Bagworms are very numerous in central Illinois as well as in other areas of the state. Heavy populations have been noticed on arborvitae, spruce, honey locust, and linden. Although stripped branches of deciduous trees such as honey locust and linden will releaf later this year or next, defoliated branches of arborvitae, spruce, junipers, and other needled evergreens usually die. If enough branches die, the entire tree or shrub may die.
Many of the bagworms being seen are still relatively small, 3/4 to 1 inch long. Although many insecticides are not effective at this size, bagworms should be controllable with sprays of Bacillus thuringiensis kurstaki (Dipel, Thuricide) or cyfluthrin (Tempo). Other synthetic pyrethroids besides Tempo should also be effective. Be sure to get thorough coverage of the foliage, particularly if using B.t.k. Caterpillars should be most numerous at the top of the tree.