Bacillus Thuringiensis (Bt): What it is and How to Use it

Bacillus thuringiensis is a commonly used organic pesticide. But what exactly is Bt? If you're having an issue with caterpillars, mosquitos, or other bugs in your garden, we provide insight into these useful soil bacteria!

Close-up of a blue gloved hand holding a plastic bottle of Bacillus thuringiensis.

Contents

Bacillus thuringiensis (Bt) is a naturally occurring bacteria used as a pesticide. It’s commonly used for caterpillars and mosquitoes, arguably two of the most annoying pests in the garden. You can find Bt in many forms, including granules, sprays, and dunks.

There are a few different strains that target specific pests, so it’s wise to keep a few different kinds on hand. Bacillus thuringiensis is considered safe to use around people and pets, and some insect species can’t be harmed by it. There are only a few concerns regarding its safety, but most concerns can be worked around or avoided while applying it to your garden.

We’re going to dig into the details of Bt so you can learn how to use this natural wonder in your garden effectively.

What Is Bacillus Thuringiensis?

A person in protective clothing walks through a vast green field, using a backpack tank with a long hose to apply liquid to the lush, healthy rows of plants.
It’s produced commercially with mediums such as fishmeal, soybean meal, and other ingredients.

Bacillus thuringiensis is a bacteria found in soil. It’s produced commercially with mediums such as fishmeal, soybean meal, and other ingredients to increase the production of the spores. It can also be found as the active ingredient in a number of organic pesticides.

There are several different species that conveniently target different insects but remain harmless toward others.

Spray is the most popular form because it’s easy to use. Handheld bottles are available for backyard gardeners, but it’s also sprayed on a large scale via aerial application. Other forms are available, such as powder, granules, or cakes you put in water. Sprays are good for foliage, powders and granules for soil, and cakes for water.

Always read the instructions on the label to make sure you use your particular brand correctly.

How Does Bt Kill Insects?

Several black-and-white-striped caterpillars line up in perfect rows along the veins of a large, vibrant green leaf, creating a striking pattern against the smooth surface.
If a caterpillar eats Bt, the bacteria creates protein toxins (Cry and Cyt toxins) that kill cells.

Bt kills insects by destroying their intestines once it’s ingested. If a caterpillar eats Bt, the bacteria creates protein toxins (Cry and Cyt toxins) that kill cells, eventually leading to an infection. It can also cause insects to starve due to their intestines getting damaged or paralyzed, as is the case with caterpillars that eat Bacillus thuringiensis var. kurstaki, or BTk.

Each Bt toxin is effective against certain insects because of the highly specific proteins in those strains. It won’t affect birds or other mammals, nor hurt human stomachs.

Most Bt won’t hurt the intended insects if it just lands on their bodies. They have to eat the toxin for it to be effective. A notable exception is spraying larvae with liquid forms, as they may absorb some of it through their skin if thoroughly coated. Still, this control method is far more effective when consumed.

Types Of Bacillus Thuringiensis

A bright green cylindrical tank with a black lid sits on the ground among dense, leafy crops, surrounded by lush greenery under the warm glow of sunlight.
There are two common forms available.

Bacillus thuringiensis var. israelensis (Bti) and Bacillus thuringiensis var. kurstaki (Btk) are the most common forms available, but they’re not the only ones. Let’s take a look at some of the different types of Bt and what they can be used for.

Bacillus thuringiensis var. israelensis (Bti)

A gardener wearing gloves sprays a leafy plant with an orange bottle, the dark green leaves standing out against a wooden fence in a backyard garden, with droplets misting over the foliage.
Since it’s not toxic to humans and animals, you can safely apply it to pools, rain barrels, and other areas.

Bti is used to kill black flies, mosquitoes, and fungus gnats. It is effective on plants, soil, or standing water to kill the insect larvae before they reach their adult stages. Since it’s not toxic to humans and animals, you can safely apply it to pools, rain barrels, and other areas that harbor annoying mosquitoes. Also use it on houseplants to keep the flies away.

Bti comes in many forms, including liquid, granules, dunks, tablets, pellets, and briquettes. It won’t leave behind any residue. Once floating on the surface of the water, mosquitos will not touch down to lay their eggs, so it’s a very effective control for those populations. For flies or gnats, cover houseplant soil with the granular form to reduce the frequency of infestation.

