The brown tail moth (Euproctis chrysorrhoea) is an invasive forest pest causing significant concern in North America, particularly across New England and Maritime Canada. This insect damages trees through defoliation and poses a direct public health hazard due to its unique larval defense mechanism. Caterpillars spend approximately nine months overwintering in communal nests, making early intervention the most effective strategy for population control. Managing an infestation requires specific, seasonal actions, combining mechanical removal with targeted treatments.
Identifying the Brown Tail Moth and Health Risks
Accurate identification is necessary to distinguish the brown tail moth (BTM) from other common caterpillars like the eastern tent caterpillar or fall webworm. The BTM caterpillar grows to about 1.5 inches, exhibiting a dark brown body with a broken white stripe running along each side. Its most distinguishing features are two conspicuous, reddish-orange spots located on its hind end, visible on the final larval segments.
The winter web, which houses the hibernating larvae, is typically a tightly woven silk structure, 2 to 5 inches long, found firmly attached to the tips of host tree branches, often incorporating a single leaf. Unlike the loose, messy tents of other species, the BTM web is dense and securely anchored, frequently appearing on oak, apple, cherry, and birch trees.
The primary danger to human health comes from the microscopic, barbed hairs, known as setae, shed by the caterpillars from their third larval stage onward. These tiny hairs contain a toxin that causes a severe, itchy skin rash known as dermatitis, similar to a reaction from poison ivy. Hairs can become airborne or embedded in the environment and remain toxic for up to three years, presenting a risk even after the caterpillars are gone. Inhaling these hairs can also lead to respiratory distress.
Manual Removal of Winter Webs
Manual removal of the winter webs is the safest and most effective initial control method, as the caterpillars are dormant inside the silk nests. This should be performed during the dormant season (October through mid-April), before the larvae emerge to feed on new foliage. Targeting the webs during this period prevents hundreds of caterpillars (each nest containing 25 to 400 larvae) from emerging in the spring.
To remove the webs, use long-handled pole pruners or hand snips, which allow you to cut the small branch or twig just below the web. It is only necessary to remove the web itself, not a large section of the limb, to minimize stress on the tree. Always ensure you are working safely from the ground and not attempting to remove webs near power lines, which requires a licensed arborist.
Once the webs are clipped, they must be immediately collected and destroyed to ensure the hibernating caterpillars are killed and the toxic hairs are contained. The recommended disposal method is to place the webs into a bucket of water mixed with dish soap. The webs should soak for three to five days before disposal in the trash. Where local ordinances permit, incineration in a contained fire is an alternative, but this must be done with caution to avoid releasing hairs into the air.
Targeted Chemical and Biological Control Methods
For large infestations, trees that are too tall for safe manual pruning, or properties with high numbers of inaccessible webs, a targeted application of a control agent becomes necessary. Treatment must be precisely timed to target the young caterpillars when they are actively feeding (mid-April until late May). Treatment after the end of May is significantly less effective and will not mitigate the public health risk, as the caterpillars will have developed their full complement of toxic hairs.
A preferred biological control agent is Bacillus thuringiensis kurstaki (Bt), a naturally occurring bacterium. Bt is a stomach poison, toxic only to caterpillars that ingest it while actively feeding on treated foliage. It is considered a biorational choice because it is harmless to mammals and other beneficial insects, as its toxin requires the highly alkaline gut environment unique to certain caterpillars to become active.
Chemical controls, such as those containing pyrethrum, permethrin, or spinosad, offer another option for managing active larval populations. These applications should be limited to the early spring feeding period to ensure maximum efficacy. For any application on large trees or near sensitive areas, it is recommended to hire a licensed commercial applicator or arborist. Professionals possess the specialized equipment and knowledge to ensure the product is applied correctly, following state regulations, especially restrictions for pesticide use within 250 feet of marine waters.
Safety Precautions and Handling Infested Areas
Protecting yourself and others is paramount during any interaction with brown tail moth caterpillars, webs, or contaminated debris. When performing manual removal or working in an infested area, always wear extensive personal protective equipment (PPE). This includes long pants, a long-sleeved shirt, gloves, and eye protection, but should also feature a disposable suit cinched at the wrists and ankles, and a respirator or filtered mask. Working on damp or wet days is advisable, as moisture helps keep the microscopic hairs from becoming airborne.
After completing work, all protective gear must be handled with care to prevent contaminating indoor spaces. Immediately remove the outer layer of clothing, and wash all exposed clothes separately from other laundry in hot water. Tools and exposed equipment should be hosed down to remove any clinging hairs.
If caterpillars are found on structures or pavement, they should be sprayed down with a hose to limit airborne exposure, then vacuumed up using a wet/dry vacuum equipped with a HEPA filter and soapy water in the tank. In the event of contact resulting in a rash, the affected area should be washed with mild soap and cool water, as hot water can aggravate the irritation. Over-the-counter remedies like hydrocortisone creams, calamine lotion, and oral non-sedating antihistamines can help manage the symptoms, but a medical professional should be consulted if respiratory symptoms develop or the rash is severe.