Going with the glow

Things that fluoresce under blacklight: the security thread in $100 bill, a forged “Old Master” painting, those groovy posters from the 70s, and the caterpillar of the frosted elfin butterfly.

In fact, according to a paper published by David Moskowitz of Rutgers University, “Many caterpillar species fluoresce, or at least stand out brightly against the background, when exposed to ultraviolet light.”

a green caterpillar at the end of a plant stem

A glow worm IRL: a frosted elfin caterpillar fluoresces under ultraviolet light.

Moskowitz’s 2019 study documented promising results from trials using ultraviolet, or UV, flashlights to survey for frosted elfin caterpillars at night.

Last year, U.S. Fish and Wildlife Service fellowship biologist Ryan Bell picked up the torch.

Using the findings from the study, he developed a standard blacklight-survey protocol for partners who are helping to look for the rare butterfly across its native range, which spans from Florida west to Texas, and north to the Great Lakes states.

The blacklight surveys for caterpillars don’t replace the surveys for adults during their flight period, which have taken place range-wide since 2019. Rather, they offer a way to make up for missed opportunities. Maybe you couldn’t get to a survey site during the flight period — only about a two-week window in the spring. Or maybe you were at a site during the flight period, but didn’t see any frosted elfin because it was raining.

“They have such a wide range and limited flight window, it can be hard to have people on the ground in the exact right location, at the right time,” Bell explained. “Adding UV light lets us extend surveys three weeks beyond the flight period.”

The leaves on a bush appears purple under blacklight

Can you spot the glow worm in this photo?

This summer, the Service’s Science Rapid Response Team, which comprises up to 12 fellowship biologists (including Bell), piloted the approach at sites in Massachusetts, Connecticut, and Rhode Island. The team is engaged in collecting new data to understand the status of a number of species, like the frosted elfin, that appear to be in decline.

Did it work? Fellowship biologist Kathryn Nolan pointed to their experience at The Nature Conservancy’s Carter Preserve in Rhode Island, where they looked for frosted elfin during the adults’ flight period to no avail.

When they went back a couple of weeks later to search after dark using blacklights, she said, “I could see caterpillars glowing from six feet away.”

Light my fire

Partners are using the blacklight survey method, too, including in northern Florida at the Apalachicola National Forest, home to one of the largest known populations of frosted elfin in its range.

“When you have a whole forest to search through, blacklight may be the most efficient way to look for these guys,” said Robert Meyer of Tall Timbers, a research station and land conservancy based in Tallahassee, about 50 miles northeast of Apalachicola.

He had a hunch blacklight would be an efficient way to gather other information on the species, too.

Meyer specializes in red cockaded woodpecker conservation, but because frosted-elfin activity falls outside the breeding season for this bird, he saw an opportunity to contribute to research needs for the butterfly.

“We have the best population of the species an hour away from the station, why shouldn’t we help out?” he said. “Especially because the species will be up for Endangered Species Act consideration in a couple of years. This is the best time to gather information.”

A brown butterfly with colored spots on its wings

A male frosted elfin butterfly that researchers captured and marked with colored dots. The pattern is a key to where in the forest he was found. Dave McElveen/Tall Timbers

Meyer and colleague Dave McElveen submitted a research proposal to the U.S. Fish and Wildlife Service and the Fish and Wildlife Foundation of Florida (funded through sales of the Conserve Wildlife license plate) to investigate several key issues for managing frosted elfin populations across their range. Their research spans the spectrum, from capturing, marking, and recapturing adults to see how far they disperse — and if they will disperse cross a utility line corridor — to experimenting with novel techniques for distinguishing frosted elfin caterpillars from those of the gray hairstreak, a look-a-like species.

One of the research questions relates to pupation — the stage when butterflies transform from caterpillars to adults. Although the frosted elfin spends the majority of its life as a pupa, scientists don’t know much about what the species looks for in a pupation site.

Knowing where they prefer to pupate could directly inform decisions about habitat management, especially because their habitat requires periodic disturbance. Frosted elfin depend upon wild lupine and wild indigo, plants that grow in fire-tolerant oak-pine barrens, oak savannas, prairie, and dry oak woodlands.

“We know pupa have a tendency to bury themselves, but we want to know where, and how that influences whether they survive fire,” Meyer said. “Maybe there’s something we can do about the environment to increase their potential to survive when we burn.”

Because using prescribed fire to restore frosted-elfin habitat is key to ensuring a future for the species, understanding the potential for pupa to survive fire will help managers use this approach effectively.

To investigate the caterpillars’ pupation preferences, the researchers decided to follow them. That’s where the blacklight comes in.

Although the caterpillars glow on their own, Meyer and McElveen proposed coating them with hot pink biodegradable UV fluorescent powder so they would also leave a glowing trail behind them on the forest floor.

A caterpillar painted with UV powder glows pink in the dark under blacklight

A “pinked up” caterpillar, painted with biodegradable UV fluorescent powder, stands out in the dark under blacklight. Dave McElveen/Tall Timbers

“When we’d see a caterpillar munching on some lupine in the daytime, we’d go over and pink them up, and then come back at night to follow the little breadcrumb trail of powder to see where they went,” Meyer explained.

While the tracking method worked, they didn’t find what they expected at the end of the hot-pink trails.

“We found a lot of shriveled bodies with puncture wounds,” Meyer said, explaining that spider populations tend to boom a few years after an area has been treated with fire.

Fortunately the trail wasn’t completely cold. They did find some pupa that evaded spiders and buried themselves just below the surface of the sand or tucked themselves into a bit of grass.

“My running hypothesis is that they look for someplace dark,” Meyer said.

That could mean the ideal pupation habitat is about two years after a burn, when vegetation is still sparse enough that the caterpillars need to dig down a bit to feel secure, rather than just hiding under a leaf or a pinecone leaving them susceptible to both fire and predation.

They’ll need more data to reach a conclusion, but there’s always next season to pick up the trail. And blacklight will help shed light on the issue.

The effort to conserve at-risk wildlife and recover listed species is led by the Service and state wildlife agencies in partnership with other government agencies, private landowners, conservation groups, tribes, businesses, utilities, and others. It has drawn support for its use of incentives and flexibilities to conserve rare wildlife, reduce regulations, and keep working lands working.

This article is courtesy of the author, Bridget Macdonald and first appeared August 10 in Medium/Conserving the Nature of the Northeast.

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