What is Btk?

Btk (Bacillus thuringiensis kurstaki) is a naturally occurring bacteria. It commonly occurs in soils and on plants throughout the world. It is employed in a biological insecticide used to eradicate the spongy moth. Widespread use of Btk to control the spongy moth began in the 1970's. Since then, it has been applied to millions of acres of forest in the East and Midwest. For example, over 500,000 acres were treated in the eastern U.S. in 2008 - more than twice the area of Mount Rainier National Park. Washington State University wrote an extensive article that examined the research, effectiveness, and safety of Bt, including Btk. Click here to view WSU's Bt article. You can also learn about Btk from the National Pesticide Information Center. 

Btk is the most safe and effective eradication method for the spongy moth. First used in Europe in 1938, Btk is used worldwide in both forestry and agricultural applications. Numerous studies have shown Btk to be very low risk to human health. Btk only impacts caterpillars and will not harm other beneficial insects such as bees, lady beetles, ants or parasitic wasps. It is also not harmful to animals, birds or fish.

The Organic Materials Review Institute (OMRI) has determined that the formulation of Btk "Foray48B" is certified for use in organic agriculture and gardening. OMRI is a national nonprofit organization that approves products that are used on operations that are certified organic under the USDA National Organic Program.

Btk is applied in an inactive form via aerial application (helicopter or small plane) or, when the treatment area is very small and easily accessible, by ground. The treatments take place in the spring, usually in April and May, when the spongy moth caterpillars hatch and begin to eat trees and shrubs. When the caterpillars consume Btk on the vegetation, it activates in their digestive system and is lethal to the caterpillar within hours.

Bees are not harmed by Btk. It only affects caterpillars of moths and butterflies.

Washington is not a primary migration area for monarch butterflies; the primary migration path is in the Midwest. Eastern Washington has a handful of monarch sightings each year, but sightings are somewhat rare in Western Washington. (According to Learner.org, which tracks monarch sightings, only 1 adult monarch was sighted in Western Washington in mid-July in 2015. No eggs or larva were sighted. There were no monarchs sighted in Western Washington in 2014.) However, even should some monarchs migrate through, they would not be impacted by our treatments for one simple reason: the monarch butterfly does not have the same life cycle as the spongy moth.

The monarch caterpillar or larva begins feeding in our state in late June at the earliest. Spongy moth treatments generally end in May. Because Btk degrades so rapidly, it would not affect any caterpillar beyond 10 days after the final treatment. Adult monarchs are not affected by Btk. Therefore, both because of the low numbers of monarchs that migrate to Washington and because their lifecycle is different than the spongy moth, WSDA's spongy moth treatments will not impact monarch populations.