Extension Educator, Horticulture
Spring has arrived which means tasty tree leaves for hungry caterpillars. One of the first ones we see are Eastern tent caterpillars. As the name implies they build a thick silken web or tent in the crotches of tree branches. Favorite trees include wild cherry, hawthorn, apple, crabapple and other trees and shrubs in the rose family. They also feed on ash, birch, willow, maple, oak, and poplar. So wander your yard now to scout trees for new residents.
Eastern tent caterpillar use the silken web as a tent. They leave the web to feed on leaves. Like hikers leaving breadcrumbs in the forest, they follow a silken strand back to the tent at night and on cloudy or rainy days. The tent enlarges as the caterpillars eat. Fall webworm, another caterpillar that forms a web, is a little different in that they never leave the web as they feed. Their webs also start at the ends of branches and occur later in the season.
Sometimes people call eastern tent caterpillar bagworms. What most horticulturists call bagworms are quite visible now as individual dangling brown ornaments from the bare branches of evergreens such as eastern red cedar, other junipers, spruce or arborvitae. 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. These bagworms can be picked off now or wait until late June to control the newly emerged caterpillars. The same insecticide for eastern tent caterpillar can be used later for bagworms.
Ok, what's the bottom line here with tent caterpillars. Most of the time the feeding damage is cosmetic but it can weaken young trees or trees under stress for other reasons. The best time to control caterpillars is when they are young. Right now they can be rubbed off with your hand. Ok I know it's gross, so use a glove. Also a hard spray of water can dislodge them when they are young. As the season progresses tearing or pruning out the webs and their resident caterpillars from the tree is effective. 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.
Insecticides can also be useful for control. If necessary use high spray pressures to break up the web and get the insecticide inside to the caterpillars and the leaves.
Sprays of Bacillus thuringiensis kurstaki (sold as Dipel, Thuricide or Caterpillar Attack) are effective when the caterpillars are young. Btk has low toxicity, is specific to caterpillars and does not harm beneficial insects. Once caterpillars are mature (about two inches long), then the insecticides acephate (Orthene), carbaryl (Sevin) or spinosad (Conserve) can used. Research at Purdue University showed that spinosad (Conserve) is very effective at controlling mature caterpillars.
According to Mark Hoard, U of I Extension IPM educator, the University of Kentucky College of Agriculture is currently conducting research into the Mare Reproductive Loss Syndrome (MRLS). Preliminary conclusions suggest that the eastern tent caterpillar is the primary cause or common factor of MRLS. While the results of this research are not conclusive, it is recommended that exposure of horses to the caterpillar and its droppings (frass) should be limited.
Contact U of I Extension, Champaign County for the Fact Sheet "Controlling Tent Caterpillars." PH: 217-333-7672 or firstname.lastname@example.org.