Extension Educator, Horticulture
When is a bag really a tent? And when is a worm really a caterpillar? Intriguing questions, I know. The term bagworm conjures up different images for different people. Many people use the term to describe the fall webworm appearing now in a tree near you.
Fall webworm as the name implies usually occur late in the season, becoming especially noticeable in August and September. Although here in central Illinois and in southern Illinois, there are two generations of fall webworm. The first generation usually appears in late June. What we see now is the second generation which tends to be more numerous.
The caterpillars are pale green to yellow in color, may have black spots, and are covered with long, white hairs. Adults are two-inch-wide white moths with brown wing spots. Fall webworm caterpillars build large, protective nests (webs) that usually start on the ends of branches. The webs keep hungry birds from devouring the caterpillars. In April we see a similar web building caterpillar, the tent caterpillar, but the web starts in the crotches of trees. Fall webworm nests increase in size as caterpillars feed. The caterpillars do not leave the nests until they are ready to pupate. Fall webworm overwinters as pupae in loosely webbed cocoons. Heavily infested trees can be completely covered with nests up tothree feet long. Fall webworm feeds on over 120 different species of deciduous trees including crabapple, ash, oak, elm, maple, hickory, sweet gum, and black walnut. They generally don't feed on conifers.
What's the bottom line here with fall webworm? Although it doesn't look great, late season defoliation of deciduous trees as with fall webworm or other caterpillars is generally not life threatening especially on mature trees. So try to ignore them. Control is more critical on first generation webworm that appear on young trees in late June.
If the webs are within reach, then they can be torn or pruned out. Trim branches to maintain the aesthetics of the tree. Do not try the cigarette lighter method. You may get some primal satisfaction out of defeating the enemy with a flaming inferno, but it's not healthy for the tree.
If the caterpillars are still feeding, then a pesticide spray is another option. Get up close and personal with the web. These bag ladies and gentlemen don't bite, although they may make funny jerky motions in perfect rhythm. (Who said disco is dead?) If they are still feeding, insecticide sprays may be useful. Bacillus thuringiensis var. kurstaki sold as Btk in Dipel or Thuricide is effective especially on young caterpillars. 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.
What most horticulturists call bagworms are also quite visible now as dangling brown ornaments from the bare branches of evergreens such as eastern red cedar, junipers, spruce or arborvitae. Bagworms left unchecked can devastate evergreens. The bagworms seen now have been feeding all summer, since mid to late June. Bagworm caterpillars construct individual silk cases covered with bits of leaves from their last meal. An actively feeding caterpillar will have green leaves at the top of its bag.
Usually bagworms quit feeding and pupate in mid- to late-August when they are about 1-1/2 inches long. Once they quit feeding, spraying with insecticides is useless. More than likely it is too late to spray this year. That's when you introduce your family to the exciting new reality show of "Let's see how many bags we can pick off our trees." Best time to spray with Btk for bagworms on evergreens is in late June when caterpillars are young.