For Release June 8, 1999

Time to Spray Bagworms

by Chuck Otte, Geary County Extension Agent

If you had bagworms on your evergreen trees and shrubs it’s time to start spraying them again. If you had bagworms on your trees and shrubs and you sprayed them last year, it’s still time to spray them again! Bagworms are voracious feeders and can kill an evergreen tree or shrub if left unchecked. Even if you did spray them last year, all you have to do is to have missed one or two of them and you can have a bag full of 500 to 1,000 eggs that have now hatched and are starting to feed!

Bagworms are the larval stage of a rather nondescript member of the moth family. Their greatest claims to fame are that the female is wingless and the larvae are very destructive to coniferous trees and shrubs. They have one generation per year and while they will feed on over 200 species of plants, their favorite is definitely members of the juniper family.

The bagworms start out in late May and early June hatching from the eggs that were laid in the old bag of the female parent last fall. For the first six weeks, or so, of their life they crawl around feeding on the host plant very unnoticed. They form the protective bag quite early, but it is so small it is easily overlooked. It is constantly adding new leaf material to the bag so it stays green and can be difficult to detect. When the bagworms are small they are very easy to control.

But by late July or early August their size, and their appetite, have increased greatly. This is the stage where you can drive by you house at 30 mph and see that you have a problem. When the bagworms have gotten to this size they become more difficult to control and are doing a lot of damage to your plants.

Any coniferous plant that had bagworms last year needs to be sprayed regardless of how well you think you may have sprayed it last year. I have found that you normally need two consecutive years of good spraying to get the job done. I would not waste time spraying deciduous trees and shrubs unless they are right next to a juniper or other evergreen. Deciduous plants are much less preferred by bagworms and have the ability to sprout new leaves to overcome the bagworm feeding damage. But, they can serve as a source of infestation for adjacent conifers.

How about waiting until later in the summer to see if a problem develops? I would only do that if you did not have bagworms last year. Also keep a close eye for a developing problem now. The sooner they can be found and treated, the less the damage will be to your plants.

Are there any organic or biological controls? Yes. I have noticed that House Finches will feed on bagworms, but they seem to be the only bird that does. There are a few predatory insects that will also feed on bagworms, but neither of these seem to put a significant dent in the population. You can also use sprays of the biological control, Bacillus thuringiensis, which is sold under such brand names as Bt, DiPel, Thuracide and many others. These products take much longer to work than traditional pesticide so it is important to start early and make several applications 10 to 14 days apart.

Many common garden insecticides are effective against bagworms IF you start early (now) and apply with a high volume of water. Use a hose end sprayer. A pump up canister sprayer simply doesn’t apply enough water to get the job done. Two treatments two weeks apart is probably sufficient if the first treatment is near or after June 10th. If you had or have a heavy infestation you may want to use three treatments just to be sure. Preferred pesticides include: Orthene, Malathion, Sevin, Tempo, Dursban and diazinon. We have an excellent bulletin on bagworms available at the Extension Office, 119 East 9th in Junction City.


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