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The Homeowners Column
What are bagworms?
State Master Gardener Coordinator
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 thirty 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, arborvitae or douglasfir. Branches of evergreens that have been stripped bare of needles may die. Bagworms may also be found on deciduous trees such as honey locust and oak but do little damage compared to evergreens. These bagworms feed all summer starting mid-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-late August when they are about 1-1/2 inches long. Once they quit feeding, spraying with insecticides is useless. By the time we get into August and September it is too late to spray. That's when you introduce your family to the exciting new dance "The Bag Picking Boogie." Best time to spray with Btk products (Bacillus thuringiensis var. kurstaki) sold as Dipel or Thuricide is when the caterpillars are young in late June.
Some people also refer to eastern tent caterpillar and fall webworm as bagworms. Eastern tent caterpillar occurs early in the season usually in late April. The larvae 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) 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 and rarely feeds on evergreens.
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 Bt products are 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.
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. For obvious reasons do not use the commonly listed solution of fire to destroy the webs. More harm will be done to the tree from fire than from the caterpillar feeding. Although it is aesthetically unpleasing, relax late season defoliation of deciduous trees as with fall webworm is generally not life threatening for the tree.