For Release August 6, 2006

Fall Webworms



     by Chuck Otte, Geary County Extension Agent

            I’ve been expecting them to show up any day, and in recent weeks they’ve just started popping up everywhere! The Fall Webworm is often, incorrectly, called bagworm because the colony of caterpillars makes a webby “bag” around leaves at the ends of branches on deciduous trees. Bagworms are the small caterpillars that live, singly, in a small bag that they construct out of leaves from their host plant, usually cedars or junipers.

            The fall webworm eggs are all laid at the same location and the small caterpillars all hatch out at the same time. The small caterpillars construct a web mass at the ends of a branch and enclose leaves as food. Inside the relative safety of this web mass, the caterpillars eat and grow. When all the leaves within the web have been consumed they extend the web mass to enclose more and more leaves.

            We have two species of fall webworm in Kansas; the blackheaded race and the redheaded race. Both overwinter as a pupa (cocoon) on the ground underneath trees. The black-headed race emerges first, about mid-May and begin laying eggs. The red-headed race emerges about a month later. The first generation webworm broods are easily overlooked as they are constructing webs, and feeding, as the trees are putting on more and more new growth. But later in the season, as there become more broods, the webworms grow larger and the trees stop putting on new growth, these web masses can become very apparent and very disconcerting. When the caterpillars are ready to pupate, they rappel out of the tree to the ground on a silken thread.

            As homeowners start looking into these web masses they notice that all the leaves appear dead. The leaves have either had all the green material eaten off them or the leaves are totally missing. The branch ends appear dead. The normal reaction for most homeowners is to cut off the end of the branch and throw the entire web mass away. Other homeowners may take the rather drastic, but somewhat satisfying, approach of burning off the web mass and any remaining caterpillars with a torch.

            The truth of the matter is that while the web mass and feeding damage are aesthetically unpleasing, the damage is quite minimal. While the web masses are unsightly, the total leaf area consumed is very small. Even if there are a lot of web masses in a tree, it is too late in the season to cause a loss in carbohydrate production. By early August, the new leaf and flower buds are already developed and the tree has pretty well stored up all the food reserves that it needs for the winter and early spring growth. In fact, trying to cut out, or burn, the web masses can result in more damage and disfiguration to the tree than the caterpillars ever would.

            If you just have to do something, you can use almost any lawn or garden chemical to spray them. First make sure that there are still caterpillars in the web mass. The web mass will persist well into the winter so the mere presence of the web mass doesn’t mean that there are still caterpillars present. If there are caterpillars present, then either tear the web mass open with a pole or stick, or use a high pressure sprayer to penetrate the web mass and ensure that the pesticide spray contacts the caterpillars. Ultimately, spraying will only serve to make you feel better. If it was my tree, I would not waste the time and effort in spraying it. Summer is short, there are many better things to spend you time doing!


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