The brown tail moth, Euproctis chrysorrhoea, is a species whose larval stage poses a public health hazard due to its microscopic, barbed hairs. These caterpillars are covered with urticating hairs that contain a toxin, which causes a persistent, intensely itchy rash upon contact with human skin. Effective management depends entirely on correctly identifying the pest and precisely timing removal or treatment efforts to coincide with vulnerable stages of its annual life cycle.
Identifying the Problem and Life Cycle Timing
The brown tail moth completes one generation each year, spending approximately nine months in the larval stage. Eggs are laid in late summer (July and August), and the small caterpillars hatch soon after to feed before constructing their communal winter nests. These larvae are dark-bodied with a broken white stripe along each side, covered in brownish hairs, and feature two distinctive red spots near their tail end.
The most telling sign of an infestation is the winter web, or hibernaculum, which shelters the overwintering caterpillars. These nests are tightly woven, palm-sized structures found at the tips of branches, particularly on host plants like oak, apple, and cherry. The web is composed of dense silk spun around a few leaves and is often bright white, making it easier to spot after the tree’s foliage has dropped (October through mid-April). The caterpillars emerge from these nests in mid-April to begin actively feeding on new spring leaves, continuing to grow until they pupate in June.
Non-Chemical Removal of Winter Nests
The safest and most effective non-chemical control method is the mechanical removal of the winter nests while the larvae are dormant. This window spans from late fall, after leaves have dropped, through mid-April, before the caterpillars emerge to feed. Pruning the branches with the webs during this time eliminates hundreds of caterpillars before they can become active and spread their toxic hairs.
To perform the removal, use a pole pruner or hand snips to cut the branch tip just below the tightly bound silk nest. It is important to remove only the nest itself rather than an entire limb, which can stress the tree unnecessarily. Even though the risk of hair exposure is lower during the winter, you must wear protective clothing, including gloves, eye protection, and long sleeves, as old hairs and cast skins may still be present.
Once the nests are clipped, they must be immediately collected; dropping them on the ground allows the caterpillars to emerge in the spring. The collected webs must be destroyed before disposal. This is best accomplished by soaking the nests in a bucket of water mixed with dish soap for three to five days. Alternatively, the nests can be burned in a contained fire, provided local regulations and permits allow for it.
Targeted Chemical and Biological Treatments
When winter nests are too high to reach safely or an infestation is widespread, a targeted application of a biological insecticide is often necessary. The preferred biological treatment is a product containing Bacillus thuringiensis kurstaki, or Btk, a naturally occurring soil bacterium. Btk is a stomach poison that is highly selective, only affecting the larvae of moths and butterflies that ingest it.
For Btk to be effective, it must be applied when the caterpillars are small and actively feeding, which typically occurs from mid-April through the end of May. The tiny larvae must consume the treated foliage for the bacteria’s toxin to become active in their alkaline digestive tract. Applying Btk later in the season, after the caterpillars have grown larger, is less effective and will not reduce the spread of toxic hairs already shed.
For very high trees or severe infestations, synthetic broad-spectrum insecticides like pyrethroids may be used, although they are applied by a licensed professional. These chemical treatments are also subject to strict timing requirements, needing to be applied before the end of May to prevent the spread of hairs.
Protecting Yourself from Toxic Hairs
The microscopic barbed hairs are the main public health concern, causing a painful, poison ivy-like rash. These hairs are most prevalent from May through July when the caterpillars are active, but they remain toxic and can become airborne for up to three years, even after the caterpillars are gone. Hairs can be stirred up by activities like mowing the lawn, raking leaves, or gardening in an infested area.
Personal Protection Measures
When working near infested trees or performing yard work in an area with a history of brown tail moth presence, wear personal protective equipment. This includes a respirator or mask, goggles, gloves, and clothing that covers all exposed skin, such as long sleeves and pants. After working outdoors, take a cool shower and wash your clothing in hot water to remove any clinging hairs.
Rash Treatment and Relief
For immediate relief from a rash, the first step is to wash the affected area with soap and water to remove any remaining hairs. Symptoms can often be managed with readily available over-the-counter products. These include topical cortisone creams, calamine lotion, and oral antihistamines. If the rash is severe, covers a large portion of the body, or if any respiratory symptoms develop, consult a healthcare provider.