Ministry of Environment
Integrated Pest Management
Fact Sheets: BT: An Alternative to Chemical Pesticides
What is BT?
BT is an abbreviation of the scientific name of a species of bacteria, Bacillus thuringiensis, that kills some groups of insects.
BT occurs naturally and is commonly found in soils worldwide. It was first discovered infecting silkworms over 90 years ago in Japan, where it became known as Sotto disease.
For many years, the possibilities of using this naturally occurring organism were not recognized, but in the 1950s research began in earnest on its effectiveness as a pest control product.
A commercial BT product was first registered in the United States in 1958; by 1960 it was cleared for use on food crops and in 1961 it was registered for use in Canada. It is now the most widely used naturally occurring pest control product in the world.
It is important to realize that BT is not a synthetic chemical. BT products contain the highly specialized protein crystals and dormant spores of bacteria. These are only activated when they are eaten by a susceptible species of insect. Unlike broad spectrum insecticides, BT is highly specific—that is, it affects only certain species of insects and has no effect on others.
How Does It Work?
BT begins to work after a caterpillar eats a piece of leaf with BT crystal proteins and spores on it (caterpillars are the immature stage of butterflies and moths).
Susceptible caterpillars have a strongly alkaline digestive tract (in contrast, humans and other animals have acidic digestive tracts). When the crystals reach the caterpillar's gut, they dissolve in the alkaline conditions and release the proteins contained in the crystal.
Through a series of complex biological processes, the proteins disrupt the lining of the gut, which causes the caterpillar to starve. Infected caterpillars may not die for several days, but they usually stop feeding immediately because their digestive tract is paralyzed by the activity of the crystal proteins.
What are BTK and BTI?
Since BT was first discovered, researchers have found many different subspecies of BT, some of which are now commercially useful because they infect different groups of insects.
BTK is the abbreviation for Bacillus thuringiensis var. kurstaki, the subspecies that is currently used most widely for control of caterpillars. BTI stands for Bacillus thuringiensis var. israelensis, a subspecies that kills young larvae of mosquitoes and black flies. BTI is used widely in British Columbia in mosquito control programs. A new subspecies that has been registered recently is Bacillus thuringiensis var. tenebrionis, which affects the larvae of some species of beetles.
Is BT Safe to Use in Yards and on Food Plants?
BT is not a synthetic chemical—it is a naturally occurring organism. It is non-toxic to humans, other mammals, birds, fish and most insects, therefore it is a good choice for use on food crops. BT products have been approved for use on organically grown food for many years.
Does BT Harm Earthworms, Honeybees or Other Beneficial Species?
Each BT strain infects a relatively narrow range of species. Studies have shown that predatory insects, such as lady beetles, are not at risk if they eat an insect that has been infected with BT. BT does not infect earthworms, nor does it kill honeybees or many other species of insects. BT is an excellent choice for control of many pest caterpillars because it does not harm predatory insects, birds, snakes and other animals that keep pest numbers low in the yard and garden.
Does BTK Harm Butterflies?
Many caterpillars, which are the immature stage of butterflies and moths, are potentially susceptible to BTK. Although the effects of BTK have not been tested on the caterpillars of every species of butterfly, we do know that it does not harm the adult butterflies, their eggs or the chrysalis stage (an immobile stage, when the caterpillar turns into a butterfly within a silken case).
Generally, only the younger caterpillars of susceptible species are killed by the product and even they must eat a sufficient dose of BT to be affected. To be effective on pest caterpillars, sprays must be timed to coincide with the most susceptible age of insect because BT only lasts for a short time in the environment. Unless butterfly caterpillars are in the same place at the same time as the target caterpillars, they would not be at risk of being infected.
To minimize the impact of sprays on non-target caterpillars in a home garden, homeowners should spray only if they are sure the caterpillars are causing significant damage (caterpillar damage often looks worse than it is; plants usually recover quickly by growing new leaves).
How Long does BT Last in the Environment?
There have been numerous studies on how long BT lasts in the environment. In general, it has been found that there is a rapid decline in the ability of the bacteria to infect insects within 12 to 48 hours after it is applied.
After the product has been mixed with water and sprayed onto leaves, it is destroyed by the ultraviolet radiation in sunlight within a few days. It is also degraded rapidly by high temperatures and substances on plants leaves. It is also washed from leaves into the soil by rainfall.
In the soil, where BTK spores are protected from sunlight, they remain dormant. The spores of BTI may also remain dormant in the mud at the bottom of a stream or pond for several months. The spores cannot germinate in either soil or water—to do this they must be eaten by a susceptible insect.
What is in Commercial BT Products?
To enable BT to be made into a commercial product, the bacterial cultures and the food medium the bacteria are grown in are mixed with other ingredients. These are added to make the products stable, mix easily in water and stick to leaves. BT is so fragile in the environment that ultraviolet protectants to shield it from the sun are usually also added.
These ingredients make up approximately 5% of the concentrated product, which is later diluted with water to make a spray. The largest component of any BT spray is water; for example, a common BT product sold for gardeners is diluted 150 - 300 times before spraying.
These additional ingredients are part of the registered product formulation that is submitted to the federal Pest Management Regulatory Agency for registration. Under current regulatory practice in Canada, all ingredients in pesticides are known and are tested for toxicity in the whole formulation. They are approved by federal health professionals when the product is approved for registration. BT products registered in Canada are required to meet stringent quality standards similar to those used in the food industry.
Why Should We Use BT Products?
Using BT instead of a chemical insecticide to control caterpillars is an important step in reducing your exposure to chemicals in the environment. It is also the best choice to avoid harming the many native beneficial species of insects and other animals.
As with all pest control products, BT should be used only when you know that control measures are necessary. BT lasts for only a few days once it is sprayed, therefore spraying it before caterpillars appear is a wasted effort.
Are BT Products Available to Home Gardeners?
At any garden centre, you can find one or more products containing BTK to control caterpillars such as cabbage loopers, imported cabbageworm, fruit tree leafrollers and other caterpillars that attack garden plants and trees. The products are in powdered or liquid concentrate form that must be mixed in a larger volume of water to make a spray for plants and trees. Products containing BTI are for commercial applicators for use in community mosquito control programs.
Always follow the directions on the label to mix and use the products correctly.