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Bacillus Thuringiensis (Bt): What Is It and How to Use it?

By: Eric Vinje Published: 3 Nov, 22

Bacillus thuringiensis (Bt) is a naturally occurring, soil-borne bacteria that has been used since the 1950s for natural insect control.

You have probably already heard a number of different people recommend using Bacillus thuringiensis, or Bt pest management, in your own backyard garden. But what exactly is it, and how does it work in the garden?

This article lays it all out, including tips to use it in your home garden as an organic pest control method.

What is Bacillus Thuringiensis (Bt)?

Bacillus thuringiensis, shortened to Bt, is a gram-positive bacterium that, during the process of sporulation, produces parasporal crystal proteins having insecticidal activity. These proteins are known as cry toxins.

Simply put, it consists of a spore, which gives it persistence, and a protein crystal within the spore, which is toxic. That toxic protein differs, depending on the subspecies of Bt producing it, yielding a variance of Bt toxic to different insect species (or none at all).

When the bacteria is consumed by certain insects, the toxic crystal is released in the insect’s highly alkaline gut, blocking the system which protects the pest’s stomach from its own digestive juices.

The stomach is penetrated, and the insect dies by poisoning from the stomach contents and the spores themselves. This same mechanism is what makes Bt harmless to birds, fish, and mammals whose acidic gut conditions negate the bacteria’s effect.

History of Bacillus Thuringiensis

A Japanese researcher who was looking into the fall in the number of silkworm moths made the initial discovery of application Bt in 1901. He linked the decline to the rod-shaped, Gram-positive bacterium.

A German scientist rediscovered it in 1911, and a solution of crystallized Bt toxins was shown to be particularly efficient against some crop pests such as corn borer, corn rootworm, corn earworm, and bollworms.

The product was initially used commercially as an insecticide spray in the United States in 1958, and numerous strains of the bacterium are now utilized to control a variety of agricultural insect pests and their larvae.

As a spray or, less often, as granules, Bt toxin can be used on crops like potatoes, corn, and cotton.

How Does Bt Work?

To be impacted, susceptible insects must consume Bt toxin crystals. Unlike toxic insecticides that target the nervous system, Bt works by generating a protein that hinders the digestive system of the insect, effectively starving it.

A Bt-infected insect will stop feeding within hours of ingestion and die within days, usually from starvation or a ruptured digestive tract, making it a fast-acting insecticide.

Each Bt strain is efficient against a specific group of insects, whether applied as a spray or through genetic engineering. The kurstaki or Btk strain of Bt, which is the one that is most commonly used, is only effective against particular kinds of caterpillars.

Bt in Genetically Modified Food Crops

Bt is also the source of the genes used to genetically engineer a variety of food crops so that they produce the toxin to repel certain insect pests.

Recently, B.t. has been questioned because of its inclusion in Monsanto’s genetically modified corn and cotton. The difference between the Bt used by organic farmers around the world and that genetically inserted into Monsanto’s corn is dramatic.

Naturally occurring Bt is contained within the bacterium. The Bt gene inserted into genetically-modified corn contains only the final toxin without its containment.

Bt has a short half-life when exposed to sunlight and the elements. By the time the insects that have consumed it are gone, so is the Bt. Its genetic counterpoint persists within the corn.

Insects have developed immunity to the genetically-modified Bt–containing corn when the GMO corn has, against the best agricultural practice, been planted in the same plot year after year.

Targeted use of Bt insect control products used on appropriately managed plots have not resulted in insect resistance.

Depending on which Bt strain is used, it continues to be effective on cabbage worms, tent caterpillars, potato beetles, mosquitoes, black flies, nematodes, and a variety of other insect pests.

Is B.t. Toxic for Humans?

Although harmful to certain insect species, Bt toxin is deemed low in toxicity to humans and other animals since they lack the digestive enzymes required to activate the Bt protein crystals whether used as an insecticide or consumed with GMO food crops.

However, every addition of additional genetic material has the potential to introduce allergies; as a result, some strains of Bt are not permitted for use in food consumed by humans.

Different Strains of Bacillus Thuringiensis

The toxin kills several orders of insects, such as Lepidoptera (butterflies, moths, and skippers), Diptera (flies), and Coleoptera (beetles). However, there are a number of Bt strains that can be used to kill specific insects. Let’s look at these in more detail:

Bt kurstaki (Bt-k) – Bacillus thuringiensis var kurstaki

Bt-k is a naturally occurring soil bacteria ideal for controlling tent caterpillars, gypsy moths, cabbage loopers, tomato hornworms, and other leaf-eating caterpillars on trees, shrubs, tomatoes, and other vegetables.

