Where to buy bt?

How to Use Bt Pesticide as an Organic Pest Control

Which Pests Does Bt Control?

Most caterpillars seen eating leaves can be controlled by Bt when applied at the proper time. In vegetable gardens, armyworms, cabbage worms, diamondback moths, melon worms, corn earworms, green cloverworms, pickleworms, tomato fruitworms, tomato hornworms, grape leafrollers, grapeleaf skeltonizers, salt marsh caterpillars, and various webworms and budworms are candidates for treatment with Bt.

How to Use Bt

Sunlight degrades Bt after a few hours, so it is best applied late in the day so it can be consumed during the nightly feeding. Keep in mind that your objective is to place the substance where the caterpillars will eat it. In the case of corn earworms, this means squirting the Bt solution into the tips of young ears of corn. When using Bt to control leaf-eating pests, repeat treatment every seven to 10 days, or until it is no longer needed.

Always follow label directions for diluting concentrated products of Bt and other natural pesticides. Some Bt products include genetically modified strains; products listed by the Organic Materials Review Institute (OMRI) include only naturally occurring forms.

How to Store Bt

Mix only as much concentrate as you will need. If not used within a few days, dispose of unused solution by diluting it with water and pouring it out in a sunny spot. Store Bt pesticides in their original containers on a high shelf, out of the reach of children or pets, in a cool place where temperatures will not exceed 100 degrees Fahrenheit. Under good storage conditions, powdered or other dry Bt products may last five years, while liquid products should be replaced after two to three years.

Grape leaffolder caterpillars munching away on our young vineyard.

If caterpillars are devouring your garden, one safe and effective way to get rid of them is by applying the naturally occurring bacteria known as Bt, or Bacillus thuringiensis. Bt can be applied as a liquid or powder, and many of the varieties are certified organic.

Here’s what you need to know about using Bt to control caterpillars in your garden.

About Bt (Bacillus thuringiensis)

Bt (Bacillus thuringiensis) is a natural bacteria that is commonly found in soil. When eaten by caterpillars, the bacteria produce proteins that paralyze the caterpillar’s digestive system, which causes them to stop feeding and die.

Bt is safe for use in organic gardens because it has a specific target and is nontoxic to humans, animals, and beneficial insects. It’s approved for use in vegetable gardens up to the day of harvest, though all vegetables and herbs should still be thoroughly washed before eaten.

Certified Organic Bt

While Bt is a natural bacteria, not all commercial Bt products are certified organic. Some products are made with genetically modified Bt (which is not allowed in organic gardens), and many of the liquid forms are mixed with a non-organic solvent. If you want organic Bt, be sure to look for the certification stamp of the Organic Materials Research Institute (OMRI). Certified organic Bt is easiest to find in powdered form.

Types of Bt

There are a variety of Bt products available, from powders to baits to liquid concentrates. Different strains of Bt are also available for different purposes, such as:

    Liquid Bt concentrate.

  • Bt (var. kurstaki): This is the most common strain of Bt. It kills leaf and needle eating caterpillars such as bagworms, cabbage worms, gypsy moth larvae, tent caterpillars, leafrollers, tomato and tobacco hornworms and European corn borers. It’s commonly found under such brand names as Dipel and Thuricide.
  • Bti (var. israelensis): This strain of Bt targets mosquitoes, gnats, and black flies. It’s commonly sold as Mosquito Dunks, Bactimos, or Gnatrol. This strain is applied to standing water or as a soil drench, rather than as a leaf spray.
  • Bt (var. tenebrionis/San Diego): This special strain of Bt targets the elm leaf beetle, cottonwood leaf beetle, and potato beetle. It’s sold under names such as Trident, M-One, and Novodor.

Follow instructions when mixing Bt.

How to Apply Bt

The concentrated or wettable forms of regular Bt can be mixed in a small spray bottle or garden sprayer, then carefully applied to the foliage of affected plants. Be sure to follow application instructions for proper dosage and mixing.

When applying Bt:

  • Apply Bt Thoroughly: In order for Bt to work, the caterpillar has to eat it. Be sure to cover both the tops and bottoms of the leaves, since they feed on both, with an even coating. It’s not necessary to drench plants to the point of dripping.
  • Apply Bt Carefully: Be sure to limit your spraying to the affected plants, so that you only target the problem caterpillars.
  • Apply Bt thoroughly to plants.

  • Give Bt Time to Work: Bt takes several days to work, so be patient.
  • Apply Multiple Doses: Bt degrades quickly in sunlight and only lasts about a week or so. Apply as soon as you spot an infestation, and reapply every week if the caterpillars return.
  • Use Bt Only When Needed: To help reduce the buildup of resistance to Bt, use it only if absolutely necessary. Use Bt as part of integrated pest management program that includes beneficial predators and other natural controls.
  • Store Bt Properly: Bt products only last a couple of years, with the powdered forms lasting longer than liquids. Store Bt in a cool, dry, shaded place for maximum shelf life.

Protect the Butterflies

The major strain of Bt kills all leaf-eating caterpillars, which unfortunately includes the larvae of monarch butterflies and other important – and beneficial – butterflies and moths. Each species of butterfly larvae feeds on specific plants; for example, monarch butterfly larvae feed on milkweed. When applying Bt, it’s important to be extremely careful to spray only affected plants. Don’t overspray or allow the product to drift over to other plants that might be hosting an insect that’s harmless and beautiful!

