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

Spraying Garden Plants

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:

  1. 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.
  2. 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.
  3. 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.
  4. Since timing is everything, it’s important to not spray the moths or eggs, but simply the leaves that the larvae will devour.
  5. 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.
  6. Since Bacillus thuringiensis compounds are easily degraded by sunshine, spraying the garden in the early morning or late evening is preferable.
  7. 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.
  8. 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.
  9. 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.
  10. 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.
  11. 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.

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44 Responses to “Bacillus Thuringiensis (Bt): What Is It and How to Use it?”

  1. Maxwell Sandford on March 17th, 2013 at 12:10 am #

    No link or supplier is listed for Bacillus thuringiensis var. san diego.
    Is this strain available for home gardeners?

  2. pnatural on March 17th, 2013 at 9:42 am #

    Maxwell –

    At one time we did offer a product containing Bacillus thuringiensis var. san diego for home gardeners. However, it wasn’t very popular and was eventually discontinued by the manufacturer. Spinosad is sold as an effective replacement for the organic control of Colorado potato beetle.

  3. Deb on April 4th, 2013 at 1:15 pm #

    Do you have an alternative OMRI approved product for control of fruit tree caterpillars?

    • pnatural on April 5th, 2013 at 4:09 am #

      Deb –

      Monterey Garden Insect Spray contains spinosad, an OMRI listed biological pesticide for use on fruit trees.

  4. Tom G on April 30th, 2013 at 11:39 am #

    i used to get bt san diego in a slurry and i would dilute it and inject into my zucchini plants to control borers and i would get enough zucchini’s to feed the neighborhood, now i hav’t gotten 10 zucchini’s in 3 yrs and well over 90 plants, they die before i get any food, is there anything i can use now i havn’t found any bt in 3 years, i want to eat zucchini’s again

    • Tom G on June 22nd, 2014 at 8:17 pm #

      OK I guess im the only one with a squash vine borer problem I an gonna have to dig up the 8 plants I have and destroy then plant more maybe I’ll get a dozen zucchinis this year, cant believe there is nothing available anymore to help this problem

  5. abolghasemi.roya on October 20th, 2013 at 3:08 am #

    I quite like the production of Bt products. How Bt toxin is produced? Please guide me.

  6. Ric Sanders on June 6th, 2014 at 9:45 pm #

    Is Bacillus thuringiensis effective in the control of iris bore, Macronocture onusta?
    Or is some other form required?

  7. Bird on May 22nd, 2015 at 7:15 pm #

    Spinosad is toxic to bees whereas BT sd is not. Please do not recommend Spinosad and bring back the BT sd.

  8. Ekta on June 16th, 2015 at 7:12 pm #

    Hello, I want to get a list of Bti products in market worldwide. I need to submit this data in my project report and I searched a lot but couldnt get. Can you please send me the list of current Bti products in market?

    • Sofia Villalaz on October 18th, 2016 at 10:16 am #

      I want a list of BTi products for use in US Zone 7b, against sawflies and other rose skeletonizing bugs.

      • E. Vinje on October 18th, 2016 at 10:20 am #

        Hi Sofia –

        Bt-i will not work on sawflies, but Monterey® Garden Insect Spray (Spinosad) will. Here’s a link that should help:


        Good luck!

        • Joe on May 11th, 2019 at 2:10 pm #

          Spinosad kills bees, however.

          • kygardener on July 14th, 2019 at 1:04 pm #

            Spinosad will kill bee’s, but if used in the recommended way which is 2 hours after the sun goes down it should have a very minimal if any effect at all on bee’s.

  9. Rukhsana hussain on October 7th, 2015 at 12:23 am #

    I have the powder form of bacillus thuringiensis, can someone please how I can use this on my plants, I have no instructions.

  10. Gretchen Roche on December 4th, 2015 at 4:16 pm #

    Hello I am so happy to see that this info.!! Could this bacteria possibly be applied to pets or in the home ( we live in an apt) to solve a problem with ticks? Here in Charlotte NC they are quite a problem! I am looking for an organic solution. Oh please can you offer an organic solution? We are using diatomaceous earth and Wondcercide with limited success.
    A natural predator, disease, or other biological for any solution would be wonderful.
    Thank you for any suggestion!

    Gretchen Roche

    • E. Vinje on December 4th, 2015 at 4:25 pm #


      Have you seen our How to Get Rid of Ticks page?

      Hope it helps!

    • DW Pranger on September 25th, 2018 at 11:26 pm #

      Diatomaceous earth should solve your flea and tick situation organically, without harm to pets.

  11. Diana on January 24th, 2016 at 11:16 am #

    Could I use “mosquito dunks” crumble (a small amount) in houseplant soil to get rid of Nat’s larvae? I must have over watered and just today, uprooted all my houseplants and spread them out in the sun, cleaned or changed all pots and plan to re-pot with fresh “moisture control ” soil. I have sprayed the roots with a combination of water, peppermint tea, cinnamon & sesame oil. What do you think?

  12. Mike Kennedy on February 24th, 2016 at 4:49 pm #

    I read I can get ride of the fruit fly type things that grow on my mushrooms I raise in the house with a solution of Bti if I spray it on them. Do have it in liquid form? Can it be sprayed directly or does it need to be diluted?

