Bagworms (Thyridopteryx ephemeraeformis) are a common landscape pest that you see on many evergreen ornamental plants. Most often they are found on arborvitae, spruce, juniper, cedar, and Leyland cypress. They are easily identified by their cone-shaped bag they spin from silk, parts of the host plant, and other debris. In late summer dark brown pupae about ½ inch to 2 inches long can be found in these bags. Adult female bagworms are wingless, legless, grub-like, and they never leave the bag. Males are small, brown, hairy moths with dark wings that clear with age.
In the fall the females lay 500-1000 eggs in their bag before they die. The eggs overwinter and hatch in May and June. When the larvae hatch, they are blown around by the spring winds and land on a suitable host. The worm then spins a tiny bag of silk and plant debris that looks like an upside-down ice cream cone. As the caterpillar grows the bag grows as more host plant material is added to the outside for camouflage. In August the caterpillars mature and molt into the pupal stage. During August and September, adult bagworms emerge as moths. Since adult females are wingless, they never leave the bag. The males leave the bags in search of females for mating.
New bagworm infestations often go unnoticed until late summer after the damage is done. Once conifers are damaged, they do not produce new foliage. The key for monitoring bagworms is to look for bags in the fall or early spring before eggs hatch.
Intervention and control of bagworms is possible, but as mentioned monitoring is critical. Since females do not fly, infestations can be concentrated and localized in your landscape. There are many different types of parasitic wasps that can aid in controlling bagworms as well as a number of insecticides. The following are some of the most available insecticide ingredients for control: bifenthrin, spinosad, bacillus thuringiensis, acephate, and cyfluthrin. The best time to use these insecticides would be in late May and June. It is too late to spray in the fall once the bags are formed. Bags can also be removed by hand and destroyed if heavily concentrated in one area. If left uncontrolled bagworm populations can become severe enough to kill large deciduous trees.
For more information about bagworms, or if you have other gardening questions, feel free to contact the Franklin County Cooperative Extension Office at 919-496-3344.
