A browntail moth winter web is covered with small, dead, infected caterpillars from last fall. It was clipped at the University of Maine’s Orono campus this spring. Photo courtesy of Angela Mech

Browntail moth caterpillars might be facing their own zombie apocalypse in Maine this spring, which would provide relief to people who get the itchy rash caused by the insect’s microscopic hairs.

The caterpillars are emerging now, and people are most likely to come into contact with the airborne hairs between mid-May and late June.

Angela Mech, assistant professor of forest entomology for the University of Maine Orono, said it’s difficult to predict what will happen in the coming weeks, but an unusual combination of conditions – including a roaming horde of “zombie caterpillars” last fall – might trigger a collapse in browntail moth populations this year.

The forest pest – whose tiny hairs cause rashes that can be so severe and persistent that local pharmacies have developed specialized lotions to treat the itch – is under constant threat by a natural fungus, entomophaga aulicae. The fungus, which typically spreads in rainy and cool conditions, infects the caterpillars and kills them.

The fungus will often keep caterpillar numbers in check while not devastating the population.

But Mech said if there’s a fall fungus outbreak followed by another the following spring, that combination can be devastating to caterpillar populations.


When the fungus spreads in the fall, it turns the caterpillars into carriers, or “zombifies” them, and the “crawling dead” will carry the fungus to the winter nests that are being built, Mech said.

The zombie caterpillars then die on the outside of the webs and will still be attached to the webs when another generation of caterpillars emerges in the spring, she said.

“When spring arrives, the caterpillars will come into contact with the cadavers: dead infected caterpillars,” Mech said. “On a sunny day, the caterpillars will sunbathe on the outside of the nest.”

Some of these caterpillars will become infected and die from the fungus carried by the zombies, while other caterpillars get infected from the new fungus that is spreading in Maine this spring.

That one-two punch can be a knockout blow.

“It is looking potentially promising for a population collapse,” Mech said. “A number of the nests we are clipping (in Orono) are showing evidence of fall fungus infection.”


Mech and her team of students and employees at the University of Maine clip the nests during the winter to help control the population, and to do research on Maine’s browntail infestation.

But whether the fall fungus was localized to the Orono area or has a greater range is yet to be determined. And it’s unknown if weather this spring – more wet, cool conditions – will trigger a significant outbreak of fungus.

There was a lot of rain in late May and June 2023, but it rained so many days in a row that it may have prevented the fungus from effectively dispersing and killing the caterpillars, said Thomas Schmeelk, an entomologist with the Maine Forest Service.

“The spores from the fungus need breaks in between the rain for the spores to be carried in the wind,” which is key to spreading the fungus and infecting more caterpillars, Schmeelk said.

Schmeelk said that in recent years, the fungus has helped dampen browntail moth populations in the Augusta area, as well as traditional hot spots such as Brunswick, Cumberland, Freeport and Yarmouth.

This spring, Schmeelk said, if the fungus or other viruses that kill caterpillars are ineffective at reducing populations, high concentrations of browntail moth caterpillars would be expected in Penobscot, Waldo, Knox and Hancock counties.


“I’m hoping we will see widespread mortality, but it remains to be seen,” Schmeelk said.

Whether the zombie apocalypse provides relief this year, Schmeelk said the longer-term “boom-bust” cycle of browntail moth populations should mean that Maine will see a population collapse of the forest pest within the next few years. The “boom-bust” cycle typically lasts 10-12 years, and Maine is in the ninth year of the boom part of the cycle.

Meanwhile, the University of Maine is continuing to research environmentally friendly pheromone treatments that could be sprayed in certain regions to control browntail moth populations.

Mech said one component of studying whether to deploy pheromone treatments – which aim to interfere with the breeding cycle – is determining how well the female browntail moth can fly.

A good flyer could potentially escape from sprayed areas. But initial tests show that the female browntail moth is a poor flyer, which also may explain why the U.S. population of the browntail moth has been largely contained to Maine.

When the moths are flying in the summertime, the prevailing winds are generally north and east, which helps explain why populations have not spread west or south, Mech said.

Browntail moth caterpillars tend to feed on leaves of fruit, oak, birch, elm and poplar trees. The Maine Centers for Disease Control and Prevention recommends residents in areas that are prone to caterpillar infestations cover faces and exposed skin and wear hats, long-sleeved shirts, long pants, goggles and a respirator/dust mask when outdoors. In some people, the microscopic hairs can also cause respiratory problems. Avoid using leaf blowers in areas known to have large infestations of the caterpillars.

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