Last week I stopped at a rest area on I-29 north of Hamburg IA, and noticed a dramatic display of the pinecone-like bagworm “bags” covering an ornamental evergreen. These 1 to 1 ½ inch long bags of dead needles or leaves are remnants of last year’s cases made by the larva of this interesting moth. Bagworms feed most notably on arborvitae and junipers but also can infest other conifers and some broadleaved trees and shrubs.
Bagworms are interesting moths. The larvae hatch from eggs that overwinter in the bags that remain tied to the shrubs. Once they hatch, the larvae prowl the plant feeding, and slowly build the silken bag with needles or leaf fragments woven in to provide them with camouflage and protection. Toward the end of the summer, they attach their bag to a branch with silk and molt into dark brown pupae within the bag. The habits of male and female moths are different. Males emerge as flying hairy brown moths that are nearly clear-winged, but the females are wingless, legless and grub-like, staying the rest of their lives in the bag. The male finds females to mate, followed by the female producing eggs that are left to overwinter inside the bag. Each bag may contain from 500 to 1000 eggs, and hatching and emergence usually takes place in May and early June. Newly hatched larvae generally crawl out and start feeding on the overwintering plant, but some may spin a thread of silk and “balloon” on spring breezes to infest nearby plants.
Usually, broadleaved plants suffer little lasting damage but the evergreens may be severely crippled, and can die in two to five years if the infestation repeats.
If you have valuable ornamental evergreens with bags present, there are two things that can be done:
- Cut the bags from the tree (scissors work well) before the larvae hatch (by early May). But to have success, you need to be thorough, getting them all. Carry the removed bags away for disposal, or,
- Apply an insecticide to treat newly emerged larvae in early June.
Remember that larvae must be present and actively feeding to be controlled by the insecticide. Effective products for homeowner use include Bt products like Dipel® (if available), carbaryl (Sevin® and others), malathion, permethrin (Ortho Max®, Eight®, Spectracide Bug Stop®, et al.), Conserve® and others. Two weeks after treatment, look for live bagworms to see if a second treatment is needed. These insecticides may kill beneficial insects so use them with care and follow the label instructions.
Bagworm cases from previous season. Photo by Rich Pope.