Bagworms are easily recognized by the bags they sew together from the needles or leaves of their host plant. When present in large numbers, they can defoliate and kill juniper, spruce, and arborvitae trees. They hatch in late May and quickly form tiny bags about 1/8 inch long. They eventually grow to bags over 1 1/2 inches long. The males pupate into clear-winged moths and mate with the females, which never leave their bags. The females lay up to 1000 eggs in the bag and then die. The eggs overwinter inside the bags. In summer, trees with large populations will get a brown cast as the new caterpillars feed on the foliage. Many other trees, such as crabapples and Japanese maples, can host bagworms, but conifers are generally the only trees that suffer long-term damage.
On small trees and bushes, hand-picking bagworms is very effective. Chemically, bagworms can be easily controlled when they are young by spraying with Bonides Captain Jack’s Deadbug Brew, which contains Spinosad. Nature Guard’s Caterpillar Killer Spray with Bt is also a safe and effective control if used in May or June. When the caterpillars grow larger, a product containing Permethrin, such as Bonide’s Eight, or Bifenthrin, such as Hi-Yield’s Bug Blaster 2.4, may be more effective. By late August, bagworms stop feeding, and insecticides are no longer effective.
*Please read and follow all label instructions when applying chemicals.