Bacillus thuringiensis var. kurstaki (Btk)

A translucent bottle with a black handle is being held near a raised garden bed filled with leafy green plants, next to a wooden frame and flourishing vegetables.
If you have a butterfly garden, you shouldn’t apply this in that area during the butterfly season.

Bt kurstaki (Btk) eliminates caterpillars. It handles a wide array of species, including (but not limited to) gypsy moths, tomato hornworms, cabbage worms, cabbage loopers, cutworms, and leaf rollers. It kills young caterpillars in the feeding stage and won’t kill anything in its egg or adult stages.

If you have a butterfly garden, you shouldn’t apply this in that area during the butterfly season. It will kill butterfly caterpillars just as easily as the pest moth caterpillars.

Btk is most commonly found in liquid and granules. Apply it directly to growing medium, trees, and plants where you find the caterpillars.

Bacillus thuringiensis var. san diego

A yellow spray bottle is being used over large, broad, green leaves with white speckles in a garden bed next to bright yellow squash nestled among the foliage.
It’s incredibly effective against these specific beetles, but it can’t work for every type of beetle.

Bacillus thuringiensis var. san diego is used to kill the Colorado potato beetle, cottonwood leaf beetle, elm leaf beetle, boll weevil, black vine weevil, and leaf-feeding beetle. It’s incredibly effective against these specific beetles, but it can’t work for every type of beetle.

Bacillus thuringiensis var. tenebrionis

A person holds a tube-like device pointed at a tall, spiky green plant with tiny white flowers, in front of a blurred background of a fence and a building.
As with other types of Bt, it’s safe to use around people and animals.

Bacillus thuringiensis var. tenebrionis kills the same beetles as the san diego variant. As with other types of Bt, it’s safe to use around people and animals.

Bacillus thuringiensis var. aizawai (Bta)

A person wearing gardening gloves holds a small orange container while tending to large, leafy plants with thick green stems and wrinkled, curled leaves.
Bta can potentially hurt honeybees, so you should be cautious about how you use it.

Bta is used to treat the grape berry moth and the wax moth, a moth that nests in abandoned beehives. The moths can multiply quickly.

Bta can potentially hurt honeybees, so you should be cautious about how you use it. A study published in 2010 discovered that BTa mixed in sugar water was harmful to bees once ingested, but it didn’t seem to cause harm when bees came into contact with it via pollen.

Other Forms

A gloved hand directs a white bottle toward a field filled with dense, leafy green plants bathed in natural sunlight, surrounded by a wild and open landscape.
Bacillus thuringiensis 4A4 appears to be extremely toxic to nematodes like the root-knot nematode.

There are other forms of BT that are still being researched. For example, Bacillus thuringiensis 4A4 appears to be extremely toxic to nematodes like the root-knot nematode. We don’t know every strain of this soil bacteria as of yet, and more are still being discovered. However, they need to be thoroughly studied before any safe-to-use products are developed and marketed.

Benefits

Several small, reddish-black insects crawl across a large green leaf covered in droplets of moisture, with torn edges and dark patches scattered around.
It stops mosquitoes, caterpillars, and beetles from damaging your garden.

The obvious benefit of using a Bacillus thuringiensis spray is pest control. It stops mosquitoes, caterpillars, and beetles from damaging your garden by killing the larvae before they grow into adults. It’s also suitable for use in organic gardens.

Although Bt isn’t toxic to people, you can be assured that you won’t be exposed to it for long because sunlight causes it to disperse. This means there won’t be any long-term residue on your plants and it won’t stay in water.

Data suggests it remains present and as an active control on plants for up to 22 days. Acidic soil also speeds up its dispersal. Bt doesn’t pollute groundwater or soil, so it’s a less harmful control and treatment for the environment.

Bt is only highly toxic to the larvae affected by the toxins it creates. That means you, children, pets, and most beneficial insects won’t be in danger when you use Bacillus thuringiensis products. Simply wash any produce before use and any Bt that was present will be rinsed away.

Drawbacks

A cluster of tall green stalks with fuzzy, wheat-like heads stands in a field, with small dark insects clinging to the stems under the soft, golden light of the sun.
Avoid this by spraying when the bees aren’t active, such as in the late afternoon when they’ve returned to their hives.

