A little ingenuity helps endangered caterpillars 'chill out' at Minnesota Zoo

Endangered butterflies at the Minnesota Zoo were struggling in the heat. The solution? Find a way to help the caterpillars chill out so they can eventually reproduce.

The zoo's efforts to raise and release the Poweshiek skipperling, an orange-and-brown endangered butterfly once plentiful in Minnesota, faced a challenge in 2016 — the butterfly's one-year life cycle had sped up dramatically, likely due to warmer weather. That shortened their lives and made their chances of growing up, being released and breeding less likely.

"These guys are critically endangered. They're about to fall off the face of the Earth," said Cale Nordmeyer, butterfly conservation specialist for the Minnesota Zoo.

So the zoo's butterfly conservation team started thinking about how to cool the growing insects, he said, and enlisted the staff that maintain the zoo's aquariums to help. Aquarium staff created a "cooling table" for the future butterflies using chilled water and PVC pipe.

"Cale just kind of approached me with this brainteaser of a problem," said Matt McLaughlin, the zoo's life support systems coordinator and the table's inventor. "It was very experimental at the beginning."

The caterpillars' houses — cylindrical structures covered with pantyhose — sit between rows of pipes carrying cold water. This lowers the air temperature by 3 degrees Celsius, which keeps the temperature closer to their natural habitat and buffers them when it's scorching out. A chiller similar to an air conditioner cools the water. The cooling table debuted in 2017. Adding a second row of pipe in 2018 produced even better results, McLaughlin said.

The Poweshiek skipperling caterpillars make their homes in a hoop house on the Apple Valley zoo's property near the bison exhibit. About 275 future butterflies spend their days munching on blades of prairie grass. The tiny green caterpillars are now 8 millimeters long and barely visible. The hoop houses aren't open to the public.

The zoo's efforts seem to be working. This was the staff's most successful year since 2016 for collecting skipperling eggs, Nordmeyer said.

"We do suspect that the cooling table has been helping that survivorship [from caterpillar to pupae]," he said.

Poweshiek skipperlings once were common in Minnesota, with millions inhabiting the prairies. The population crashed between 2000 and 2009. With a population of 150 to 500 in the wild, they were listed as endangered.

Wild skipperlings are now found in just three areas — in Manitoba and two small Michigan sites.

"We used to be at the heart of it here in Minnesota," Nordmeyer said. "The fact that we seem to have lost ours is horrifying."

Less than 2% of Minnesota's native prairie habitat remains today, he said. Butterflies, in general, are a great indicator of ecosystem health, Nordmeyer said.

The two biggest threats to the skipperlings are habitat loss and new types of pesticides. Poweshiek skipperlings are especially sensitive to pesticides because they don't build shelters, he said.

The Poweshiek skipperling is one of two main species in the zoo's Prairie Butterfly Conservation Program, which started in 2012. The other is the Dakota Skipper.

The zoo partners with the Minnesota Department of Natural Resources, the U.S. Fish and Wildlife Service and the nonprofit Nature Conservancy, along with other zoos, on butterfly conservation.

The project, which had a 2020 budget of $285,000, is funded by private donations, zoo admissions, the U.S. Fish and Wildlife Service and state grants.

She said seeing the cooling table's success has prompted her to be more interdisciplinary in her thinking.

"My mind was blown," she said of her first glimpse of the contraption. "I'd never seen anything like it."

Erin Adler • 612-673-1781