The common bagworm, Thyridopteryx ephemeraeformis, has emerged and started building bags. The eggs overwintered in the bags that were built last year, and hatched in late April or early May. And as soon as they emerged, the young larvae immediately began to feed and construct bags for the next generation.
The first evidence of an infestation is normally a small bag, about 1/4-inch long, which is what I am seeing on my trees now. As the larvae grow, they use their silk and collect fragments of the host plants foliage. They continue to build the bag until it reaches 1 1/2 to 2 inches long. Then when the larvae are mature, they fasten the bag to a plant stem with silk.
Bagworm larvae damage their hosts by feeding on the foliage. Heavy infestations can completely defoliate small plants. Defoliation can kill the host plant, which is usually red cedar and other junipers. Broadleaf hosts aren’t typically killed, but are weakened and become more susceptible to other insect damage and pathogens.
Infestations can be reduced by handpicking the bags in the fall, winter or spring before eggs hatch. Eggs remain viable within the bags, so be sure to destroy bags upon removal by burning them. When larvae become active, bagworms can still be removed by hand if the numbers are small and the affected host plants are small enough to reach the canopy. And remember to destroy the bags once you remove them from the host plant.
Bacillus thuringiensis var. kurstaki, a bacterial insecticide, is a good control method for bagworms. Other products that contain the active ingredient spinosad, a microbial agent, are also effective. To be most effective, apply early when the larva is small. These insecticides must be ingested by the caterpillars in order to achieve kill, so be patient, as it will take some time to see results. Repeat applications may be needed later in the summer in order to keep susceptible plants free of bagworms.
For more information on bagworms and other garden pests, see an OSU factsheet online at https://extension.okstate.edu/fact-sheets/ornamental-and-lawn- pest-control-for-homeowners.html.