Bacillus thuringiensis, or BT, is a naturally occurring bacteria used as a pesticide for various pests that attack gardens. It’s commonly used for caterpillars and mosquitoes, which are arguably two of the most annoying pests in the garden. You can find BT in many forms, including granules, sprays, and dunks. BT spray is one of the more common varieties available for home gardeners or commercial use.

There are a few different BT strains that can target specific pests, so it’s wise to keep a few different kinds on hand. BT 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 BT?

BT spray
BT spray, powder, or granules are common organic pesticides. Source: OSU Master Gardener

Bacillus thuringiensis is a bacteria that can be found in soil. There are several different species that conveniently target different insects but remain harmless toward others. Although it can be 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 be found as the active ingredient in a number of organic pesticides.

Bacillus thuringiensis (BT) spray is the most popular form because it’s easy to use. Handheld bottles are available for backyard gardeners or can be sprayed on a large scale via aerial application. Other forms are available, such as powder, granules, or cakes you can 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?

BT kills insects by destroying their intestines once it’s ingested. If a caterpillar eats BT, the bacteria creates a protein toxin that causes the gut to break down, 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. Bacillus thuringiensis var. israelensis, or BTi, will kill fungus gnat larvae in soil and mosquito larvae in water.

Each BT toxin is effective against certain insects because of the highly specific proteins that are 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 because they have to eat the BT toxin to get killed by it. 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 BT

BT through a microscope
A view of a BT strain (in this case 4A4) at 1000x magnification through a microscope. Source: BlueShift 12

Bacillus thuringiensis var. israelensis (BTi) and Bacillus thuringiensis var. kurstaki (BTk) are the most common forms of BT control methods 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)

BTi can be used to kill black flies, mosquitoes, and fungus gnats. It can be used 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. You can 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 so you can safely use it on plants. 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, a granular form also can be used to cover the surface of your houseplant potting mix to reduce the frequency of infestation.

Bacillus thuringiensis var. kurstaki (BTk)

BT kurstaki (BTk) is effective at eliminating 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 that is in its egg or adult stages. If you have a butterfly garden, you shouldn’t apply this in that area during the butterfly season, as it will kill butterfly caterpillars just as easily as the pest moth caterpillars.

BTk is most commonly found in liquid and granules. You can apply it directly to growing medium, trees, and plants where you find the caterpillars.

Bacillus thuringiensis var. san diego

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

Bacillus thuringiensis var. tenebrionis kills the same beetles as the san diego variant. Just as with other types of BT, it’s safe to use around people and animals, so you can use it in your garden as needed.

Bacillus thuringiensis var. aizawai (BTa)

BTa is used to treat the grape berry moth and the wax moth, which is a moth that nests in abandoned beehives. The moths can multiply quickly, so utilizing BTa is a wise choice.

BTa can potentially hurt honeybees, so you should be cautious of how you use it. In a study published in 2010, it was 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

There are other forms of BT that are still being researched. For instance, a strain currently known as Bacillus thuringiensis 4A4 is being currently studied as it 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, but they need to be thoroughly studied before any safe-to-use products are developed and marketed. As scientists learn more about BT and its many variations, we will see additional potential uses over time.

Benefits Of Using BT Spray

Tobacco hornworm
Hornworms (like this tobacco hornworm) can be treated with BTk. Source: Gardening Solutions

The obvious benefit of using a Bacillus thuringiensis spray is that you can stop mosquitoes, caterpillars, and beetles from damaging your garden by killing the larvae before they grow into adults. BT is natural so you don’t have to worry about being exposed to chemical pesticides in your garden. It’s better for the environment and your health when you can stick to natural methods.

Although BT isn’t toxic to people, you can be assured that you won’t be exposed to it for long because exposure to sunlight causes it to disperse. This means there won’t be any long-term residue on your plants, and it won’t stay in the water. Data suggests that it remains present and as an active control on plants for up to 22 days. Acidic soil also speeds up its dispersal, so it won’t be left behind in your soil. BT doesn’t pollute groundwater or soil, so it’s a harmless control and treatment for the environment that you can feel good about using.

BT is only highly toxic to the larvae affected by the toxins it creates, so 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 Of Using BT Spray

One of the very few issues that come with using BT spray is that some forms, such as Bacillus thuringiensis var. aizawl (BTa) can be harmful to bees and possibly other beneficial insects like wasps. It’s only harmful if consumed or if the spray is sprayed directly onto the insect. This can be easily avoided by spraying when the bees aren’t active, such as the late afternoon when they’ve returned to their hives. Once dry, this is not harmful to your beneficials.

Many caterpillars are harmed by Bacillus thuringiensis var. kurstaki (BTk) which will be an issue if you have a butterfly garden that’s meant to attract butterflies. 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 adult butterflies.

That brings us to the issue of BT being highly dependent on 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 insect 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 as chemical insecticides do. It can take a few days for the larvae to die of starvation, so some patience will be required to use Bacillus thuringiensis products.

How To Use Bacillus Thuringiensis

Closeup of dyed bacillus thuringiensis
Another closeup look at Bacillus thuringiensis. Source: TheRubinLab

Applying Bacillus thuringiensis products to your garden is easy. Liquids are available as ready-to-use formulas or as concentrates. For the ready-to-use options, you simply have to spray it on your plants or soil. Be sure to follow the directions on the package to make sure you apply it to the right areas.

Concentrates need to be added to water and are generally intended to be used on larger areas. You have to add the liquid to plain water and mix it before applying it for this type of BT spray control. Read the label to find out the ratio of how much liquid to water. It may vary between brands, but a little goes a long way, so you’ll likely only need a few teaspoons per gallon.

Liquids will need to be applied regularly since BT disperses quickly in the sunlight. Once a week to every two weeks should suffice for as long as you notice pests. If it rains, you’ll need to reapply again once it dries out, as it will be rinsed off leaves. Apply BT in the early morning or late evening to avoid exposure to direct, strong sunlight while you apply it. Afternoon treatment with this control is generally easier as there’s a lessened risk to pollinators.

When you apply BT, be sure to coat the entire area you’re finding 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 the toxicity to come into play, so spray whichever part of the garden you find an insect.

BT powder pesticides work in a similar way, but 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. You may find that 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 since they’re little pebbles of BT that won’t stick to plants. For powder and granules, you’ll need to reapply when it’s gone or after it rains.

Some BT products come in the form of cakes, which are best known as Mosquito Dunks. Mosquito dunks are meant to float in standing water, and they’re safe to use in ponds with fish or in livestock troughs. Mosquito larvae living in the water will eat it and die before they grow to be adults. Add another dunk once it’s used up. They should last about a month before they need to be replaced.

Frequently Asked Questions

Q: Is BT spray safe?

A: BT is an insecticide that occurs in nature and is completely safe to use on all plants. BT is a bacteria that 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.

Q: Is BT toxic to humans?

A: 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. Once dry, its toxicity is limited solely to the particular genus of insects that consume the organic pesticide. It’s popular in agriculture as one of the safest organic control methods available to the general public.

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



BT Spray: Useful Bacterial Organic Pesticide



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