Tent-Making Caterpillars: Friends or Foes?
Browntail Moth (Euproctis chrysorrhoea)
Maine is home to many different tent-building caterpillars—some native, like the Eastern tent or forest tent caterpillars, or non-native or introduced species, like the spongey and browntail moths. Even though all these species can defoliate trees, none of them regularly kill healthy specimens. Unfortunately, one of these species, the browntail moth, has specialized hairs that cause itching and can be hazardous to human health.
Browntail moth webs look quite different from some of Maine’s native species and can often be mistaken for leaves stuck to the end of branches that persist through the winter. Eastern tent caterpillars are far more likely to make their nests in the crooks of branches, and their tents break down in late summer. Forest tent caterpillars do not make webs. Identifying browntail moth caterpillars is easy: browntail moth caterpillars have a two orange dots on their backs. Adult moths are white with brown tufted abdomens and typically fly in July and August.
Origin and Transport
Originally from Eurasia, browntail moths are a non-native species. These moths were first introduced in the 1890s to the mid-Atlantic region of the United States, most likely through egg masses on lumber products.
Populations go through a boom-and-bust cycle, maintaining stable populations in coastal Maine and Massachusetts, and spreading when conditions are right. Browntail moths have been a larger problem since 2015, and, with the effects of climate change, populations may have the opportunity to expand their range. Although they do cause harm, browntail moths are considered semi-naturalized and have a few predators that help to control their populations. Over time it is likely that the number of predators will expand, providing a much-needed check to their overpopulation.
Webs and Removal
Browntail moth webs are primarily found on broadleaf trees: oaks, beeches, birches, and plants in the rose family (cherry, plum, crabapple, serviceberry). Browntail moth caterpillars build nests at the tips of branches that can often be spotted in late autumn, winter, and early spring. These nests often appear as lone leaves still attached to the tree. Nests are best physically removed from the trees whenever possible, although some systemic insecticides can be used.
There are several methods used to destroy the nests after they are cut out by hand. Many people opt to burn them, but this can release the itchy hairs into the air and can cause intense respiratory issues. A more lung-friendly approach is to drown the nests in soapy water, which ensures the hairs are not released into the air. If you are dealing with a major multi-year infestation, it is best to call one of the many arborists across their range that deal with browntail moths. Fortunately, this species rarely impacts a single area for more than a year or two at a time due to their boom-and-bust lifecycle.
Here at the Gardens, we focus on using integrated pest management (IPM) to help control our browntail moth populations. We rely on monitoring our known problem areas and physically removing winter webs for destruction.
While there are many species that share our woods, only a few are out of balance with the rest of our environment. If you are stewarding land, make sure the nests you remove belong to browntails and not one of our native species.
Hazards and Treatment
Browntail moth caterpillars and nests are covered in urticating hairs— extremely itchy hairs that cause both a physical reaction as well as a chemical one. Similar, but nontoxic, hairs are used by many species of North and South American tarantulas as their primary form of defense. These hairs cause a red itchy rash with many pinpricks that can look much like poison ivy.
While your best bet is trying to avoid browntail moth rash altogether, treatment for browntail can be simple. When working outside in an area known to have browntail moths, it is best to work after rain when the hairs are significantly less effective and to wear long pants and sleeves. If you do get the hairs on you, they can be removed using tape or a sticker, but be careful not to rub or itch as that can drive the hairs deeper. Many local pharmacies in downeast and midcoast Maine carry a salve that can help if you do encounter the hairs. You can also make your own using the following recipe from Maine General Health.
Browntail Moth Rash Treatment- Maine General recipe
- 1/2 30 gm tube of hydrocortisone 1% cream (e.g., Cortizone-10)
- 1/2 30 gm tube of diphenhydramine hydrochloride 2% and zinc acetate 0.1% cream (e.g., Extra Strength BENADRYL®)
- 1/4 30 gm tube lidocaine 4% cream (e.g., Aspercreme®)
- 1/4 cup witch hazel solution
Refrigerate: good for 30 days (about 4 and a half weeks)
For additional information on the browntail moth risk, identification, and removal, visit the State of Maine’s website at https://www.maine.gov/dacf/knockoutbtm.
Look-Alikes
Forest Tent Caterpillars (Malacosoma disstria)
Forest tent caterpillars are a native species that can occasionally rise in population to destructive levels. The tents from this species are most prevalent in the forks of trees and appear in the late summer and fall. Fully grown caterpillars are roughly 2 inches long and have hairs along the length of their body and a row of white dots along their back. Caterpillars will emerge from egg masses in spring when they feed on aspen, birch, and other broadleaf trees.
Eastern Tent Caterpillars (Malacosoma americanum)
This native species can have outbreaks with primarily aesthetic effects every 8-10 years. Eastern tent caterpillars build nests in the forks of branches but, unlike the closely related forest tent caterpillar, these nests show up in the spring and early summer. Young caterpillars have paired yellow lines down their back while older caterpillars have a white stripe down the middle with a yellow line on each side. Adult moths are fuzzy tan and have two white stripes across the wings. Egg masses can look much like black knot fungus but have a smooth and shiny appearance. Host plants are rosaceous tress including apples, cherries, and crabapples.
Fall Webworms (Hyphantria cunea)
Native to Maine and much of North America, webs of the fall webworm moth are loosely constructed and include branches. Unlike the other species on this list, the caterpillars feed from within the web. Webs will primarily be seen in late summer and fall. Caterpillars are either yellow with dark stripes or dull black and are very hairy. Egg clusters are covered in hairs. Preferred host trees include walnut, elm, hickory, and maples.
Spongey Moths (Lymantria dispar dispar)
Spongey moths are an introduced, or adventive, species that do not cause human harm but can be a regular defoliator of broadleaf trees. These moths have a classic boom and bust cycle where their population regularly dips and spikes. Healthy trees can survive a few seasons of defoliation, so generally this species does not cause lasting harm to an area. If the outbreak persists, however, there can be long term consequences.
More Information from Maine’s Department of Agriculture, Conservation and Forestry
For information and seasonal updates on the browntail moth, check out Maine’s Department of Agriculture, Conservation and Forestry’s website at http://maine.gov/dacf/knockoutbtm. Their “Knock Out Browntail” campaign features the latest information from the state on the spread of browntail moth as well as best practices for identification and removal for individuals, towns, and businesses.
Sources:
https://extension.psu.edu/eastern-tent-caterpillar
https://extension.unh.edu/resource/fall-webworm-eastern-tent-caterpillar-fact-sheet
https://www.invasivespeciesinfo.gov/terrestrial/invertebrates/spongy-moth
https://extension.umaine.edu/home-and-garden-ipm/fact-sheets/common-name-listing/brown-tail-moth/
https://www.maine.gov/dacf/////mfs/forest_health/documents/browntail_moth_faqs_11172021.pdf
https://www.maine.gov/dacf/mfs/forest_health/invasive_threats/browntail_moth_info.htm