This strain of B.t. is most effective when applied to caterpillars during their 1st and 2nd instars when they are still small. It must be ingested by the insect, as it is a stomach toxin. Plus, it’s harmless to humans, animals, and beneficial insects.

Bt-k biodegrades quickly in sunlight and may require reapplication under heavy insect pressure. To maximize effectiveness apply in the late afternoon. Several vendors offer Bacillus thuringiensis kurstaki that is approved or use in organic production.

Bt israelensis (Bt-i) – Bacillus thuringiensis var israelensis

Bt-i is a highly specific biological pesticide for use against mosquitoes, black flies, and fungus gnat larvae.

It may be applied safely to irrigation and roadside ditches, pastures, marshes and ponds, water gardens, flower pots, bird baths, rain gutters…any place there is standing water!

Once ingested, Bt-i kills 95-100% of mosquito larvae within 24 hours. It is highly effective because it kills these pests before they become biting adults. Fortunately, it doesn’t harm people, pets, wildlife, or fish. Mosquito Dunks is a commercial form of Bt-i.

Bt san diego (Bt-sd) – Bacillus thuringiensis var san diego and Bt tenebrionis (Bt-t) – Bacillus thuringiensis var tenebrionis

The Colorado potato beetle has developed unprecedented resistance to multiple applications of chemical insecticides. Bt-sd and Bt-t are toxic to a limited range of leaf-eating beetle species and are now considered to be the most effective control for this destructive insect pest.

It can also be used to control the elm leaf beetle and may be used on potatoes, eggplant, tomatoes, and elms. These biological pesticides should be applied to the young larval stages, as they have no effect on adult beetles. This strain is safe for people, pets, wildlife, and fish.

Bacillus popilliae

The milky spore disease of the Japanese beetle was the first microbial control to be developed commercially.

Milky spore is the name of the disease to which Japanese beetle larvae succumb when attacked by Bacillus popilliae. The bacteria spreads naturally as each infected beetle larvae dies, decomposes, and releases billions of new spores into the soil.

Time must be allowed for this process to completely saturate a treated area, but only one application is required and the spores continue to multiply on their own, as long as larvae are present.

When there is no longer a grub infestation, the spores remain dormant waiting for subsequent populations.

How to Use Bacillus Thuringiensis for Effective Pest Control (Top Tip for Home Gardeners)

Now that you know what Bacillus thuringiensis is, you might think that Bt pest control is your only option. However, there are a few things you should know about Bacillus thuringiensis products before you start using them:

Be sure to check the label before using it. If you don’t have the pests that Bt kills, you don’t need to use it in the garden. Bacillus thuringiensis products are quite selective in terms of which insects they will or will not kill.
There is always the risk of insects developing immunity to any pesticide, whether it be synthetic or natural, and you don’t want to exacerbate the problem by using too much of either.
Bt only affects insects that consume it, so spraying your corn crop after the larvae have made their way inside the ear will be ineffective.
Since timing is everything, it’s important to not spray the moths or eggs, but simply the leaves that the larvae will devour.
Be warned that starving can take days for those specific insects that do consume the Bt substance. For this reason, most gardeners who have only used chemical pesticides in the past aren’t convinced that Bt pest management is effective since the insects can still be seen roaming around after being treated.
Since Bacillus thuringiensis compounds are easily degraded by sunshine, spraying the garden in the early morning or late evening is preferable.
The majority of B.t. compounds have a brief persistence on the plant’s leaves, typically lasting less than a week after application and decreasing in length when exposed to rain, a sprinkler system, or overhead watering.
The best place to keep Bt pest control solutions is a cold, dark area because they have a shorter shelf life than most chemical insecticides.
It’s better to only purchase what you can utilize in one season, even if brands state that the product can last up to 2 to 3 years. This time frame is even shorter for liquid applications.
Bt pest management may be an option to think about if any of the susceptible insects are infesting your yard. Bacillus thuringiensis pest control can be an effective and environmentally safe approach to treating your yard. Understanding what Bacillus thuringiensis is and how and when to employ it is critical to its success.
Considering Bt crops only kill a certain subset of the insects that feed on them, supplemental insecticides are frequently necessary in order to protect plants from the destruction of a wider variety of plant-eating insects.

There you have it – that’s everything you need to know to effectively use Bacillus Thuringiensis (Bt) in your home garden effectively.