What is Btk?

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

On this page

• Is Btk safe?
• How does Btk work?
• How long does Btk last in the environment?
• Will the Spongy moth or other pests develop resistance?
• How are Bt products made?

More than 20 varieties of Bt exist. The "kurstaki" variety (Btk) is used to control moth caterpillars. There are many strains within each variety. The HD-1 strain of Btk is used for caterpillar control.

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

Is Btk safe?

Btk has no known toxic effects on humans, other mammals, plants, birds, fish, or honeybees or other beneficial insects, except moth and butterfly caterpillars. After more than 30 years of Btk use and research around the world, Canada, the United States and most other countries where Bt products are currently registered consider it safe and have declared it exempt from pesticide residue tolerances on food crops.

Consuming Btk

Btk is used to control pest caterpillars in the production of many food crops. Even organic food growers use this biological insecticide. Because it is not toxic to humans, Btk can be used right up to the time of harvest. Nevertheless, any sprayed fruits or vegetables should be washed before they are eaten.

Breathing in Btk

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. Close windows the evening before aerial spraying takes place and stay indoors until the Btk droplets have cleared the air (usually within 30 minutes to 1 hour, sooner in windy conditions). Multi-lingual health information is available online or by phoning HealthlinkBC at 811.

Btk residue on belongings

The spray does not damage paints or finishes on automobiles, houses, boat or trailers. 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 is only on their skin. Once inside the caterpillar’s digestive system, the bacterium multiplies and releases toxic substances. The caterpillar’s gut is paralyzed and it 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 in the environment very quickly, in three to seven days — faster with sunlight. For Btk to be most effective, it is usually applied to infested areas more than once. The caterpillars must be in an early larval stage for the Btk to work, and not all caterpillars hatch at the same time. Because the product breaks down quickly, it will not be available to caterpillars that hatch after a single spraying; hence the timing of sprays is critical and not all caterpillars will be affected due to differences in development timing.

Will the Spongy moth or other pests develop resistance?

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

How are Bt products made?

The bacterium is grown in sterilized tanks, on an artificial diet mix. The bacterial solution is mixed with water or a mineral oil similar to baby oil. The manufacturer then adds a proprietary mixture of inert ingredients to improve the effectiveness of the formulation. These ingredients include stabilizers, stickers, preservatives and UV protectants – all are considered of minimal risk to human and environmental health by Canadian and U.S. pesticide regulators. 

Spongy moth eradication programs use only water-based formulations.