Browntail Moth Update #9: June 14, 2024
This week at our monitoring sites, we observed pupating caterpillars, pupation is the life stage between caterpillar and adult moth. There were still a handful of caterpillars active and feeding on host foliage at most sites. In preparation for the next life stage, caterpillars will silk together leaves, creating a packet.
There can be more than one caterpillar in each pupal packet with some having 5 or more individuals inside. Caterpillars will pupate in many different locations including the foliage of host plants, sheltered areas on building such as roof overhangs, siding and decks but also vehicles, where they pose a risk of hitchhiking to new areas. To prevent spreading browntail to new areas, check these areas for caterpillars and cocoons that may be trying to hitch a ride.
The silky cocoons/packets surrounding the pupae contain the last cast skin of the caterpillar and are full of toxic hairs. People in areas with browntail moth populations should take precautions to minimize exposure to the caterpillars’ hairs. Use protective measures to avoid skin contact with the toxic hairs when carefully removing caterpillars or cocoons. Eliminate caterpillars and cocoons by moving them to a plastic bag or container, then freeze them or soak them in a bucket of soapy water before disposing of them in the trash.
Browntail moth caterpillars constructing a pupal packet on host foliage (left) and a completed packet (right).
Pupation means we will soon see browntail moth flights. Nearly every year, we receive the first reports of adult moths sometime during the last week of June or first week of July. Usually, the peak of browntail moth adult activity is somewhere in mid-July and the last flights are toward the end of July and early August. Although some moths stick close to where they developed, others can travel long distances.
To reduce the attractiveness of your landscape to these travelers, limit outdoor lighting in July. If you can’t eliminate lighting, consider switching to yellow-spectrum lights. Research by the Mech Lab at the University of Maine indicates that these yellow-colored lights are less attractive to browntail moth. Preliminary results from the lab also indicate that higher density browntail caterpillar populations develop near lights. Limit the lights to have a better spring next year.
We continue to see fungal and viral mortality at some of our monitoring sites and other areas including Bradley, Brunswick, Old Town, Richmond, Dresden, and Turner. Through public reports, we have also been able to confirm diseased caterpillars in Belgrade, Durham, Freeport, Leeds, Swanville and Waterville. Although we are seeing some diseased caterpillars, it should not be assumed this will lead to a widespread die-off of browntail caterpillars. More typically, the fungus or virus may be able to spread in localized areas, like the population on a tree or a cluster of trees.
Some signs of pathogens may be:
- White or yellow fungal spores on caterpillars (Fungus)
- Caterpillars hanging onto branches with just their rear prolegs (leg-like extensions of the exoskeleton) (Fungus)
- Liquid expulsion of the caterpillar guts (often the caterpillar rests in an upside-down "V" shape) (Virus)
- Swollen, puffy caterpillars (Virus or Fungus)
- Dead caterpillars (Virus or Fungus)
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.
Fungal-killed caterpillars observed in, Brunswick ME.
Predatory stink bugs, spiders, hornets, ground beetles and other insects have been observed feeding on browntail moth caterpillars. This week, we observed parasitic wasps probing pupal cocoons some of our monitoring sites. Birds may also eat browntail caterpillars, however, many birds will avoid them because of browntail's irritating hairs. Although some of our native insects and birds can eat browntail caterpillars, they are not effective at controlling the population at high densities.
A parasitic wasp in the genus Pimpla was seen investigating browntail pupal cocoons, Bridgton ME.
Check out more helpful advice on management solutions for browntail on our frequently asked questions page:
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