The increase in bagworm populations noticed on arborvitae, juniper, red cedar and other trees in southern and central Iowa over the past few years appears to be continuing. Large populations of "miniature" bagworms were reported this week from Iowa City. Miniature bagworms, of course, are early instar caterpillars and the small bags that they construct from silk and bits of plant needles. Bags collected were about one-half inch in length. By the end of the summer, fully-grown bags will by approximately 2 inches long.
Bagworms have been an occasional pest on evergreen and deciduous trees and shrubs in southern Iowa. Infestations are rare north of Interstate Highway 80. Over 128 plant species have been reported as host plants for bagworm, though arborvitae, red cedar and other juniper species are most commonly attacked.
Each bagworm caterpillar produces an individual protective silken bag or case around its body. With its head and legs free, it is able to move about the plant as it feeds on the foliage. Attacked plants usually are partially defoliated, weakened and rendered unsightly. It is not uncommon for complete defoliation to occur in the course of a summer.
If you have had bagworm infestations in the past, now is the time to carefully insect trees and shrubs for small bags. Even if you handpicked last summer's bags during the winter there could still be bagworms present. On a limited number of small trees if may be possible to handpick the entire population while the bags are small. If handpicking is not practical infested plants should be sprayed as soon as the eggs hatch and small larvae begin feeding. Chemical control becomes less effective as the season progresses because of the increased size of the larva and its bag. Insecticides that can be used against the bagworm include Bt (Bacillus thuringiensis), Sevin, permethrin and other pyrethroids, Orthene, or malathion. Apply according to label directions.