Extension Educator, Horticulture
People aren't the only creatures to use sleeping bags and tents. Some caterpillars have similar methods to protect them from the elements and from the voracious appetites of birds. But when is a bag really a tent? After fifteen years of helping home gardeners with their questions, I have come to the conclusion that the term bagworm can conjure up different images.
What most horticulturists call bagworms are quite visible now as dangling brown ornaments from the bare branches of evergreens such as eastern red cedar, other 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 for bagworms is in late June when caterpillars are young.
Sometimes eastern tent caterpillar and fall webworm are also called bagworms. Eastern tent caterpillar occurs early in the season usually in late April. Caterpillars gather at a fork of a tree and build a web or "tent," but they leave the web to feed except on cloudy or rainy days and at night. The tent enlarges as the caterpillars eat. Tent caterpillars feed on crabapple, hawthorn, mountain ash, flowering cherry, and other trees and shrubs in the rose family.
Fall webworm as the name implies usually occur 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. 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.
Ok, what's the bottom line here with tent caterpillars and fall webworm? Figure out if the caterpillars are 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, than insecticide sprays are useless. At that point just good tree maintenance including watering during drought periods is recommended.
Tearing or pruning out the webs and their resident caterpillars from the tree can also control tent caterpillar and fall webworm. 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. Although it doesn't look great, late season defoliation of deciduous trees as with fall webworm is usually not life threatening.