Alternatives to Chemical Pesticides

One species of bacteria called Bacillus thuringiensis, (or BT) is known to kill some groups of insects.

BT occurs naturally and is commonly found in soil. It is the most widely used naturally occurring pest control product in the world. It reduces the need to put chemicals in the environment and it doesn’t harm native beneficial species of insects and other animals. 

BT is not a synthetic chemical. BT products contain protein crystals and dormant spores of bacteria that are only activated when they’re eaten by target insect species. Unlike broad spectrum insecticides, BT is highly specific –it only affects certain species of insects and has no effect on others. Here’s an overview of how the product works:

  • A caterpillar eats a leaf treated with BT product
  • BT crystals reach the caterpillar's gut and dissolve in the alkaline conditions
  • BT proteins contained in the crystal are released
  • The proteins disrupt the lining of the gut, causing 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.

Safe to Use

BT is a natural alternative that is non-toxic to humans, other mammals, birds, fish and most insects – it makes an excellent pest control option for use on food crops. In fact, BT products have been approved for use on organically-grown food for many years.

Each strain of BT infects a relatively narrow range of species. Other, more beneficial species are not negatively impacted by its use – for example:

  • Predatory insects, birds, snakes and other animals that keep pest numbers low in the yard and garden are not at risk if they eat an insect that has been infected with BT.
  • BT also does not infect or kill earthworms, honeybees or many other species of insects. 

BT products can be found at local garden centres to help control caterpillars that attack garden plants and trees. The products are in concentrate form that must be mixed in a larger volume of water to make a spray for plants and trees. Always follow the directions on the label to mix and use the products correctly.


Since BT was first discovered, researchers have found many different subspecies of the bacteria that are commercially useful because they infect different groups of insects.

BTK: Bacillus thuringiensis var. kurstaki is the subspecies most widely used to control caterpillars. 
BTI: Bacillus thuringiensis var. israelensis is the subspecies that kills young larvae of mosquitoes and black flies. It’s widely used in B.C. for controlling mosquitos. 
BTT: Bacillus thuringiensis var. tenebrionis is a newly registered subspecies that affects the larvae of some species of beetles.


Many caterpillars 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 adult butterflies, their eggs or the chrysalis stage (an immobile stage, when the caterpillar turns into a butterfly within a silken case). Generally, only younger caterpillars of susceptible species are killed by the product and even then, 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. 

Only spray a BT product if you’re sure the caterpillars are causing significant damage. This will minimize the impact on non-target caterpillars in a home garden. Remember that caterpillar damage often looks worse than it is – plants usually recover quickly by growing new leaves.

Treatment Frequency

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 (12-48 hours) once it is sprayed, therefore spraying it before caterpillars appear is a wasted effort.

After the product has been mixed with water and sprayed onto leaves, it’s destroyed by ultraviolet radiation from sunlight within a few days. It is also rapidly degraded by high temperatures and substances on plants leaves. It will be washed from leaves into the soil by the rain.

BT spores can remain dormant for several months in places where there is no sunlight like in the soil or mud at the bottom of a stream or pond. Spores will not germinate on their own in soil or water – they must be eaten by a susceptible insect.

Commercial Products

Bacterial cultures and the food medium the bacteria are grown in are mixed with other ingredients to make commercial products stable and easier to mix with water or stick to leaves. Ultraviolet protectants are usually also added to shield the product from the sun. The largest component of any BT product is water. For example, a common BT product sold for gardeners is diluted 150 to 300 times before spraying. 

All ingredients of any pesticide product are registered with the federal Pest Management Regulatory Agency. Pesticide ingredients must be tested for toxicity and approved by federal health professionals before the the product is allowed to be used. Any BT products registered in Canada are required to meet stringent quality standards similar to those used in the food industry.