Browntail Moth in Maine

September 1, 2023

By Tim King

In 1903, the Massachusetts State Board of Agriculture commissioned a report on the browntail moth following a substantial outbreak of the pest, which had been introduced to Cape Cod, Massachusetts, only a few years earlier. Among other things, in “A Report on the Life History and Habits of the Imported Brown-Tail Moth,” the authors wrote of another outbreak of the moth — in 1543, in its native Europe. People at that time believed that the inexplicable spread of insects, which were defoliating orchards and causing horrid skin rashes, were a punishment for human shortcomings. A member of the city council at Grenoble, France, introduced a resolution begging the local church official to “excommunicate these pests and censure them, in order to check the damage they were doing daily.”

During the decades following the report, the invasive moth (Euproctis chrysorrhoea) expanded its North American territory into coastal Maine and the Canadian Maritimes and south to Long Island. Efforts to mitigate browntail moth’s spread were made, including the spraying of DDT and a quarantine of nursery products grown within the moth’s expanding range. By mid-20th century the population had decreased to a relic population on Cape Cod and several islands off coastal Maine. The quarantine on nursery products was lifted and the afflicted humans relaxed for a couple generations. Although there were sporadic pockets of browntail moth population growth every 10-15 years, these were quickly eradicated through insecticides and winter web clipping. 

Browntail Moth Populations Increase in Maine

Spring browntail moth caterpillar
Spring browntail moth caterpillar. Note the distinctive two orange dots that are characteristic of the species. Courtesy of Angela Mech

When increasingly large browntail moth populations began spreading more widely across Maine in 2015 and 2016, citizens, local government officials, scientists and even public health experts were caught off guard. Among the questions being asked were: Why now? Where did all these caterpillars come from all of a sudden?

Eleanor Groden and her colleagues at the University of Maine set out to determine if there was a relationship between the rapidly multiplying moth population, the moth’s expanding range, and Maine’s warming temperatures. Other insect populations had been affected by the changing climate, Groden, now retired, pointed out.

After assessing 23 years of weather patterns overlaid with browntail moth population expansion and contraction, Groden and her colleagues concluded that increasingly warmer fall temperatures were resulting in more older and larger browntail moth caterpillars emerging from winter hibernacula (protective silk webs) in the spring.

“Climate trends indicate continued increases in fall temperatures since browntail moth resurgence,” Groden et al. published in the October 2021 issue of Environmental Entomology.

Cool temperatures and adequate spring precipitation can drive outbreaks of Entomophaga aulicae, a fungus that can infect and kill browntail moth larvae, the entomologists pointed out. However, the healthy populations of larvae coming out of the warmer autumns will be more resilient and able to bounce back more quickly during dry springs.

One of the mitigation efforts taken during the outbreak of the early 20th century was to inoculate browntail moth caterpillars with Entomophaga aulicae. “Entomologists began rearing browntail moth larvae in outdoor insectaries in 1908 and infecting them with the naturally occurring entomopathogenic fungus, Entomophaga aulicae,” Groden and her colleagues wrote. “These infected larvae were then transported and released into local populations throughout Massachusetts over four years.” Groden reported that some data showed that the E. aulicae releases were successful.

Browntail moth caterpillar infected
Browntail moth caterpillar infected with Entomophaga aulicae. Courtesy of Mech

In 2023, when browntail moth larvae infected with the fungus began dying on a 17-acre island in Casco Bay during the cool and wet spring, the Maine Forest Service offered a very cautious note of hope.

“In the past few weeks, we have been finding some evidence of pathogen-related death at a few of our monitoring sites,” the Forest Service reported in a browntail moth update on June 12. “We responded to a request from the manager of Eagle Island State Historic Site and confirmed high mortality of browntail caterpillars from the fungus Entomophaga aulicae. The recent cloudy and rainy days are helping contribute to the spread of fungal and viral pathogens that attack browntail caterpillars. Although pathogens can regulate some populations of browntail caterpillars, disease outbreaks can only happen with co-occurrence of rainy weather and the presence of the pathogen. Therefore, we should caution that pockets of disease like this epizootic event (disease outbreak) may be quite isolated.”

