most animals, insects have a 24-hour circadian rhythm. Diurnal insects like butterflies rest during the night and are active during the day. Nocturnal insects like moths are the opposite. One thing Adler wanted to discover by observing the insects was just how dominant is this circadian rhythm?

moth on a flower

Butterflies are typically seen during the day, and moths at nighttime.
Image Credit: Carmony Corley

“For millions of years, insects have known the sun was setting. It’s such a predictable cue that organisms respond to. What totality showed us was that the environmental cue is more commanding than the 24-hour cycle for these insects,” Adler said.

Suellen Pometto, Adler’s graduate student in entomology, agreed with this conclusion. Pometto created her own experiment to test the insects’ response to the eclipse, using a light trap at the Cherry Farm.

“During the day, the light trap is on, but it’s not doing anything. It catches some incidental insects that fly by, but not much else,” Pometto said. “I wanted to see if any insects came by during the totality of the eclipse, so I collected two different ways: one with jars set up underneath the trap and one with what I saw flying by on a white sheet set up behind the trap.”

Pometto was able to capture crescent-shaped slivers of sunlight during the eclipse.

Pometto was able to capture crescent-shaped slivers of sunlight during the eclipse.
Image Credit: Suellen Pometto / Clemson University

Pometto observed her setup at four separate times during the day: once in the morning, once from 2 to 3 p.m., again from 4 to 5 p.m. and then again at dusk. When insects would fly up to the light trap, they’d often hit the glass behind it and drop to the ground into Pometto’s jars. This allowed her to quantify the amount of insect activity happening throughout the eclipse. The white backdrop provided by the sheet gave Pometto an easy way to visualize the dark insects as they encountered the light trap.

“If the circadian cycle is stronger than the environmental cue of darkness, then I could expect to see no change in the number of insects collected during the hour of the eclipse as compared to the samples that I took before and after the eclipse,” Pometto said. “If the darkness results in an increase in the number of insects coming to the light trap, what insects are coming? Are they nocturnal species becoming active ahead of schedule because of the darkness? Or are they diurnal species that are confused by the darkness and the bright light?”

As totality began, Pometto noticed a moth – usually a nocturnal species – fly up to the light trap immediately, along with wasps and flies. The darkness was also accompanied by a drop in temperature and an increase in relative humidity. Pometto is still sifting through the results of her experiment and quantifying her data, but the preliminary results seem to support Adler’s belief that the environmental cue of light intensity is stronger than that of a 24-hour circadian rhythm.

Adler says he won’t be recreating this experiment any time soon, given the rarity of total solar eclipses in Clemson. However, his observations will provide fascinating anecdotal evidence for the insect behavior class he teaches.

“This is the sort of thing I can share with students and talk about. It will be of interest to them, and I think it will be something they can relate to because they likely saw the eclipse, too,” Adler said.

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