How to use Bacillus thuringiensis (Bt) to control insect pests

What is BT?

Bt is the abbreviation of bacteria that was named Bacillus thuringiensis in 1911 after it was discovered in the German city of Thuringia that it could kill Mediterranean flour moths.

These common soil bacteria have been used as microbial insecticides for the past century.

Farmers in Europe were the first to recognize their potential in 1920, and American farmers began using them in 1958. The Environmental Protection Agency (EPA) registered them as a pesticide in 1961.

We link to suppliers to help you find relevant products. If you buy through one of our links, we may earn a commission.

They can be used on foliage, food storage facilities, soil or water environments.

Why are these bacteria so widely used? They occur naturally, affect very specific insects and are relatively cheap and safe for humans, birds, fish and most beneficial insects.

However, for them to work effectively, you need to know what type of insect you want to target and make sure you purchase a species that kills that particular species.

The National Pesticide Information Center reports that there are more than 180 registered pesticide products containing Bt! And many are approved for use in organic farming.

We at Gardener’s Path cover the types of Bt that are commonly used. With thousands of species out there, we can’t discuss them all!

We’ll list the types you’re likely to encounter and describe how to use them.

What types of insects does Bt kill?

Each strain of Bt will kill different types of insects. Some strains are highly specific to a few species, while others kill a large class of insects. Listed below are the most commonly used types:

Since the insect must eat the poison to be affected by it, this means that insects that do not feed on the surface of plants are unlikely to come into contact with the poison and are therefore not affected by it.

Soil bugs, for example, are unlikely to eat Bt. Insects that bore quickly into plant tissue, such as corn earwigs, may not be exposed to enough Bt to be affected. And beneficial insects — grazing on other insects, not plants — are unlikely to be affected by the bacteria.

How does Bt kill insects?

Bt only kills insects that eat the poison and are susceptible to it. This is not a poison that kills on contact like most insecticides.

Once eaten, the poison affects a specific switch in the stomach, such as a lock and key. This switch (a receptor) activates a response that makes holes in the stomach. The contents, including the toxin and normal gut bacteria, pass into the rest of the insect’s body, and it dies from infection and starvation.

It may take hours or days for the insect to die. Usually the insect will remain on the plant as it dies, so don’t be alarmed if there are still insects on your plants even after treatment. They are probably on their deathbed.

The insects are usually most susceptible when the larvae are small.

How to kill caterpillars with Bt

Bacillus thuringiensis kurstaki (Btk) is the best known and most commonly used strain of Bt. It kills the larvae of many species of moths and butterflies.

This means you can fight most caterpillars with Btk. This includes many types of caterpillars known as “worms” that feed on broccoli, cabbage and other cruciferous plants.

Monterey Btk Liquid via Arbico Organics

Other caterpillars commonly controlled by Btk include the European corn borer, tent caterpillars, gypsy moths, and other wood caterpillars. Formulations have even been developed to control Indian mealworm larvae in stored grain.

Several Btk formulations are available from Arbico Organics.

Another strain of Bt, Bacillus thuringiensis aizawai (Bta), kills wax moth larvae in honeycombs.

How to kill mosquitoes and flies with Bt

Fortunately, there is a strain of Bt that controls the larvae of many stinging flies and mosquitoes! Bacillus thuringiensis israelensis (Bti) is widely used by governments or non-governmental organizations to control mosquitoes on a large scale.

For example, the World Health Organization switched to using Bti to kill mosquitoes as part of its anti-malaria campaign instead of using chemical pesticides.

AQUABAC® 200G Granular Bti Mosquito Repellent

You can buy Bti from Arbico Organics to kill mosquitoes on your property. Mosquito dunks are a very popular product for killing these creatures in your water garden or rain barrels.

Combi set for mosquito bites and dunks

Bti is very effective against Aedes and Psorophora mosquitoes. However, if you’re trying to kill Anopheles or Culex, you’ll need to use higher amounts of this insecticide.

As for flies, you can use Bti to kill blackflies, but not houseflies, blowflies, or stableflies.

You can also use some species to control fungus gnat larvae in greenhouses or in mushroom cultivation. In this case, you would apply the Bti as a drench to the soil (for plants) or growing media (for mushrooms).

Killing beetles with Bt

You may or may not know that beetles make up the largest number of species in the world, and there are over 350,000 of them! Fortunately, most are not garden pests.

However, the available Bt strains affect only a few species of beetles. However, some of them are major pests. The bacteria used to control beetle larvae are Bacillus thuringiensis san diego or Bacillus thuringiensis tenebrionis.

Beetles that can be controlled include the Colorado potato beetle, elm leaf beetle, and willow leaf beetle.

Tips for applying Bt

First, make sure you have the right strain for the pest you want to control! You also need to make sure the insect is in the larval stage when you apply the Bt.

You have better control when the larvae are small. There will be less damage to the plants because the small insects will eat less. If you treat larger grubs, they will eat more of the plants and do more damage. Also, they can pass into the reproductive phase and become desensitized to the toxin.

And avoid water with a high pH! A pH higher than 8 activates the toxin in the insect’s gut, and you don’t want it activated beforehand and then useless by the time you apply it.

The spray will be more effective if you add a spreader or sticker to the tank mix. Use the spray within 12 hours of mixing. Be sure to spray both the top and underside of the leaves.

Although it can survive in the soil for years when adsorbed to soil particles, Bt is quickly inactivated by the UV radiation in sunlight. Many people spray their plants in the evening so that the poison can work overnight before being inactivated by the sun the next day.

The bacteria are sensitive to temperature and should be kept at 50-60 F. Do not expose the bacteria to high or cold temperatures, which can kill them.

Take Precautions

Bt has been tested extremely well in studies of toxicity or its ability to cause cancer in animals – even at extremely high doses.

This microbe actually seems to be safe for humans – even if eaten. Human stomachs are acidic, not alkaline, so they digest the Bt toxin.

However, some products are used in the form of dust and there have been some cases of allergic reactions to the dust when the product was applied.

To be on the safe side, wear gloves, long sleeves and pants when applying the microbial insecticide. And then wash thoroughly.

Take steps to minimize resistance

Like any pesticide, the targets of Bt can develop resistance, although this rarely happens. The diamondback moth, a pest of cabbage and other cruciferous plants, is the prime example of an insect that has developed resistance in the field.

Many insects have shown resistance in the lab and are being studied.

If you use Bt on your farm, you should take some steps to prevent resistance, according to the EPA. One way is to alternate their use with synthetic insecticides.

Another is to rotate your crops. Since different types of insects feed on different crops, you would use a different type of Bt.

The wide variety of Bacillus thuringiensis strains available enables horticulturists, farmers and mosquito control experts to control a range of insect pests.

Since the strains are highly specific to the target insects, residual effects on other organisms are not a problem.

The widespread use of Bt has been a boon to farmers and to people who are now being sprayed with these bacteria to kill mosquitoes instead of DDT!