Issue 14, September 8, 2014

Fall Webworm

Fall webworm is numerous throughout the state, particularly in the southern half. It lives as a group of caterpillars that spin a communal silk web. This silk nest typically encloses the end of the branch and associated leaves. The caterpillars remain in the webbing, feeding on these enclosed leaves. When the leaves inside the web are eaten, the silk webbing is expanded to include more leaves. Webs of mature caterpillars are typically 2 to 3 feet long. Multiple colonies frequently occur on a single tree, so many branches can be involved. Occasionally, entirely webbed trees are found.

Fall webworm damage to crabapple.

The caterpillars are yellowish and hairy. There are two races of fall webworm. The redheaded race has a red head and a yellowish body. The blackheaded race has a black head and a yellowish body with many black spots and a wide black stripe running down the back.

This insect has an extremely large host range, being found on almost any deciduous tree and some shrubs. It is most commonly found in Illinois landscapes on crabapple, walnut, hickory, pecan, redbud, sweet gum, maple, and oak. There does not appear to be any separation of hosts based on the races of fall webworm.

In the southern half of Illinois, fall webworm has two generations per year. The first generation typically occurs in June, with the second generation in August and September. In the northern half of the state, only the August and September generation occurs.

Only the spring generation of these caterpillars is considered to be important to the health of the tree. The generation that occurs in August and September eats leaves that have already produced most of the energy that they will for the tree. As a result, the loss of those leaves is not a major problem to the plant. However, if the tree responds to the loss of these leaves by breaking buds and growing new leaves, then there is a health impact. Usually, this doesn't happen.

Pruning off the branch with its webbing and disposing of it is an effective control. Many insecticides are effective in controlling fall webworm. Bacillus thuringiensis kurstaki (Dipel, Thuricide), carbaryl (Sevin), pyrethroids, and other labeled insecticides are effective. However, the webbing is waterproof, making it spray resistant. Enough spray pressure is needed to break into the web and get the insecticide onto the leaves within the nest. Nest webs are typically expanded only every week or so, so insecticide deposited on leaves outside the webs is likely to break down before the caterpillars expand the webbing over treated leaves. (Phil Nixon)

Phil Nixon

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