One of the few issues that come with using Bt is that some forms (such as Bta) can be harmful to bees and possibly other beneficial insects like wasps. It’s only harmful if consumed or sprayed directly onto the insect.

Avoid this by spraying when the bees aren’t active, such as in the late afternoon when they’ve returned to their hives. Once dry, it is not harmful to your pollinators.

Many caterpillars are harmed by Bacillus thuringiensis var. kurstaki (Btk) which can lower butterfly populations. Avoid spraying the garden if you don’t want to harm those butterflies. Keep in mind that BT only kills young caterpillars, so the beneficial butterflies won’t have as much of a risk if you’re able to spray the garden once all the beneficial larvae have matured into adults.

That brings us to the issue of timing. Unlike chemical insecticides, Bt has to be applied at the right time. This control method won’t kill eggs or adults, so you have to apply it when the pests such as caterpillars or other larvae are actively feeding. If you’re new to gardening or haven’t closely studied the habits of the insects in your area, it can take some practice to get it just right. 

Something else to consider is that Bt doesn’t kill right away. It takes a few days for the larvae to die of starvation, so some patience is required.

How To Use Bacillus Thuringiensis

A person wearing tall boots and holding a black hose tends to rows of healthy, leafy plants in a field at sunset, with the vibrant green crops glowing in the warm light.
It’s important to read the packaging of your chosen product carefully before applying.

As there are different forms, it’s important to read the packaging of your chosen product carefully before applying. Follow these general guidelines to get it right.

Preparation

A worker in a large, open field wears protective gear and holds a long rod pointed at tall plants, as the golden light of the setting sun bathes the crops and horizon.
Concentrates need to be added to water and are generally used on larger areas.

Liquids are available as ready-to-use formulas or as concentrates. For the ready-to-use options, simply spray it on your plants or soil. Follow the directions on the package and apply it to the right areas. 

Concentrates need to be added to water and are generally used on larger areas. Add the liquid to plain water and mix it before applying. Read the label to determine the ratio of liquid to water. A little goes a long way, so you’ll likely only need a few teaspoons per gallon. 

Liquids need to be applied regularly as Bt disperses quickly under sunlight. Once a week to every two weeks should suffice for as long as you notice pests. If it rains, reapply again once it dries out.

Application

A yellow container is held by a gardener near a collection of white, pink, and yellow flowering plants, with lush green stems and leaves illuminated by soft sunlight.
Late afternoon to evening treatment is generally easier as there’s a lessened risk to pollinators.

Apply in the early morning or late evening to avoid exposure to direct, strong sunlight while you apply it. Late afternoon to evening treatment is generally easier as there’s a lessened risk to pollinators.

When you apply, coat the entire area where you find caterpillars, mosquitoes, or beetles. Cover the tops and bottoms of leaves, stems, and the soil around the plant. The pests have to eat the Bt for it to work.

Powder pesticides work in a similar way, just in a different form. Squeeze out some powder directly where you have an affected area, or use a duster to puff it lightly onto foliage. This pesticide type works best for ears of corn or on the soil surface, but you can still use it wherever needed.

Granules are best used in the soil as they won’t stick to plants. For powder and granules, reapply after it rains.

Some products come in the form of cakes, typically to kill mosquitos. They float in standing water and are safe to use in ponds with fish or livestock troughs. Mosquito larvae living in the water will eat it and die before they grow to be adults. Add another once it’s used up. They should last about a month before they need to be replaced.

Frequently Asked Questions

Is Bt spray safe?

Bt is an insecticide that occurs in nature and is safe to use on all plants. The bacteria is only toxic to certain insects. They don’t cause harm to people, mammals, or fish, so you can use them anywhere in your garden.

Is Bt toxic to humans?

BT has no toxicity to humans and isn’t known to cause any severe health problems or long-term disease. It’s completely safe to use, but if you’re concerned about using it, avoid exposure to the area you applied it to for up to thirty minutes to allow the BT to settle and dry. It’s popular in agriculture as one of the safest organic control methods available to the general public.

The National Pesticide Information Cesaranter has noted that some people with hay fever have experienced a few symptoms such as nose and throat irritation after aerial spraying in farm regions. However, it is undetermined if the reported symptoms are related to seasonal pollen or Bt exposure. If you are sensitive to pollen, wear a mask and gloves to avoid being exposed during application.

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