Fall Webworms are Coming

Fall webworm (as the name implies) occurs later in the season becoming especially noticeable in August and September. They build large, protective nests (webs) that usually start on the ends of branches, unlike tent caterpillars. Nests increase in size as caterpillars feed. They do not leave the nests until they are ready to pupate. Heavily infested trees can be completely covered with nests up to three feet long enclosing the ends of branches. Fall webworm feeds on over 120 different species of deciduous trees including crabapple, ash, oak, elm, maple, hickory, sweet gum, and black walnut. Fall webworm generally doesn't feed on conifers.

Ok, what's the bottom line here with tent caterpillars and fall webworm? Figure out if the caterpillars are present and still feeding. Get up close and personal. None of these bag ladies and gentlemen will bite, although the fall webworms may make funny jerky motions in perfect rhythm. (Who said disco is dead?) If they are still feeding, insecticide sprays may be useful.

Luckily Bacillus thuringiensis var. kurstaki sold as Dipel or Thuricide is effective especially on young caterpillars. With fall webworm and tent caterpillar use high spray pressures to break up the web and get the insecticide inside to the caterpillars and the leaves. If the caterpillars are not there, then insecticide sprays are useless. At that point just good maintenance, including watering during drought periods, is your best tactic.

Tearing or pruning the webs and their resident caterpillars out of the tree can also control tent caterpillar and fall webworm. Obviously be aware of maintaining the shape and health of the tree by not removing large branches. Although it is aesthetically unpleasing, late season defoliation of deciduous trees as with fall webworm is usually not life threatening for the tree.

For more information, contact your local extension office.