For Release June 10, 2003

It's Time To Spray For Bagworms

by Chuck Otte, Geary County Extension Agent

When it comes to the plant pest known as the bagworm, confusion reigns supreme! First of all we have the confusion between bagworms, webworms and tent caterpillars. Tent caterpillars normally spin little webs in the forks of tree branches. They are most visible early in the year. They move out of the webbing to feed during the day. They seldom cause serious problems other than being unsightly.

Webworms are that group of caterpillars that form large webs on the tips of branches late in the season, usually August and September. They are very unsightly and cause a lot of concern. The caterpillars form a web over the leaves that they are going to eat. They seldom feed outside the webbing. While they do cause concern for homeowners, they do very little damage. Homeowners probably do more damage burning the ends of the branches or clipping the branch ends off. Some people call these bagworms because they make a web bag at the branch end. Webworms are not bagworms.

The common bagworm is the insect pest that feeds primarily on junipers. They are easily overlooked early in their life cycle, but are very destructive if not noticed until late in the season. The adult bagworm is a moth. The moth does little other than mate, lay eggs and die. The caterpillar forms a little bag that it decorates, or camouflages, with foliage from the cedar, or whatever the host plant is.

The bagworm overwinters as an egg in the old bag. While many of last year's bags will be empty, you will find a few of them are full of 500 to 1,000 eggs. These eggs hatch in late May and early June. For the first few weeks of their life cycle, the bagworms are very small, about one fourth inch long, and they don't eat very much. They are easily overlooked even if you are looking for them. But as they eat their way through June and July they grow in size and appetite. By late July and early August, the bags become very visible as does their damage. In fact, junipers can appear okay, and in just a few days damage becomes very obvious.

Many homeowners have the mistaken idea that the problem occurred overnight. In reality, the bagworms have probably been there all summer. If a neighbor has a severe infestation, the bagworms can crawl a hundred feet or so, but most of the time, severe infestations this year started with a few unnoticed bagworms last year. Bagworms will kill an evergreen if they are not controlled. While they'll also feed on deciduous trees and shrubs, over 200 plant species are known to host bagworms, we don't worry about them as much since deciduous plants can generate new foliage.

Controlling bagworms is easy if you know they are there. Many products are labeled for bagworm control, including the biological product known as Bt or Bacillus thuringiensis. The trick is not so much the product but the application. The presence of the bag creates an extra challenge. To be effective, any product needs to be applied with a lot of water. Therefore pump up canister sprayers are not effective. You need to use a hose end sprayer. Timing is also critical. Spraying too early is ineffective as there is nothing to control yet. The larger the bagworm becomes, the harder they are to kill. Now is the time to spray while they are still small. Spray two times, ten to fourteen days apart. If you had bagworms last year or if a neighbor had bagworms last year, I would plan to spray. If you have never had bagworms on your junipers, don't spray yet, but check them closely every week during June and July. Bagworms are very destructive and they can sneak up on you, so be alert and be diligent!


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