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Public Health - Seattle & King County

Gypsy moths and the use of Bacillus thuringiensis kurstaki (B.t.k.) pesticide

Why are gypsy moth caterpillars a problem?
How is the pesticide Bacillus thuringiensis kurstaki (B.t.k.) used to control gypsy moths?
Are there any health risks associated with B.t.k.?
When will the State Department of Agriculture spray B.t.k.?
Do we need to take special precautions?
Additional resources

Why are gypsy moth caterpillars a problem?

Gypsy moth caterpillars can damage and kill trees and shrubs by eating leaves and needles. They are considered one of America's worst forest pests and have defoliated millions of acres of trees in the eastern U.S. Gypsy moths can spread widely in a region, and infestations can cause extensive environmental and economic damage by destroying forests as well as orchards and residential trees.

There are two types of gypsy moths that have been a concern in the Seattle area. The Asian gypsy moth (AGM) tends to eat both leafy (deciduous) and needled (coniferous) trees, while the European gypsy moth (EGM) prefers to eat leafy trees. During a later period of its caterpillar growth cycle the EGM may eat evergreen needles.

The Asian gypsy moth most likely came into the Seattle area off ships that had previously visited ports in eastern Russia. The AGM female can fly up to 20 miles before laying her eggs. Each female may lay up to 1000 eggs. In contrast, the EGM tends not to travel such wide distances and will generally stay in one location.

The EGM was introduced to North America about 100 years ago and is an established pest in the Northeast and upper Midwest regions of the U.S. They were first detected in Washington State in 1974 but a permanent population has never been established due to detection and control efforts.

More information about gypsy moths and the Washington State Department of Agriculture's Gypsy Moth Eradication Program can be found at www.agr.wa.gov or by calling their Gypsy Moth Hotline at 1-800-443-6684.

Other web sites with information on gypsy moth threat and control are:

How is the pesticide Bacillus thuringiensis kurstaki (B.t.k.) used to control gypsy moths?

The Washington State Department of Agriculture uses a pesticide spray known as Bacillus thuringiensis kurstaki (B.t.k.) for gypsy moth control B.t.k. is a naturally occurring bacteria found in soil. An inactive, or spore, form of the bacteria is approved by the Environmental Protection Agency for use as a pesticide to control gypsy moth. It is commonly used by organic gardeners to control pests, and is approved for use on more than 200 food and grain crops in the United States.

B.t.k. is harmful to moths and butterflies only at their caterpillar stage of development. Spores are activated in the stomachs of caterpillars that eat vegetation sprayed with B.t.k. causing the catepillars to die in 7 to 10 days.

The commercial B.t.k. spray used by the Department of Agriculture is called Foray®. In addition to B.t.k.,the product also contains ingredients to make it stick to plant leaves, and has residues of food crops and preservatives that are approved for use on food. When diluted for ground application, the spray is 99% water.

Are there any health risks associated with B.t.k.?

B.t.k. is not considered toxic for people, animals, birds, fish, and other insects such as bees and ladybugs. Also, it does not harm water supplies. Despite widespread use, B.t.k. has not been shown to cause infections in persons exposed through aerosol spraying either in the general population, children, or people with immune system disorders.

A small number of persons have reported symptoms including skin rash, irritation of the eyes, nose and throat, and hay fever-like symptoms after exposure to B.t.k. It is not known whether exposure to B.t.k. was responsible for the symptoms; however it is possible that these symptoms might be related to allergic reactions or irritation to a component of the pesticide spray.

As a general precaution, Public Health - Seattle & King County recommends all persons in spray areas minimize exposure to B.t.k. Persons who are more susceptible to infections or respiratory irritation should pay particular attention to the precautions above. This includes people with an underlying illness such as leukemia, AIDS or other immune system deficiency, people receiving radiation or chemotherapy treatment, and people with asthma, emphysema or allergic sensitivities.

When will the State Department of Agriculture spray B.t.k.?
Spraying is generally done in the spring when the gypsy moth caterpillars emerge from the egg masses. Information about when spraying will take place can be obtained by visiting the Washington State Department of Agriculture Gypsy Moth webpage or calling the agency's toll-free hotline at 1-800-443-6684. The Department of Agriculture routinely offers to notify residents via telephone or e-mail when they intend to spray the next day, weather permitting. This service is available to anyone who asks.

Do we need to take special precautions?

Even though B.t.k. has an excellent safety record, as a precaution, Public Health recommends that people in the spray area take the following steps to minimize their exposure:

  • Remain indoors for at last 30 minutes after the spray application. It's a good idea to keep family pets inside too.
  • Children should wait until moisture from the spray and dew has dried on grass and shrubs before they play outside and they should wash their hands after playing outside. Gardeners should follow the same precautions.
  • If you come in contact with the wet spray, wash the affected skin with soap and water. If wet materials should get into the eyes, flush them with water for 15 minutes.

Persons who are more susceptible to infections or respiratory irritation should pay particular attention to the precautions above. This includes people with an underlying illness such as leukemia, AIDS or other immune system deficiency, people receiving radiation or chemotherapy treatment, and people with asthma, emphysema or allergic sensitivities.

People with concerns related to exposure to B.t.k., their health, or their immune system should contact their health care provider for advice.

Additional resources

For more information about the Gypsy Moth Eradication Project, including the dates of spray applications, call the Washington State Department of Agriculture's Plant Protection Hotline at 1-800-443-6684. You may also request to be notified by telephone or e-mail the day before any spray application is scheduled to occur.

To report an illness you think may be associated with gypsy moth spraying, call the Washington State Department of Health toll free at 1-888-586-9427.

To contact Public Health - Seattle & King County, you may call 206-263-9566 Monday - Friday, 8 am - 5 pm.

Call the Washington State Department of Health, Pesticide Section at 360-236-3368.