Browntail Moth Update #4: May 22, 2025
The browntail moth caterpillars have put on some noticeable growth since our checks last week and most are now fourth instar. We have not observed heavy feeding damage yet, but we expect feeding to pick up over the next week or so, especially if we have warm and sunny days.
At some of our sites and across much of Maine, our native web-building eastern tent caterpillars (Malacosoma americanum) are making their presence known due to the size of their webs. This species is commonly mistaken for the non-native browntail moth, especially at this time of year.
Some key differences between these species in late May are:
| Browntail Moth Caterpillars |
Eastern Tent Caterpillars |
- Webs tend to stay on the smaller side
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- Webs start small, but grow to the size of a football or larger
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- Webs found on the tips of branches
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- Webs found where branches meet the trunk
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- Caterpillars are dark brown with two red-orange dots on their tail end
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- Caterpillars lack two red-orange dots on tail end
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- Caterpillars have two broken white stripes down body
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- Caterpillars have one connected white stripe down body
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Image: A side-by-side comparison of a browntail moth caterpillar web (left) and an eastern tent caterpillar web (right). Note the difference in size compared to a human hand as well as the placement on the tree, branch tips for browntail and the junction of trunk and branches for eastern tent caterpillar.
We have not observed evidence of any pathogens causing disease in browntail caterpillars at our monitoring sites this year to date. We welcome any reports from the public of diseased caterpillars. Please include a photo or two in your report so we may confirm pathogen presence.
Some signs of pathogens may be:
- White/yellow fungal spores on caterpillars (Fungus)
- Swollen, puffy caterpillars (Virus & Fungus)
- Liquid expulsion of the caterpillar guts, often the caterpillar rests in an upside-down "V" shape (Virus)
- Caterpillars hanging onto branches with just their rear prolegs (leg like extensions of the exoskeleton) (Fungus)
- Dead caterpillars
See the photos below for examples of pathogen killed browntail moth caterpillars.
Image: Examples of pathogen killed caterpillars, note the upside down "V" shape indicative of the virus in the top right photo.
FAQ: I have browntail moth caterpillars crawling all over my house. What can I do?
To remove crawling browntail moth caterpillars, you can dislodge them with a strong jet of water, or fill a wet/dry vacuum with a few inches of soapy water and vacuum them. If using a wet/dry vacuum, make sure you have a HEPA filter installed to prevent the hairs from blowing back out of the unit and let the caterpillars soak for a few days to ensure they die.
Remember to cover up exposed skin when in areas with browntail moth caterpillars to prevent a rash from the stirred hairs in the environment.
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