Browntail Moth Update #3: May 16, 2025
Despite the rain and chilly mornings this past week in Maine, browntail caterpillars are still thriving thanks to the functional structure of their webs. These new thread-like expansions are made from silk, which is water repellent and helps keep the caterpillars dry during our rainy spring seasons.
Browntail caterpillars at some monitoring sites are developing faster than those at other sites. The largest caterpillars were at our monitoring site in Brewer, with a body length of about 0.58 inches (14.89 mm), while caterpillars at other sites were roughly 20% smaller. This pattern has happened in previous years and is likely linked to local weather differences between the sites.
Image: Browntail caterpillars continue growing and expanding their winter webs in Old Town. Note the bud damage from caterpillar feeding.
Some of the larger browntail caterpillars at the Brewer monitoring site had developed broken white stripes down their back, indicative of the caterpillar nearing later instars (life stages). These more mature caterpillars will begin to make excursions from their host trees, leading to more encounters.
Additionally, as caterpillars eat and grow, they need to shed their outer skin, a process called molting. The number of toxic hairs in the environment increases with each caterpillar molt. Please take caution while performing outdoor yardwork near trees that have browntail caterpillars to avoid contracting a rash from the toxic hairs - wear long sleeves, eye protection, and gloves.
Image: Comparison of different browntail moth caterpillar instars. Fourth instar larvae have broken white stripes down their back, whereas third instar and younger do not have these markings.
FAQ: Is it too late to remove browntail winter webs from my trees?
Yes, it is too late to remove browntail winter webs. The caterpillars have already emerged and begun eating foliage, so removing their webs now will not remove most of them and instead poses a risk to you contracting a rash and a risk to the tree from pruning stress.
Instead, consider treatments for trees with browntail caterpillars close to your house or in your dooryard to prevent most impacts. If treatment isn't an option, plan ahead to prevent the rash: avoid the areas when possible and cover up if you can't avoid the area.
FAQ: Is it too late to treat my trees for browntail caterpillars?
No, it is not too late, but time is running out - treat trees with browntail caterpillars before the end of May. Treatments after May are not as effective because the caterpillars will already have molted several times and eaten most (or all) of the leaves, both impacts that treatments aim to avoid.
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