June 20, 2018
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Return of ‘gross’ caterpillars to Maine spurs fears of infestation

By Julia Bayly, BDN Staff

Denise Duperre still remembers her father coming home from a day of cutting trees around Madawaska when she was 11 or 12. He shook caterpillars out of his hair into the bathroom sink.

“The caterpillars would fall out of his hair and wash down the drain,” Duperre said. “It was so gross. You didn’t want to look, but you couldn’t look away.”

They’re back

It seems everyone who lived in northern Maine in the late 1970s to early 1980s — the time frame of the last major tent caterpillar infestation — has their own creepy-crawly story of caterpillar encounters.

Some talk of hosing down entire sides of homes covered with the caterpillars. Others remember using snow shovels to fill barrels with them as they crawled across driveways. And, once heard, no one can forget the “popping” sound made when they are stepped on or driven over.

These stories crept to the surface this week after reports of a fresh invasion in Blue Hill where so many of the Malacosoma disstria are congregating along a 2-mile stretch of Minds Road — also known as Route 176 or Route 15 — that they have created a driving hazard. The state has posted a traffic advisory warning motorists of slippery conditions.

“This is a low-level infestation now that is starting in Blue Hill,” Dave Struble, entomologist with the Maine Forest Service, said. “These things are cyclical, and this sounds like a classic infestation.”

Struble said this is the second year the caterpillars have been observed in large numbers in Blue Hill.

“We mapped them in a 70-acre area last year,” he said. “It was a small but intense population. It sounds like it’s as intense now.”

It’s a wait and see situation

But will the numbers reach the massive levels of three decades ago? And will they continue to spread out from Blue Hill?

“We really have to wait and see what they will do,” Struble said. “There was an outbreak in western Maine around 1976 to 1978 that collapsed on its own without turning into what they had in Aroostook County, so it could be either situation.”

For those who lived through it, the situation in Aroostook is remembered as horrific.

According to Struble, tent caterpillars were first spotted in eastern Quebec and into New Brunswick. By 1979, roughly 20,000 acres of woodland in the St. John Valley was infested.

The next year, Struble said, they had spread to 120,000 northern Maine acres and by 1982 reached their peak population density infesting around 800,000 acres in northern Maine.

“In 1981 they had begun to spread into central Aroostook and the next year they were in Washington County and were getting into the blueberry fields,” Struble said.

A friendly ally

There was much talk at the state level on how to combat the caterpillar, which was denuding trees of leaves and starting to munch on blueberries. Insecticides were used on trees. Some infested trees were also cut down and chipped. In the end, Mother Nature stepped in to control the situation.

“As the caterpillars spread, we began to see what people often call ‘government flies’ responding,” Struble said. “They respond beautifully to tent caterpillars.”

“Government flies,” or “friendly flies” [Sarcophaga aldrichi] are native to New England and are a parasite on the tent caterpillars, with their populations rising along with caterpillar infestations.

In early summer, the flies emerge from their larval stage underground and seek out forest tent caterpillar cocoons on which they lay their eggs. After hatching, the fly larvae bore into the caterpillar cocoons and feed on the pupating insects inside, eventually killing them.

Friendly flies are often called “government flies,” according to Struble, due to misguided and false rumors that began in the 1970s saying the flies were released by the government to control the tent caterpillar.

Resembling a large housefly, friendly flies don’t bite humans, spread diseases or lay eggs anywhere other than on tent caterpillar cocoons, Struble said. At worst, they may land on human skin as they are attracted by sweat.

On the move

When tent caterpillars are on the move in large numbers, it can be an impressive site, Struble said.

The caterpillars, he explained, have vision in which everything looks as if it is in silhouette and they are particularly attracted to any silhouette that looks tall and thin.

“To them, any vertical profiles that look like trees that would be a food source,” he said. “So they will crawl up to it and if it’s not something to eat they just crawl up and over and keep moving.”

That includes telephone poles, fences, cars, slow moving people and — when their numbers grow large enough — entire homes.

He recalled standing in a parking lot full of the caterpillars in the 1980s and as he was talking to another person, the entire mass started moving in their direction.

Not always an ecological crisis

For all their numbers back then, the caterpillars did little lasting damage to the forest crown, Struble said.

“They really do not kill too many trees,” he said. “But they will set the trees’ growth rates back.”

The caterpillars are not picky about where they get their meals — one deciduous tree is as good as another for the general feeders — and they can eat a lot of leaves.

But Struble said those trees, despite looking stark in the short term, are able to recover — often in the same season after the caterpillars have moved on.

“If it’s early enough in the season, they can regenerate enough to grow back enough leaves to survive,” he said “Those leaves will be smaller and probably small the next year, but it would take several years of constant tent caterpillar infestation to kill the tree.”

In Blue Hill, some trees in the infestation area have been stripped clean, and although it’s early in the season Struble said the stress of current dry conditions and thin soil of that area could make it more difficult for those trees to recover.

“It’s going to be hard on the trees,” he said. “We could see some die off.”

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