HILLSBORO, Ill. – Timing is important when it comes to treatment of caterpillar pests in trees. The earlier you find these insects, the easier it is to control them. Summer is a nice time to take a cruise in the car and you may notice some webs in the roadside trees. Most noticeable in August and September, the fall webworm will build protective nests that usually start on the ends of branches, unlike tent caterpillars which build nests in branch crotches.
The fall webworm nests increase in size as the caterpillars feed. The webs in heavily infested trees can expand to up to three feet long. Fall webworm feed on approximately 120 species of deciduous trees, with mulberry, maple, crabapples, birch, chokecherry and walnut being most susceptible. The caterpillars do not leave their protective nests until they are ready to pupate, which can make management difficult. Fortunately, the timing of the feeding of the fall webworm only leads to cosmetic issues for the tree and not a serious threat of the overall health of the tree, and management is often not warranted.
Bagworms are another insect pest where early detection is critical for chemical control. Bagworms are caterpillars that feed all summer. Usually they begin feeding in mid to late June. By the time August arrives, chemical control for bagworms is usually too late. Handpicking is another control method that can reduce the population for next year. The eggs of bagworms will overwinter in the bags. So if you don’t get them now make sure you do before the eggs start hatching. Be sure to remove the bags from the area or destroy them. Just throwing them on the ground is not enough.
Eastern tent caterpillars are sometimes confused with fall webworms but they occur in the spring. Eastern tent caterpillars, like fall webworm, also inhabit webs, but more appropriately the webs are referred to as tents. These webs are found in the fork of a tree in the early season, usually in late April. The nests have the appearance of a tent which they use for protection during cloudy or rainy days or at night. The tent enlarges as they feed on the leaves of trees. Tent caterpillars feed on crabapple, hawthorn, mountain ash, flowering cherry, and other trees and shrubs in the rose family.
When is it too late to treat? The longer you wait, the less your chance of adequate control. Bacilius thuringiensus var. kurstaki sold as Dipel or Thuricide is effective especially on young caterpillars. There are many other products available to use but be sure to read the label before use. High-pressure sprays can help to break up the nests of fall webworm and tent caterpillar. The webs are water resistant so a gentle spray will not penetrate the surface. The webs may need to be opened up in order for chemical sprays to be effective.
By the time the nest has expanded to feed on more foliage, the spray may no longer be effective, especially in the case of Bt. Removal of nests for both fall webworm and tent caterpillar can also reduce populations.
Contact your local Extension office for further recommendations and resources.
Andrew Holsinger is a University of Illinois Extension educator, horticulture.