The Return of the Browntail Caterpillars
It may have been a few weeks since you've seen browntail caterpillars, and it may be soon that you see them again. In early August, adult female browntail moths are laying eggs of the next generation of caterpillars.
These egg masses will begin hatching in August and small caterpillars will begin feeding on only the outer surface of the leaves (not the whole leaf) of their favorite trees until the end of September. This type of feeding is often called "skeletonizing", because only the structural "bones" of the leaf are left behind; this eventually causes the leaf to die and turn a bronze-copper color. Damage this time of year is generally not a serious health issue for the plants.
In addition to feeding, these young caterpillars will begin constructing their winter web to use as shelter in preparation for the winter season.
Learning when certain life stages of browntail moth are active is important in understanding what to look for on your property.
What does a browntail egg mass look like?
The first step of prevention is recognition. Browntail egg masses are:
- 1-2 inches in length.
- Brown and fuzzy (may have toxic hairs on the outside of the mass). The adult moths themselves do not have toxic hairs, however, sometimes, the moth may pick up toxic hairs from its time as a caterpillar and the hairs may be deposited on the egg mass as it is being laid.
- Often found on the underside of leaves of host trees (oak, apple, crabapple, pear, birch, cherry, and other hardwoods).
If egg masses are found, remove the affected leaves with gloved hands and soak the eggs in soapy water for a few days and dispose of them in the trash.
Browntail moth egg mass on a host leaf in Old Town, ME. During August, young caterpillars will emerge from this mass and begin skeletonizing leaves.
Small caterpillars in August: Browntail or Fall Webworm?
Our native fall webworm (Hyphantria cunea) is a moth that has a caterpillar form that is easily mistaken for the browntail moth caterpillar. Both insects are active in August and these small caterpillars can be found eating the same host plants and trees, and creating filmy silken webs. So how can you tell the difference?
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(Carefully) Check the web! Oftentimes, fall webworm webs will be larger than browntail webs during late summer.
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Are there silk 'highways'? In late summer, browntail caterpillars will build white silk highways to help them travel in their host trees. In trees with fall webworm, although there is silk, they do not create readily observed trails.
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Look at the caterpillars! If you can safely reach the area where the caterpillars are, check for two dark spots toward the head of the caterpillar, if present, these are browntail caterpillars. If not present, they are fall webworm caterpillars. Young browntail caterpillars also tend to be smaller (about a quarter of an inch) compared to fall webworm (often longer than a quarter of an inch).
Young browntail caterpillars have two dark spots near their head. Note the skeletonization of the leaf in areas without the typical green underside.
Young fall webworm caterpillars feeding and creating a web in Old Town, ME. Note the lack of two dark spots near their head which identifies them as fall webworm and not browntail moth.
If you are still struggling to identify the caterpillars, you can wait until later this fall, when the small winter webs of the browntail moth are formed. At that time, the silk highways engineered by browntail caterpillars (see photo below) are well developed and the caterpillars will wrap one or more leaves together to create their recognizable winter web. Webs of fall webworm will break down through fall and winter, as they are no longer in use by their creators.
In late fall, browntail caterpillars create silk 'highways' that let them travel in their host trees. Webs of fall webworm will break down through fall and winter and will not be as visible as browntail silk structures.
Exposure to these young browntail caterpillars are unlikely to cause a reaction in most people, however, personal protection is still recommended. The toxic hairs from the previous seasons' caterpillars are still stable in the environment, therefore, caution should be taken to avoid stirring up the hairs that have settled in areas near past infestations.
Should I treat the late summer browntail caterpillars?
Using foliar or injectable pesticides to target the young caterpillars in late summer is very difficult and is generally not recommended by Maine Forest Service:
- The skeletonizing of the leaves from the young caterpillars can be hard to see. When applying pesticides, label directions will instruct you to confirm the presence of the pest before application. Do not treat uninfested trees.
- Browntail caterpillars produce silk and feed underneath it, therefore, pesticides sprayed on leaves or other non-systemic pesticides may not be as effective.
- Trees in late summer may be less effective at absorbing and spreading injected pesticides throughout the tree's leaves and therefore may not be as effective.
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