The Forest Service is monitoring 10 sites throughout the state in 2023 and reported more mortality on June 27. This time the reported cause of death at the monitoring sites was viral — likely Euproctis chrysorrhoea Nucleopolyhedrosis Virus or EcNPV, according to research done on browntail moth mortality by the University of Maine between 2016 and 2019. “We are seeing some mortality of caterpillars at our sites, although, there is just a handful of dead caterpillars at each site,” the agency reported. “When caterpillars die from a viral pathogen, they latch on to a twig with their first set of prolegs and hang in a ‘U’ shape. Eventually, these caterpillars will liquefy and their insides will drip onto foliage below, which may allow other caterpillars to encounter the viral pathogen. Interestingly, a large proportion of caterpillars look like they died as they were pupating.”

Monitoring Moth Populations

forest entomology lab JPG
Workers in the forest entomology lab collecting browntail moth pupal packets in full protective gear due to the health risks from toxic hairs. Courtesy of Sadia Crosby

According to Allison Kanoti, director of forest health and monitoring at Maine Forest Service in Old Town, “These sites allow us to monitor phenology through the growing season across the region and communicate that information through our updates to provide information to those managing and living with browntail moth.”

Angela Mech, a forest entomologist, agrees that monitoring the browntail moth population across the state is an important part of understanding the insect. Mech leads research at the forest entomology lab in the School of Biology and Ecology at the University of Maine in an effort to better understand, monitor and control browntail moth populations. The lab had 35 browntail moth monitoring sites across the state in 2023, and had assistance observing those in Southern Maine through a collaboration with the University of New Hampshire.

Mech’s team put a lot of effort into developing a monitoring trap that would entice the male browntail moth. The main attractant is a female sex pheromone that was identified in the early 1990s. The Maine scientists worked with Trécé, Inc. of Adair, Oklahoma, to test different purity levels and concentrations of pheromone and have also been considering different trap styles and colors.

“White sticky traps work the best,” Mech said. “That makes sense because browntail moths are nocturnal.”

One reason Mech’s lab is conducting monitoring is to determine what she calls the “itch threshold” — the point at which the browntail moth population is on the cusp of a significant expansion.

Mech’s research team is working on the assumption that the current outbreak of browntail moth will behave like all of the other known outbreaks of the last 500 years. The population will eventually decline and be limited to a relic population once again. Then, likely years or even decades from now, the population will rapidly expand again. Knowing what the itch threshold is will provide a forecasting tool that wasn’t previously available.

“Male BTM catch data will be collected yearly and correlated to the number of winter webs in each trap area. The predictive model will help answer the question ‘how many males in a trap equates to a damaging population level?’ and can be used to develop risk assessments,” according to the lab’s 2022 report.

“We’re hoping we’ll have something that will alert us before the next outbreak gets to the point where we are now,” Mech said.

Where we are now is a serious public health problem combined with the defoliation of around a quarter million acres of forest land.

Browntail Moth Mitigation

The forest entomology lab at the University of Maine is juggling research projects that will respond to infestation in the future as well as the present. One of those projects involves taking a second look at the female pheromone that has been developed for monitoring traps.

The idea is to use the female pheromone to confuse males. Called mating disruption, it’s a technique that’s been used for decades to successfully keep a lid on spongy moth (Lymantria dispar). The process involves inundating a forested area with the female pheromone at a concentration that confuses the male to the extent that he can’t find a female.

Female browntail moth
Female browntail moth laying her egg mass. Courtesy of Mech

“We are trying to mimic the spongy moth results with the female pheromone of the browntail moth,” Mech said.

The pheromone can be applied by airplane or from the ground. “This method can be used over large areas, does not directly affect natural enemies, is cheaper than using chemicals, has minimal non-target effects, is environmentally friendly, and can be highly effective,” the lab’s 2022 report stated.

Mech categorizes efforts at browntail moth mitigation into two levels: a landscape level involving thousands of acres, and a backyard or urban park level. Her interest in local-scale mitigation has more of a public health focus than a forest entomology focus.

If you live in Maine, it’s very hard to avoid the rash caused by browntail moth caterpillars, she said. “I’ve had it repeatedly over the last three years and each time it gets worse. We take lots of precautions at the lab and cover ourselves from head to toe when we’re working with it but we still get it. The hairs are very small.”

The poisonous hairs can go airborne and can cause a dermatitis similar to poison ivy even without direct contact with a caterpillar. For some, the response is systemic, and a reaction spreads throughout their body. Respiratory complications are also a concern.

To help people protect themselves in their backyards and neighborhoods, the lab researched porch and yard lighting. Eliminating all lighting results in attracting the fewest browntail moth during their short adult lifecycle.

