Bagworms can be a big problem in the landscape, especially in a windbreak. But timing is critical for control of these caterpillar pests. I get calls throughout the spring most every year looking at when to spray for bagworms. We are just starting to enter into the timeframe for spraying for bagworms.
Bagworms are the immature form or the caterpillar of a clear-winged moth. The female overwinters in the bags with her eggs. In the spring or early summer, the eggs will hatch allowing the caterpillars to emerge from the bag. These caterpillars will then move around on that tree and they will move to other trees and feed on the foliage. They prefer cedars and other trees and shrubs in the juniper family, but they can also be found on pines and spruces as well as on deciduous trees to a lesser extent. The female adults will live their whole life in a bag and the males will emerge as a clear-winged moth to mate with the females.
The damage from bagworms includes foliage that turns brown or is missing. The foliage may be chewed on when the caterpillars first start feeding. You may also start to notice the bags on the trees which look like small, brown Christmas tree ornaments that develop on your trees. Most often, the damage from bagworms is minimal and will not usually kill a tree. However, if populations get large enough they can kill your trees.
Bagworms emerge most often in mid to late June. Their emergence is weather dependent. So some years it may be in late May and some years it is nearly July before they emerge.
It is best to only treat for bagworms when the caterpillars are active. Once the bags get too large, the sprays will no longer be effective. Spraying before the caterpillars emerge from the bags will not be effective either, so timing is critical. If you find bagworms on your trees you can pull them off and throw them into a bucket of soapy water, if there aren’t too many and if you can reach them all. Most years, we spray for them in the third week of June, but it is best to spray when they are just forming until they are no larger than one-half inch in length. The best chemicals to use for bagworms include tempo or a product containing Bt. Bt is Bacillus thuringiensis which is a type of chemical that is specific to butterflies and moths and will not harm other beneficial insects such as bees and beetles. However, most any general insecticide will work. Be sure to follow the label instructions for mixing and proper protection equipment when spraying. If you can’t spray to the top of your tree, it would be best to hire a professional with a pesticide license to spray the tree thoroughly.