Bagworm eggs are hatching in southern and central Illinois. These caterpillars overwintered as eggs within old female bags left on the plant. One bag may contain 500 to 1,000 eggs. Bagworms feed on many plants but are common on arborvitae, juniper, cedar, pine, spruce, and a number of deciduous trees.
As the caterpillar feeds, it constructs a bag (made of silk and plant material) that it remains inside as it feeds. This bag increases in size as the caterpillar grows and is 1 to 1-1/2 inches long when the caterpillar is mature. The bag, which is usually brown due to dried foliage on the outside, protects the worm from natural enemies (and from pesticides, to an extent). The worm feeds for several weeks, then pupates. Male moths are winged and emerge in the fall to search for the wingless females, which remain in their bags.
Early management of bagworms is essential for effective control. Look for small bags to determine when to treat. Hatch occurs over several days: the young bagworms crawl to the tops of trees, suspend on silk threads, and are blown to new hosts. Once the bags dwindle to 1/4 inch long, all the eggs should have hatched and caterpillar migration should be over. Spraying with insecticide before this time may require a retreatment to control late-hatching larvae and larvae that blow onto the trees from elsewhere.
Because young larvae migrate to the tops of trees and shrubs, look in these areas for early infestations. Branches of conifers that are stripped of foliage will usually die; deciduous plants will refoliate. Young bagworms can be controlled effectively with Bacillus thuringiensis kurstaki (Dipel or Thuricide), cyfluthrin (Tempo), spinosad (Conserve), and trichlorfon (Dylox). Old bags that can be readily reached (in other words, in shrubs and low tree branches) should be removed and burned before the eggs hatch.