“Beginning in late June and early July, adult browntail moths will become active, and you may start seeing them near light sources,” the Forest Service stated in a June newsletter, citing Mech’s research. “We recommend keeping unnecessary outside lights off between 9:00 PM and midnight until the beginning of August to avoid attracting adult browntail moths to your property. If this is not possible, consider switching to yellow-spectrum lighting to reduce property attractiveness to dispersing moths.”

“My husband and I have changed our outdoor lighting,” Mech said. Her lab found that the browntail moth particularly likes light from bulbs that emit UV and/or blue parts of the light spectrum. These include black lights as well as compact fluorescent, incandescent and cool white LED bulbs.

Another neighborhood-level control method is to be alert to hitchhiking larvae and cocoons. The Forest Service reports that browntail moth showed up in counties far outside its range in 2022. “We think these caterpillars more than likely hitched a ride to get there,” their May 19, 2023, update stated.

The Forest Service recommends not parking under or near infested trees. They also recommend checking vehicles, ATVs, boats and trailers before leaving home if in an infested area. Firewood from infested areas should never be transported out of the area it was harvested. They further caution if you find larvae or cocoons on any of these sources to remove them and dispose of them with the utmost care.

Winter Web Removal

With winter approaching, Mech recommends clipping webs out of trees. It’s a method that was widely used during the outbreak at the beginning of the early 20th century and probably 500 years ago.

“Clipping webs is still one of the most effective control methods,” she said.

Laura Sieger, an orchard specialist at MOFGA, says that the orchard crew manages browntail moth at the Common Ground Education Center in Unity, Maine, by clipping winter cocoons and spraying Bt (Bacillus thuringiensis) when the larvae are feeding.

“We prune out all of the nests we find and stick them in the wood stove,” they said. “That’s done during winter pruning from December through March or April. If we find any lingering, we typically add Bt to a spring spray.”

Sieger says that clipping nests out of tall shade trees, such as the oaks that browntail moth favor, is more challenging. “One year a few of us went out to the fairgrounds in early spring scouting and shooting nests out of the tall oaks and collecting the ones that were blasted out of the tree and drowning them in soapy water,” Sieger said.

Jack Kertesz, MOFGA’s landscape coordinator, has worked on nest removal on the tall trees at the fairgrounds for several years. He says that shooting the nests was supposed to break them up and decrease the survival of the larvae in them. 

“It appeared to work reasonably well,” he said. Though he cautioned that the shooters had sore shoulders, and there were lots of spent plastic shotgun shells littering the ground.

Sieger agreed that collecting debris afterward was more of a hassle than they would have liked it to be.

The following year, in 2022, MOFGA hired an experienced nest remover with a lift truck, which was expensive. In 2023, they rented an aerial lift that had a steep learning curve but ultimately worked well. “Pruning with a pole pruner and rented lift in 2023 seemed better since it was easier to keep track of all the nests that came out of the tree this way,” said Sieger.

The airborne nest clippers had the most success when supported by a spotter on the ground. Nests are easier to spot with the sun at your back, according to the Maine Forest Service.

In 2022 a Canadian company called DeLeaves began experimenting with aerial drones for nest removal. The drones cost around $30,000, making them competitive in price compared with bucket trucks and aerial lifts.

Like MOFGA, cities and towns across Maine face challenges in establishing a safe and effective response to the outbreak. The state of Maine requested proposals from Maine municipalities and nongovernmental organizations to develop mitigation programs. As of this writing, Jim Britt of the Maine Forest Service said nine proposals were submitted but no awards had been made yet.

“We didn’t apply because we felt the grants were for communities with fewer resources,” Bangor Public Works Director Aaron Huotari said.

Bangor has had a multi-pronged mitigation effort for several years that includes education, monitoring, clipping and, in 2023, some experimental chemical applications of Bt and an injected insecticide into a few trees in city parks.

“We also intend to have a lending library of pole trimmers that residents can borrow to clip winter nests on their own property,” Huotari said.

When it comes to managing this invasive pest, as history has demonstrated, a community-wide effort can make a difference.

About the author: Tim King is a produce and sheep farmer, a journalist and cofounder of a bilingual community newspaper. He lives near Long Prairie, Minnesota.

This article was originally published in the fall 2023 issue of The Maine Organic Farmer & Gardener.

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