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A bad year for bagworms

August 5, 2018 GMT

Windbreaks are an essential element for an acreage to block wind. However, some years we face a pesky caterpillar called a bagworm. This is one of those years where many people are facing problems with bagworms.

Bagworms are a pest that we are seeing in our landscapes right now. Bagworms are the immature form, or 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. Bagworms are most common in the middle to late June. Their emergence is weather dependent.

It is best to only treat for bagworms when the caterpillars are active. Once they get too large, the sprays will no longer be effective. 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.

It is hard to give a specific date or time of year when we should spray for bagworms, they are dependent on the environment so every year is a little different time. 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 inch in length. The best chemicals to use for bagworms include Sevin, Eight, Bifenthrin, malathion, 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.

This year the bagworms have been out and about for more than a month now. The bags are mostly all 1 inch in length or more now. Feeding by mature caterpillars slows in August before pupation into adults, so chemical control in late summer and fall is not effective. If the trees are small, it would be a better practice now, to just remove the bags by hand. It would be more effective to wait to spray until next year when the bagworms are more actively feeding.