These pesky little insects are usually found on evergreen trees and shrubs. If left untreated, the insect will increase in population each year, eventually causing the defoliation of the plant.
In May each year, bagworms are just beginning to emerge from their eggs that have overwintered. The larva will continue to hatch through early June depending on weather conditions. The larva feeds on the plant foliage. Once they are 3 to 6 mm in length, they will produce a strand of silk to hang from, to disperse them to other near-by hosts.
After reaching a final destination, they continue to feed. At this time, they construct silken bags that are covered with small twigs or foliage. They remain in the bag, but use it similar to a shell. Their head and thorax will protrude while feeding.
As the bagworms grow, their bags become brown and are easy to spot in late summer. Typically, this is when most calls are received to treat them. Unfortunately, hand picking is your best line of defense at this time, because the insecticide will not penetrate the bag.
After feeding for the summer, in mid-August to September the larva stop feeding and seal the top of the bags. During this time, the larvae then enter the pupae state. Females never turn into moths, but the males will transition into black moths. After mating the males die quickly. One female will typically lay between 500 to 1,000 eggs and will eventually die in her bag.
Treatment:
Now is the time to begin looking for the crawling larva. Insecticides should be applied when the larva is 3 to 6 mm long. The smaller the larva, the more success you will have. Because of the different times larva can emerge, applying insecticide weekly is recommended for several weeks. Recheck weekly for crawling larva to see if another application is needed.
Insecticides should be applied thoroughly in the morning or evening when larva are most active. Look for products that contain Bacillus thuringiensis subsp. kurstaki or spinosad. On small trees, a hand pump sprayer is effective to reach all parts of the plant. Treatment of windbreaks can be challenging because it is difficult to reach the tops and thoroughly cover the trees. A high-pressure sprayer that delivers a large volume of spray would be most effective.
More information can be found on K-State Research & Extension’s Bagworm publication, or Ward Upham’s Common Pest Reference Sheet – Bagworms.
