The browntail moth is an invasive insect found primarily in coastal Maine and parts of Massachusetts that causes a painful, itchy rash in people who come into contact with its caterpillars’ tiny toxic hairs. Unlike most moths, which are harmless nuisances, browntail moths pose a genuine health concern: their caterpillar hairs can trigger skin reactions and respiratory problems even without direct contact, because the hairs become airborne and linger in the environment.
How to Identify Browntail Moths and Caterpillars
The caterpillars are the life stage that matters most, since they carry the toxic hairs. Young browntail moth caterpillars are pale yellow, less than half an inch long, with a black head and two dark spots near the head. As they mature through spring, they grow to about 1.5 inches and develop a dark brown body covered in tufts of white hair, with a distinctive broken white stripe running along each side. Two prominent reddish-orange dots sit near the tail end, which is the single most reliable identification feature.
Adult browntail moths are bright white with a wingspan of about 1.5 inches. They have a tuft of brown hairs at the tip of the abdomen, which gives the species its name. Adults are strongly attracted to light and often show up in large numbers around porch lights and streetlamps during their flight season in July and August.
Seasonal Life Cycle
Browntail moths follow a predictable annual cycle, which is useful for knowing when the risk of exposure is highest. In mid-April, caterpillars emerge from silky winter webs built in the tips of tree branches and begin feeding aggressively on leaves. They grow through spring, reaching their maximum size in June. This April-to-June window is when caterpillars are most active and shedding the most toxic hairs.
By late June and early July, caterpillars pupate. Adults emerge in July and fly through August, mating and laying eggs. A second batch of caterpillars hatches from those eggs in August and feeds until early October, when they spin their winter webs at the tips of hardwood branches and hibernate through the cold months. These winter webs, which are tight, white, silky structures wrapped around the very ends of branches, are one of the easiest ways to spot an infestation during the leafless months.
The Rash and Respiratory Symptoms
Browntail moth caterpillars have tiny barbed hairs called setae on their bodies. These hairs contain a toxin that causes a reaction similar to poison ivy in most people: red, bumpy, intensely itchy patches of skin that can last anywhere from a few hours to several weeks. Some people develop blistering or hive-like welts. The severity varies widely from person to person, and repeated exposures tend to make reactions worse over time rather than building tolerance.
What makes browntail moths especially problematic is that you don’t need to touch a caterpillar to react. The hairs break off easily and become airborne, landing on skin, clothing, outdoor furniture, and laundry hung outside to dry. They can also be inhaled, causing coughing, wheezing, and difficulty breathing in sensitive individuals. People with asthma or other respiratory conditions are at higher risk for airway irritation. The hairs remain toxic even after the caterpillars are gone, persisting on surfaces and in the environment for up to three years.
Treating a Browntail Moth Rash
If you develop a rash after spending time outdoors in an affected area, a cool shower can help remove any hairs still clinging to your skin. Avoid hot water, which opens pores and can drive hairs deeper into the skin, making the reaction worse. Over-the-counter hydrocortisone cream and oral antihistamines like diphenhydramine can reduce itching and inflammation. Calamine lotion also provides some relief.
For more severe reactions involving widespread blistering, significant swelling, or breathing difficulty, a doctor may prescribe a stronger topical steroid. Most rashes resolve within one to two weeks, though some people report lingering sensitivity for longer. If you’re having respiratory symptoms, particularly tightness in the chest or audible wheezing, that warrants prompt medical attention.
Reducing Exposure Around Your Home
The most effective long-term strategy is removing winter webs from trees on your property between November and mid-April, before caterpillars emerge. These webs are found at the tips of branches, usually in hardwoods like oak, apple, crabapple, birch, and cherry. You can clip them with pole pruners and submerge them in a bucket of soapy water to kill the hibernating caterpillars inside. Do not try to burn them in the tree.
During caterpillar season (April through July), limit outdoor activities on dry, windy days when hairs are most likely to be airborne. If you need to do yard work in infested areas, wear long sleeves, pants, gloves, and a mask rated for fine particles. Change clothes immediately afterward and wash them separately in hot water with an extra rinse cycle. Avoid drying laundry outdoors during peak season.
Mowing, raking, and leaf blowing can stir up toxic hairs that have settled on the ground, so wearing a mask during these activities is especially important. If your property has a heavy infestation, a licensed arborist can treat affected trees with targeted biological sprays in spring when caterpillars are small and actively feeding. Timing matters: treatments work best on young caterpillars in late April and May, not on the larger caterpillars that appear in June.
Where Browntail Moths Are Found
Browntail moths were accidentally introduced to the northeastern United States from Europe in the late 1800s. Their range once stretched from Connecticut to Nova Scotia, but natural enemies and control efforts reduced the population significantly through the 20th century. Today, established populations are concentrated along the coast of Maine, from Portland north through Midcoast and Downeast regions, and on Cape Cod in Massachusetts. However, outbreaks have been expanding inland in recent years, and individual moths can show up well outside their core range.
The caterpillars feed on over 20 species of hardwood trees and shrubs, with oak and apple being their preferred hosts. Heavy infestations can defoliate trees entirely, which weakens them over successive years and can eventually contribute to tree death when combined with drought or other stressors. Healthy trees can typically survive one or two years of defoliation, but repeated stripping of leaves takes a serious toll.