Issue 14, August 26, 2013

Fall Webworm

Fall webworm 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.

The silk webbing shelters the caterpillars from rain and protects them from predators and parasites. It is common to see parasitic wasps trying to get to the caterpillars, only to be prevented from doing so by the silk webbing. Similarly, insectivorous birds are usually unable to pluck many caterpillars out of the webbing.

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.

Clientele find the webs to be unsightly; and as the season progresses, they become only more so. Not only are the webs made larger, but they tend to become littered with the cast skins and fecal pellets from the larvae. For these reasons, control is usually important in landscapes for aesthetic reasons.

Because the caterpillars are in the webbing at all times, pruning off the branch with its webbing and disposing of it is very effective. Another method is to grab onto the webbing and pull it off the branch, bringing almost all of the caterpillars along with it. This allows the branch to remain on the tree, to releaf the following spring. A popular homeowner method is to set fire to the webbing, which does an excellent job of killing and toasting the caterpillars. However, the fire damages the bark of the involved branch and those nearby, which may get cankers or have other resulting problems.

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.

Finally, fall webworm is a native insect that is attacked by several natural enemies. As is typical of native insects, it is very numerous and obvious for about 3 years, followed by several years, usually 5 to 7, where it is low in numbers. Doing nothing to control these insects will not result in overwhelming attack and damage. Their numbers will drop naturally although they are likely to be numerous for the next couple of years before their numbers drop. Control this year is likely to have little effect on how large the infestation is next year as this insect feeds on many forest trees and the adult moths are strong fliers. (Phil Nixon)

Phil Nixon

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