Bagworms overwinter as eggs within bags fastened to twigs and this year they appear to have overwintered well. I have had multiple reports the eggs have hatched this past week a little later than normal. Immediately after hatching, some of the caterpillars release a streamer of silk and are blown by the wind, establishing new infestations on nearby trees. That’s why it can be so devastating on cedar trees in windbreaks.
Others begin to spin tiny (l/8 inch) protective cases or "bags" around themselves. These bags are constructed of silk and fragments of needles or leaves. As bagworms grow, leaf fragments are added to bags, which often grow to 2 inches in length by the end of the summer. Larvae feed until late August or early September. There is only one generation per year. Female moths deposit their eggs within their own bags, drop to the soil and die in the fall. Each female can produce 500 to 1,000 eggs in a bag.
Bagworms feed on most coniferous plants and on many deciduous trees and shrubs. Common evergreen hosts include juniper, arborvitae, spruce, and pine. The earliest sign of bagworm injury in an evergreen is yellowing and then brown or stressed needles at the tips of branches. This is caused by tiny, first-stage bagworm caterpillars etching needle surfaces as they feed. These plants can be severely stressed or killed. Bagworms are especially damaging to conifers because destroyed foliage is not regenerated.
Bagworms have many natural insect enemies which feed on them during their larval or pupal stages. A recent study at the University of Illinois at Urbana-Champaign showed that planting asters and daisies near bagworm-infested trees provided shelter and nectar for beneficial insects and reduced bagworm numbers. Birds, especially sparrows and finches, are important predators of bagworms in late summer.
Timing is everything in successfully controlling bagworms before they cause irreversible damage to your windbreaks. Control measures implemented now will be most effective. Aerial application is not an option, because we don’t get enough coverage and penetration into the tree canopy.
Insecticides are most effective when applied during the early stages of bagworm development. Spray when bags are less than 1/2 inch. Safer for predators are insecticides containing Bacillus thuringiensis (Bt – which is Dipel or Thuricide), spinosad, or neem oil (azadirachtin) and insecticidal soaps are quite effective against young bagworm larvae, but may require repeated applications. These products generally have minimal impact on beneficial insects.
Other insecticides, highly effective this time of year, currently labeled for bagworm control include acephate (Orthene), bifenthrin (Talstar, Capture, Brigade), carbaryl (Sevin), cyfluthrin (Tempo), malathion, permethrin (Eight) and fluvalinate (synthetic pyrethroid chemical compound that is a miticide commonly used to control varroa mites in honey bee colonies. For bagworms a common name is Mavrik). You need to use ground equipment that delivers a high spray volume and pressure or hire a professional.
In the summer, affected plants must be thoroughly covered with the insecticide and that it is ingested by the insects as they feed. No endorsement is intended when mentioning trade names in this column nor criticism implied if a product is not mentioned. Always read and follow label directions carefully when using pesticides and use the personal protective equipment.
Commercial application is worth it compared to losing a windbreak to this pest.