Browntail Moth Update #5: May 19, 2023
Very late last week, after our monitoring site visits were complete, we received a report of some browntail caterpillars that had developed to the fourth instar (life stage), which we confirmed at most of our sites this week. Fourth instar and older caterpillars have white markings on the sides of each body segment and have more of the irritating hairs that cause human health impacts. This is a tipping point for this species in terms of the number of irritating hairs as well as the growing appetites of the caterpillars. Feeding damage is apparent on some branches, and over the next couple of weeks, as the caterpillars grow, they will defoliate host plants from the top down, seemingly overnight.
Comparison of 3rd (Top) and 4th (Bottom) instar caterpillars. Note the white tufts on each side of the body segments and the more abundant hairs.
As mentioned in previous updates, our native eastern tent caterpillar (ETC) webs can often be confused with browntail caterpillar webs. At this time of the year, ETC web development has surpassed browntail webs in size, reaching the size of a football in many areas around Maine (see photo below).
Native eastern tent caterpillar webs are now much larger than browntail caterpillar webs in many areas; also, note the web’s position on the host plant and appearance of the caterpillars, Unity ME.
Windy weather and the fact that the caterpillars are growing larger means that caterpillars may soon be found crawling on the sides of houses, decks and around your dooryard. This means that there may be an increased chance of people coming into contact with the caterpillars, so caution should be exercised in areas of high infestations. These wandering caterpillars are also quite capable hitchhikers, and care should be taken not to spread them to other areas. Look for an upcoming update for more information on browntail hitchhikers.
Fourth instar caterpillar crawling on the railing of a deck, Dresden, ME
If you wish to remove caterpillars from decks, buildings, driveways and other surfaces, do it in a way that will not increase your risk of exposure to their hairs (do not use leaf blowers! People get rashes this way because it stirs up the hairs). You can dislodge them from areas out of reach with a strong jet of water. To remove them more permanently, add a couple of inches of soapy water to the canister of a wet/dry vacuum, vacuum them up, and allow them to sit for a day or two, until the caterpillars have drowned. Then safely dispose of the contents (bear in mind, the toxin in the hairs is stable and may still cause irritation).
Fourth instar caterpillars huddled on their web. Note the feeding damage in the upper part of this photo, Brunswick, ME.
Plans for management through insecticide treatment, and some treatments, should already be made. We recommend working with a licensed pesticide applicator for insecticide treatments. In most years, treatments should be completed before the end of May to limit the buildup of toxic caterpillar hairs and host foliage loss.
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