What is Btk?
(Bacillus thuringiensis var. kurstaki)

Bacteria are present everywhere in our natural environment, including in soil, in food and even on our skin. Bacillus thuringiensis (Bt) is a bacterium that is found naturally in the soil and is known to cause illness in various insect larvae, including caterpillars of pest species such as gypsy moth, spruce budworm and cabbage looper.

There are more than 20 varieties of Bt. The "kurstaki" variety (Btk) is used for caterpillar control and other varieties are used for blackfly and mosquito control. It is not harmful to humans, birds, pets, fish, honey bees, beetles, spiders, etc.  Within each variety are numerous strains.  The Btk used for caterpillar control is the HD-1 strain.

Bt pest control products have been registered for use in Canada for over 30 years.

Is Btk safe for humans?

Btk has no known toxic effects on humans or other mammals, plants, birds, fish, honeybees, or other beneficial insects. Its safety record, and the results of more than 30 years of Btk investigation around the world, have led many countries including Canada, the United States and most other countries where Bt products are currently registered to declare it exempt from pesticide residue tolerances on food crops.

Btk is used to control pest caterpillars in the production of many food crops. Organic food growers also use this biological insecticide. Unlike many pesticides, Btk can be used right up to the time of harvest because it is not toxic to humans. Any sprayed fruits or vegetables should be washed before they are eaten.

Even though there is no evidence of harm to humans from Btk, persons with respiratory ailments or other health concerns may wish to reduce their exposure to the product during the spray operation. To deal with this, close windows the evening before aerial spraying takes place and stay indoors until the Btk product has had time to dry (which takes a few hours).

Btk has been reviewed by Agriculture Canada, Environment Canada and Health and Welfare Canada and has been on the market since 1961. There is extensive use experience without harmful effects to humans. It has been sprayed in many B.C. locations for gypsy moth eradication, including Victoria, Colwood, Vancouver, Kelowna, Parksville, Nanaimo, Chilliwack, Fort Langley, Burnaby, Delta and Courtenay.

The spray does not cause damage to automobile, house, boat or trailer paints or finishes. If it is left to harden, the spray can be removed with water but may require more effort. The sooner it is washed off the easier it is to remove.

How does Btk work?

Btk affects only the larval stage of susceptible insects; it cannot be used to kill eggs, pupae, or adults. To work, Btk must be eaten by the caterpillar (it will not harm caterpillars if it remains only on their skin). Once inside the caterpillar’s stomach, where a basic (alkaline) pH is present, the bacterium multiplies and releases toxic substances. Specific enzymes in caterpillars’ stomachs are also required to activate the product. When the Btk is activated, the gut is paralyzed and the caterpillar stops feeding and dies within one to five days. Larvae are most susceptible to Btk when they are in the early developmental stages.

How long does Btk last in the environment?

Btk breaks down quickly in the environment in about three to seven days. Sunlight will help to break the product down. For the most effective use of Btk, it is usually applied more than once in infested areas. This is because the caterpillars must be in an early larval stage in order for the Btk to work. Unfortunately, not all caterpillars hatch at the same time. Because the product breaks down quickly, it will not be available to caterpillars that hatch after the first spraying, unless several applications are made.

Will the gypsy moth or other pests develop resistance to Bt?

After over 30 years of use, there are no known instances of target insects developing resistance to Bt.

For more detailed information on Btk registration, health and environmental impact studies, go to the Gypsy Moth Spray Ingredients page

Gypsy Moth Homepage

Contact Tim Ebata if you have comments on the presentation of this information.

BC Ministry of Forests, Lands & Natural Resource Operations
Resource Practices Branch
P.O. Box 9513 Stn. Prov. Gov.
Victoria, BC
V8W 9C2

Section phone: (250) 387-8739
Section fax: (250) 387-2136