Got Webworms in Your Trees?

It is mid-August, and many honeylocust trees (young and old) are covered in brown leaves that are matted together with webs at the tips of branches. The mild winter temperatures this year have led to a large outbreak of mimosa webworm on thornless honeylocust that are planted as street, park, and landscape trees throughout Pennsylvania. Mimosa webworms are typically found in southern states, but they are surviving our more recent warmer winters, especially in urban areas, which tend to be warmer. 

Mimosa webworms are small caterpillars that spin their cocoons between the folded leaves that they have webbed together. Fully grown caterpillars are about 1 inch long and grayish brown in color with five lime green stripes running down the body. They have a dark head, and the body is sometimes tinged with pink. A second-generation caterpillar leaves the tree canopy to find a site where it can spend the winter in a cocoon protected from cold temperatures. The cocoons are white and look like puffed rice.  

There are two key questions:

• How much harm are these caterpillars causing?
• Do they need to be treated?

Mimosa webworm, like fall webworm, found on hickories and other native trees, is a late-season defoliator that typically does not cause major harm to honeylocust. Healthy trees have already set buds for next year, and losing foliage a month before the leaves naturally fall off is not a major health issue for the trees. The webs and browned foliage are unsightly, but the damage is more aesthetic than serious.

Honeylocust trees can be treated for mimosa webworm if they are discovered early enough (mid-June), before the leaves are brown and webs formed in August. The webworms are caterpillars, in the Lepidoptera order of insects, and can be treated with organically registered pesticides that contain Bacillus thuringiensis (Bt), a naturally occurring bacterium that kills caterpillars and is non-toxic to mammals (humans). Other biorational pesticides such as spinosad (Conserve), azadirachtin, and chlorantraniliprol (Acelepryn) can also be used to treat mimosa webworm. Purdue University Extension has an excellent factsheet on mimosa webworm that contains more information about treatment options.

At this point, it is probably best to just keep your honeylocust healthy by watering them through this hot and dry summer, mulching properly, and scouting for webworm cocoons (small white puffed rice-shaped) at the base of trees and on the trunks, and prepare to manage them next year if necessary.