Should I be spraying for bagworms now?
Young bagworms have begun hatching in Kansas. Recently hatched larvae are 1/25 of an inch long and spin silken bags around them. The larvae drag these bags along with them as they feed on the host plant. The larvae continue adding to the bag using leaf pieces while they’re feeding. When the larvae reach maturity, usually August, the bags can be one to two inches long. At this time the larvae attach the bag to the underside of the branch and seal themselves inside to pupate. Adult males are clear-winged moths and emerge from the bag in September. Adult females are wingless and do not leave the bag. After mating, females lay eggs inside the bag and then die. Eggs overwinter in the bag and hatch the following spring beginning the cycle again.
Knowing the life cycle of the bagworm can help you understand how to time treatments. This time of year, manually removing bags can be effective for small populations. Bags need to be placed in a bucket of soapy water. Larvae can chew through trash bags and will find a new host if not disposed of properly.
Insecticides containing the active ingredients Bacillus thuringiensis subsp. kurstaki or spinosad are effective if used when larvae are small. These are stomach poisons so the larvae must consume the residue on plant foliage. Thorough coverage of plant foliage is necessary for control.
If an insecticide is applied now, you will miss many of the larvae that are still hatching. Typically, we recommend waiting to apply insecticide until mid-June when the majority of the larvae are actively feeding. If you choose to apply treatment now, you will need to repeat applications on a weekly basis until mid to late June to ensure all the larvae have hatched. As larvae reach maturity, they eat less which reduces their exposure to insecticides. By August, when they seal inside the bags, insecticide is completely ineffective.