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Nature Notes: Browntail Moth

Ed Robinson
September 28, 2014

Harpswell has been invaded in the last few years by a pest called the winter moth, with plenty of public education to help us control their spread and their damage to our trees. For over 100 years, we have also hosted another invasive moth that can harm not only our trees, but also humans. The browntail moth arrived in Somerville, MA in the late 1890’s on flowers imported from Europe. With no natural predators, the browntail quickly expanded its range so that by 1915 it was found across most of New England and into Nova Scotia. Fortunately by the 1970’s, the range had been reduced to coastal areas of Casco Bay, and Cape Cod. It is thought that the moths have been controlled by parasites and cold, inland winters.

The adult browntail moth is quite striking, with pure white wings and body, apart from a tuft of brown hairs at the end of the abdomen. In males, the brown hair covers most of the back of the abdomen. In females, the back of the abdomen is mostly white, but the tuft of brown hair at the end is much bigger. The moths have a wingspan up to 1.5 inches. Like many moths, they are active at night and are attracted by lights. In the larvae or caterpillar stage, their coloration is red with white markings, and they are covered by hair.

The human problem with the browntail results from these tiny hairs (only 0.004″ long), which are shed into the air or contacted when we collect moths or caterpillars. They serve the browntail well throughout their life cycle, being formed into a protective layer over the eggs, and also being woven into cocoons that provide shelter in spring and all winter long. They also serve to make the caterpillars unpalatable to potential predators like birds, a good survival adaptation.

a browntail moth lays eggs on the underside of a leaf

a browntail moth laying eggs (Tulle Frazer photo)

Unfortunately, the hairs contain a toxin and enzymes that can cause human skin rashes, headaches and breathing difficulties, and they also have a minute barb that allows them to embed in the eyes or skin, causing physical irritation. People who suffer exposure to many hairs, or worse, ingest them, can suffer the effects for anywhere from hours to many weeks, especially those people with weak immune systems. Friends have reported serious rashes and discomfort after doing yard or gardening work near hardwood and fruit trees, or ornamental shrubs favored by the caterpillars. If you must work near or handle the browntails at any stage in their life cycle, wear protective clothing and eye goggles, then wash carefully when done. Avoid drying clothing outdoors in the summer if it may be exposed to airborne hairs

Caterpillars feed in April and May, and again in August and September, on leaves of hardwoods and shrubs. In this area, favored species would include oak, apple, cherry, serviceberry, rugosa rose and bayberry. After their autumn feeding, the caterpillars retreat to large silk webs that may house hundreds, thought to be a way of surviving cold winters. To control infestations of the moths and the resulting tree damage, it is recommended to clip the silk webs during winter months while the caterpillars are inside, and destroying them with soapy water or fire. There are approved pesticides for spraying but to be safe, you should hire a licensed technician to handle these chemicals.

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