  13. Linda on March 7th, 2016 at 1:04 pm #

    Is Bt-K toxic to bees??

    • E. Vinje on March 7th, 2016 at 1:50 pm #

      It is not.

  14. Tammie on March 19th, 2016 at 9:49 am #

    Milky Spore has been discontinued in this area? It worked great controlling the moles. We applied it for a few consecutive years will that be enough to control beetles, grubs and moles and how long will it last? Is there another alternative and how often should it be applied? Thanks!

    • E. Vinje on March 19th, 2016 at 4:13 pm #

      Tammie –

      Milky Spore 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 (2 to 4 years), but only one application is required as the disease continues to multiply on its own, as long as larvae are present. When there is no longer a grub infestation, the disease remains dormant, waiting for subsequent populations.

      Milky Spore can be purchased here.


  15. Sue McGrady on June 10th, 2016 at 8:19 am #

    Will the Miky Spore effect butterflies?

    • E. Vinje on June 10th, 2016 at 8:24 am #

      Hi Sue –

      Milky Spore (Bt-sd) will NOT harm butterflies… just Japanese beetles!

  16. william storm on August 7th, 2016 at 11:14 pm #

    A lady gave me some variegated blue agave plants years ago at our old house. I brought them with me to our new house in the year 2000. Agave beetles infested the plants in the last few years and killed all of the baby Agaves coming up from the base of the main two plants. Recently they both bloomed, which killed the plants, so I pulled them up but kept them to see if the seed pods they set would develop. To my surprise the formed baby plant-lets on the blossom spike!! But the beetles also killed a “volunteer” yucca plant, which came up in my hedge / flower bed and bloomed twice a year for several years. I found the larvae in the base of the plants after they died. Some kind of boring insect killed of my apple trees and several rose bushes. I think they are called cane borers. Can you recommend products for the agave beetles and the cane borers on the roses?

  17. jim Rogers on July 15th, 2017 at 7:51 am #

    We think we have mealy bugs on our ornamental plants (hibiscus, Azazel, firecracker plant). The areas affected are usually where the leaves are joined to the branch. What we see is what looks like a white fluffy powder, but when touched, it becomes a clear liquid. We haven’t been able to isolate any individual bugs from this. What can we use to combat this. It is spreading very quickly among our plants. Thanks…

    • CaseG on September 6th, 2019 at 9:01 am #

      Spray your plants with a hard water stream daily or every other day. Are there ants present? Often times ants will farm scale and mealybugs all over the plants helping them spread. Getting rid of the ants and blasting off the scale or mealy bug can take care of the problem. Ants also protect pests from predators that would normally eat or kill the scale or mealybugs. Most oil based garden sprays will work neem oil being one. Cheers

  18. Randy on July 31st, 2017 at 6:57 pm #

    What’s best to use for black gnat fly aphids? Soil born and eating my root system? It’s causing a lot of problems for me. Please respond if possible.

  19. Kathy on September 6th, 2017 at 6:39 am #

    We have a collection in an Archives in South Africa which has been infested with Book Beetles and mold please could you recommend one of your products.

  20. Eshwar on February 18th, 2018 at 6:58 am #

    I’m looking for organic products for mosquitos to kill in paddy fields.

  21. Jocelyn on July 25th, 2018 at 7:21 pm #

    I live in an arid climate but need to kill a severe infestation of earwigs which feed on my roses and clematis, particularly on young clematis vines. Sluggo plus and some other products contain Spinosad but you have to reapply after rain or watering so doesn’t seem to be practical. I use small cat food cans to contain oil etc which is effective but limited as I have to pick up before/after water and becomes time consuming. Also, I need to use mulch for water retention so creating an arid environment is great but not of much use for me.

    I am wondering if applying a bt spray such as Monterey bt would be effective. Will earwigs eat the leaves and die especially if I begin using early in the season. I currently use a Bonide systemic product to control western flower thrips but haven’t evaluated it in terms of earwigs. It does not appear to control the earwigs but does control the thrips.

  22. Chandler on September 14th, 2018 at 9:37 am #

    I was wondering what spray would be beneficial to control spider mites?

  23. Tim R on February 22nd, 2019 at 7:37 am #

    I am searching for a method to calculate the amount of a Bti product to apply to a flowing stream in order to reach a concentration of 0.75 mg/L or higher, for the control of black flies. This will be done in accordance with and with approval of the State regulatory agency. would you be able to point me in the right direction?

  24. Albion E on March 17th, 2019 at 2:16 pm #

    I’m looking for BTi to control coffee borer beetle in Organic Coffee. Do you have BTi in a dry powder form?

  25. Denny Vearrier on March 22nd, 2019 at 6:57 pm #

    I am trying to control bagworms. Which product do you recommend?

    • E. Vinje on March 23rd, 2019 at 2:47 am #

      Bt kurstaki (Bt-k) for leaf eating caterpillars on trees, shrubs, tomatoes and other vegetables.

  26. Audra on June 21st, 2019 at 3:39 pm #

    Looking for the correct BT to get rid of June bugs.

  27. Grant Erickson on December 20th, 2019 at 9:07 pm #

    Will BTI harm beneficial nematodes? I have a serious infestation of fungus gnats. It was suggested I try both treatments. Is one better than the other?

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