Colonies of browntail moth caterpillars build winter webs high up on tree branches and emerge in the spring. GETTY IMAGES PHOTO

Toxic caterpillars are running rampant

AUGUSTA — State Entomologist Tom Schmeelk was close to losing his voice on Tuesday from answering an average of 25 phone calls a day regarding browntail moth caterpillars.  

“It’s really bad in May and June, but this year has, by far, been the worst since I’ve been here for browntail moth calls,” he said in conversation with the Islander.  

Despite the many calls he receives on the subject, Schmeelk says he has little to offer by way of combating the furry caterpillars that seem to be in abundance right now.  

“This is the worst time to treat for browntail moth,” he said. “We’re a month past the treatment time.” 

For those who are not familiar with them, or have been lucky enough not to have had a run-in with them, browntail moth caterpillars are fuzzy with broken white lines that run down their backs, on which there are two distinct orange dots. They look similar to other benign, native fuzzy caterpillars, such as the eastern and forest tent, gypsy moth and milkweed tussock moths.  

Maine Center for Disease Control has declared the browntail moth caterpillar a public health nuisance for both forests and humans. Those fuzzy hairs on the caterpillar are toxic and can, upon contact, cause a skin irritation similar to poison ivy. If the hairs become airborne and are inhaled, they can cause lung irritation, which can be serious in people with asthma or other respiratory health issues.  

“On dry, windy days, they really blow around,” said Fred Mason, an arborist who, with his wife, owns Mountain Wren Tree and Garden Specialists. “When mowing the lawn, do it in the morning when dew is on the grass still. If you wait until afternoon, it’s dry and can blow those hairs around.” 

Mason recommends wetting down any outdoor spaces before entertaining company, covering sandboxes that children play in and wetting the cover before removing it, and dowsing other large equipment like trampolines with water during the time when the hairs are more toxic. According to the Maine CDC, caterpillars in their third larval stage begin to develop the toxic hairs; larger caterpillars have more of the toxic hairs. 

For the tree-working couple, dealing with browntail moth caterpillars is just part of the job. To prevent exposure, the Masons employ several different tactics that range from wearing a Tyvek suit to an extra layer of clothes that are removed before entering their vehicle or home. But if he does have a brush with the caterpillar, Mason employs fast, proper hygiene. 

“I just try and wash quickly,” he said. “It’s just part of life for us.” 

As Schmeelk explained, once the browntail moth caterpillars are out, it is too late to do a lot about it. Pesticides can be used to control caterpillars. The Maine Forest Service recommends contracting with a licensed pesticide applicator to control browntail moth caterpillars. Pesticide treatments should be done before the end of May. Later treatments will not reduce human exposures to the toxic hairs. 

“What we’re seeing now is the offspring of last fall,” said Mason, adding that there are treatments for injecting or spraying trees to try and kill the caterpillars. “They prefer oaks and fruit trees, but they will go on any tree that’s deciduous, especially after they’ve wiped out the others. They don’t attack evergreens at all.” 

Maine and a small area on Cape Cod are the only places these toxic caterpillars can be found in the United States. According to Schmeelk, in cool, damp springs, a natural fungus called Entomophaga aulicae grows and will wipe out the browntail moth caterpillars. But, this year has been a warm, dry spring, which helps them thrive. “The fungus really only affects the caterpillars,” said Schmeelk. 

Really, the time to be on the lookout for these insects is in the fall and through the winter.  

“It’s been in Maine since 1904 and is here to stay, unfortunately,” said Schmeelk, who has been with the Maine Forest Service since 2018. “This current outbreak started in 2015.” 

Although social media is abuzz on ways to capture the moths – some suggest placing tubs of soapy water at the base of trees where the caterpillars are – the best defense is an aggressive offense. Treating for browntail moth caterpillars is done from December through April by removing their silken webs from tree limbs.  

“People think they are large webs, but they’re the size of the palm of your hand,” said Schmeelk. Go out on a bright, sunny day during these months and turn your back to sun, he explains, and the nests will shine white on the tips of the branches of fruit and other leaf-shedding trees. Removing the nests and soaking them in soapy water or burning them is the best way to get rid of what is inside them.  

“Forty to 400 little caterpillars can be in one of those nests,” said Mason. “Each one of them makes 10 babies.” 

What about those silken nests found in the crotches of many tree branches during this time of year? Mason says those are the nests of the native caterpillars and moths and are not harmful. 

In July, you’ll see the super furry white moth, he added. “Their hairs are toxic too.” 

Browntail moth caterpillars are active two times a year, according to the Maine CDC. In mid-April they emerge from their winter webs and grow to their full size by June.  

“Each adult moth can lay 200-400 eggs,” said Schmeelk. 

Then, the second batch of caterpillars hatches from their eggs in August and are active until October when they enter their winter webs.  

As with other insects that transition from caterpillars to moths or butterflies, there is a cocoon stage for the browntail moth. Cocoons can be found on the sides of houses or on the side of trees.  

“They are impregnated with toxic hairs, so I wouldn’t tangle with them,” said Schmeelk.  

When the moths emerge, they are attracted to lights and will hang around buildings where they are kept on through the night. To catch the moths, some people put out tubs of soapy water with a light shining close to the water.  

Winter weather used to do more to keep the browntail moth population in check.  

“Our winters aren’t cold enough or extreme enough to kill a moth,” said Mason. “You won’t see brown tail nests until winter. Those nests are pretty darn tough and the hairs in those nests are toxic for a couple of years.”

For those looking for relief from a run-in with a browntail moth caterpillar, there are a few recommendations. One, offered by the Maine General Medical Center, includes a mixture of witch hazel, hydrocortisone cream, diphenhydramine cream and Aspercreme shaken in a spray bottle and applied topically. Another involves mixing equal parts of hydrocortisone cream, diphenhydramine, witch hazel and Vicks VapoRub and applying it three to four times a day. Ice packs can also be soothing.  


Sarah Hinckley

Sarah Hinckley

Former Islander reporter Sarah Hinckley covered the towns of Southwest Harbor, Tremont and neighboring islands.

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