The Ups and Downs of Browntail Moths

Left to right: In autumn, browntail moth caterpillars build webs by wrapping clumps of leaves with silk; they spend winter in these webs. Photo by Allison Kanoti / Maine Forest Service. In early spring, caterpillars leave the webs and are highly social. Photo by Tom Schmeelk / Maine Forest Service. By late spring, caterpillars are larger and begin to wander. Photo by Brittany Schappack / Maine Forest Service.

A non-native invasive moth has been lurking in the Northeast for more than 120 years and has recently become a pest again – of trees and people – in parts of Maine. Browntail moth (Euproctis chrysorrhoea), which causes extensive defoliation of hardwood trees, also poses a threat to human health. The caterpillars’ toxic hairs cause rashes (similar to those resulting from contact with poison ivy) and respiratory problems. The hairs can break off the caterpillars when disturbed, become airborne, and remain in leaf litter for many years. Outdoor activities such as mowing, leaf blowing, or raking can stir up the hairs, which may also attach to air-dried laundry hung on windy days. While adult browntail moths do not produce toxic hairs, the hairs may be present on adults after they emerge from their pupation cocoons, which also contain toxic hairs.

Native to Europe, parts of Asia, and North Africa, browntail moth was accidentally introduced to Massachusetts around 1897 in shipments of rose bushes from France and Holland to a florist’s greenhouse near Boston. By the early 1900s, the moth had spread and caused significant defoliation to hardwood trees – oak, apple, cherry, and elm – throughout New England and southeastern Canada.

A federal quarantine on nursery plants in 1912 sought to limit the spread of browntail moth. In addition, there were numerous control efforts including spraying, biocontrol releases, tree removal, and the clipping and burning of tens of thousands of browntail moth winter webs. Biocontrol efforts included inoculating browntail moth caterpillars with spores of the naturally occurring fungus Entomophaga aulicae and releasing the inoculated caterpillars into local populations throughout the outbreak. This allowed the fungus to spread and infect other caterpillars. Scientists also observed additional control from the parasitic fly Compsilura concinnata, which had initially been released as biocontrol for spongy moth in 1906. These measures eventually reduced the infestation area to a small region along the Maine coast and Cape Cod by the 1930s.

Left to right: Adult browntail moths have white wings and brown abdomens. Browntail moths lay eggs in brown hairy masses on the undersides of leaves. In late summer, after eggs hatch, larvae feed and build webs. Photos by Allison Kanoti / Maine Forest Service.

The browntail moth continues to be a concern in southern Maine, where its populations have recently rebounded. While trees usually recover from a single year of defoliation, multiple consecutive years of heavy defoliation can cause tree dieback and mortality. Factors such as drought or other pest infestations combined with defoliation from browntail moth may increase the chances of tree mortality. Forest health specialists in Maine map forest damage from the air annually, including defoliation from browntail moth damage. This helps them track browntail moth populations and the amount of damage in a given year.

Browntail moth populations in Maine began to rise again in the 1990s, resulting in a 4-year outbreak that peaked in 2003, when browntail moth caterpillars defoliated nearly 11,000 acres of trees. Factors contributing to this resurgence included milder temperatures, which help over-wintering caterpillars survive, and a lack of effective natural predators. In 2015, browntail moth populations once again rebounded, resulting in another, much larger outbreak that peaked in 2021, when the pests defoliated more than 198,000 acres in southern Maine. Annual defoliation dropped to 150,000 acres in 2022, 46,000 acres in 2023, and only 2,000 acres in 2024.

The decline in 2024 was due partly to fungal outbreaks during the fall of 2023 and the spring of 2024. Entomologists also detected Nuclear Polyhedrosis Virus (NPV) – a naturally occurring deadly viral disease – in the caterpillar populations during that period. In addition, the Maine Forest Service provided $150,000 to landowners and organizations for browntail moth mitigation projects including outreach, tree injections, and web clipping. Other New England states are also monitoring for a resurgence in moth populations. The New Hampshire Division of Forests & Lands found and removed winter webs on the Isles of Shoals in the Gulf of Maine in June 2024. This was the first confirmed incident of the species in New Hampshire in 75 years.

Most reports of human health impacts are from late-May through July. However, toxin in hairs is extremely stable (lasting 3+ years); exposure is most likely in dry conditions. In infested areas use protective measures when conducting activities that might stir up the hairs. Graphic by Allison Kanoti / Maine Forest Service.

Browntail moth eggs hatch in August from hairy egg masses laid on the undersides of leaves. The early instar larvae feed on the leaf tissue between the veins throughout autumn. This “skeletonizing” of the leaves causes them to die and turn bronze in color. As temperatures drop in late fall, caterpillars make winter webs by enclosing several leaves in silk. After spending winter in this webbed hibernaculum, late-stage caterpillars emerge the following spring and can consume entire leaves before pupating during the latter half of June. Caterpillars are hairy and dark brown in color, with two conspicuous red-orange dots on their tail end, and pupate within cocoons made of clumps of leaves that are tied together with silk. Adult moths emerge, mate, and lay eggs from July through August; they have white wings and brown, hairy abdomens.

While oak and apple trees are primary hosts for browntail moth caterpillars, they may also build winter webs in coastal shrubs such as beach plum and cherry, as was the case on the Isles of Shoals. It is easy to confuse browntail moth webs with other web-making caterpillars such as fall webworm and eastern tent caterpillar. Fall webworms, however, build large nests encompassing foliage on the ends of tree branches from late summer through autumn, while eastern tent caterpillars make tents in branch crotches in spring.

One way to help control browntail moths is to clip the winter webs between November and mid-April and either drop them into a bucket of soapy water for a few days or burn them. (Be sure to check with your local fire department or state forestry agency to obtain a fire permit if needed.) Toxic hairs are also present in the winter webs. People should not handle any life stage of the moths without wearing proper gloves and, when possible, a mask. Insecticides can also be effective at controlling browntail moth populations, and treatments should be completed by June. It’s best to consult a licensed pesticide applicator in fall to schedule spring treatments. You can also reduce local populations in your yard by limiting outdoor lighting, which attracts mating moths, in July and August.

For more information on browntail moth and the ongoing situation in Maine, click here.