If you have seen two-inch bags hanging on your plants this winter or if you look outside one morning and see what looks like little pearls hanging on the limbs of your Leyland Cypress or other junipers, beware of the bagworms.
When trying to control bagworms, the saying, “Know your enemy” from Sun Tzu’s book The Art of War is very applicable. They are called bagworms because most of their life is spent incased in a chemical resistant bag of silk, camouflaged with bits and pieces of the plant they feed on.
It is the larval or caterpillar stage of the moth Thyridopteryx ephemeraeformis that damages the plants. They can feed on a wide range of plants. Bagworms are most destructive on arborvitae or juniper species where they strip the plant clean of all foliage.
Eggs overwinter in the mother’s bag and hatch in May and June. The larva spins a short thread of silk and launches itself into the spring breeze where it can float to a new plant to begin feeding. If the larva lands on a plant different from the parent’s host plant, the larva may struggle to survive or it may not survive at all. Many of the larva land on the original host plant where they begin to create their own camouflaged, silk bag.
One caterpillar doesn’t do much damage; however, the females can lay between 500 and 1,000 eggs each. These sheer numbers can do a lot of damage, especially to needled evergreens. The larva finishes feeding and pupates sometime around the end of July or August. Adult females are wingless and never leave the bag, while adult males, having wings, fly from female to female mating. The adults are only active from late August through September. Once they have mated and the eggs are laid, the female dies and the cycle begins again.
Knowing the lifecycle of the bagworm helps us determine the best times to control this pest. In fall and winter bags can be removed using a sharp pair of scissors or a sharp knife. Worms seen moving around on plants from May through July may also be removed by hand.
For those who are squeamish about touching insects, there are chemical controls. The earlier these chemicals are applied after the caterpillars appear, the more effective the chemicals will be. The least toxic products are going to be those containing Bacillus thuringiensis kurstaki (examples: DiPel and Worm Whipper) or spinosad (example: Conserve), as these are biological controls. As caterpillars increase in size, something more toxic may need to be applied such as acephate or carbaryl (example: Sevin).
Bagworms also have some natural enemies including birds that feed on the young caterpillars and parasitic wasps that lay their eggs inside the caterpillars. Sometimes the natural enemies aren’t as abundant as the damaging caterpillars and we must step in or risk loosing the plants.
Chemical recommendations are in the 2010 North Carolina Agricultural Chemicals Manual. Always read and follow label directions when applying pesticides.
More information about bagworms can be found in the Ornamentals and Turf Insect No. #81 on the web at www.ces.ncsu.edu/depts/ent/notes/O&T/trees/ort081e/ort081e.htm, or call your local office of North Carolina Cooperative Extension.
Shawn Banks is the Consumer Horticulture Agent with the NC Cooperative Extension Service in Johnston County. You may reach him at firstname.lastname@example.org.