Further Information

  • Organic Pest Control (video)
  • Organic Mosquito Control (video)
  • Bacillus thuringiensis (Colorado State University Extension)
  • Bacillus thuringiensis (Cornell University Resource Guide)

Gram stain of Bacillus thuringiensis under 1000 X magnification by Dr. Sahay on commons.wikimedia.org

Bacillus thuringiensis – frequently referred to by its initials of Bt – is a naturally occurring, ubiquitous species of spore-forming, rod-shaped bacterium that is used as a biological pesticide for insect control. Bacteria in the genus Bacillus commonly occur in soils, and most types with insecticidal activity have been isolated from soil samples. Bt can be found almost everywhere in the world, sparsely distributed in the soil in all types of habitats, including beaches, desert, and tundra.

This species of bacterium was first recognized in 1901 when a Japanese biologist, Shigetane Ishiwatari, isolated it as the cause of the disease that was killing large populations of silkworms. The name he gave this species, Bacillus sotto, was later ruled invalid and we now know this bacterium as Bacillus thuringiensis, after the German town of Thuringia where a diseased Mediterranean flour moth – from which Ernst Berliner isolated the bacterium – was found in 1911.

Farmers in Europe had started to use Bt as a pesticide by 1920, with a commercial spore-based formulation in France called Sporine in 1938 used primarily to kill flour moths.

Crystals of Bt-toxin from Bacillus thuringiensis. Microscopy by Jim Buckman on commons.wikimedia.org

When producing its spores, many Bt strains make crystal proteins, which Berliner had noted in 1915. It wasn’t until 1956 that researchers discovered that the main insecticidal activity was due to those proteins. These proteins are toxins – called endotoxins – that kill some insects, but not others. Different endotoxins may be active against an entire order of insects, or they may be effective against only one or a few species. These proteins are not toxic to humans or other mammals, and little to no direct toxicity to non-target insects or other wildlife has been observed. It is these proteins that are the basis of the insecticidal activity of the bacterium.

In the US Bt is the most widely used microbial insecticide. It was used commercially starting in 1958 and has been registered for use in pesticides by the US Environmental Protection Agency (EPA) since 1961, and now Bt strains are found in over 180 registered pesticide products. Some are for use on crops and ornamental plants, while others are used in and around buildings, in aquatic settings, and in aerial applications. Some of these products are approved for use in organic production.

Soybean loopers infected with Bt. Bottom: healthy larva. Middle: larva infected for 24 hours appears healthy, but is very sluggish and has stopped feeding. Top: dead larva 48 hours after initial infection. Photo by Daniel L. Mahr.

Bacterial insecticides must be eaten by target insects to be effective; they are not contact poisons. When the bacterium is ingested by a susceptible insect, the high pH and enzymatic activity in the insect’s gut dissolves the crystal, releasing the proteins which bind to specific receptors in the gut. The toxins paralyze and destroy the cells of the gut wall, allowing the gut contents to enter the insect’s body cavity. Poisoned insects stop feeding within hours, may die quickly from the activity of the toxin or may die within 2 or 3 days from the effects of septicemia (blood-poisoning). These toxins are not activated in mammals, so they cannot harm humans. (Related bacteria, such as B. cerus, produce toxins that do cause gastroenteritis (food poisoning) in humans.)

Bt does not reproduce and persist in the environment in sufficient quantities to provide continuing control of target pests. Spores do not spread to other insects or cause disease outbreaks on their own. The bacteria may multiply in the infected host, but because few spores or crystal protein toxins are produced, few infective units are released when a poisoned insect dies. Bt cells last only a few days on the soil surface (although they can survive for several months when covered by soil) and the toxins are rapidly broken down by sunlight and other microbes. Consequently, Bt products are applied much like synthetic insecticides. Bt treatments are inactivated within one to a few days in many outdoor situations, and repeated applications may be necessary for some crops and pests.

Bt products are produced commercially in large industrial fermentation tanks. The bacterial cells usually produce a spore and endotoxin as they develop. Most commercial Bt products contain the protein toxin and spores, but some contain only the toxin component.

Each type of Bt creates endotoxins that bind to different receptors in the gut, so can only be activated by certain insect larvae, so each isolate or subspecies is highly specific to different target insects. There are thousands of different Bt strains, producing over 200 proteins that affect a wide range of insects and some other invertebrates. Initially the strains of Bt known were effective only against caterpillars, but isolates that kill other types of pests were eventually identified and developed, with the first subspecies toxic to flies discovered in 1977 and strains toxic to beetles in 1983. Bt formulations that are commercially available fall into the following broad categories.

  • Thuricide is one of several brands of Bt used for caterpillar control.

    Bt Formulations That Kill Caterpillars. The best-known and most widely used Bt insecticides are formulated from Bacillus thuringiensis var. kurstaki (Btk) isolates that are pathogenic and toxic only to larvae of the butterflies and moths (Lepidoptera). Many such Bt products have been registered under a variety of trade names. They are used to control many common leaf-feeding caterpillars, including pests on vegetables, especially the “worms” that attack cabbage, broccoli, cauliflower, and Brussels sprouts; bag worms and tent caterpillars on trees and shrubs; larvae of the gypsy moth and other forest caterpillars; and European corn borer larvae in field corn. Some products are used to control Indian meal moth larvae in stored grain.Bacillus thuringiensis var. aizawai is another Bt that kills caterpillars. It produces slightly different toxins and is also highly toxic to honeybees.

    Bt products that kill caterpillars are not effective against other types of pests. Even certain caterpillars are not controlled by Bt, especially those that live in the soil or bore into plant tissues without consuming a significant amount of the Bt applied to plant surfaces. The peach tree borer in stone fruits, corn earworm in corn, and cutworms that clip off garden plants are examples of caterpillars seldom controlled by Bt treatments. Most Bt products are not labeled for the control of codling moth larvae that attack apples and pears because these larvae do not feed on fruit surfaces.

  • An example of a granular formulation of Bti.

    Bt Formulations That Kill Fly Larvae. Bacillus thuringiensis var. israelensis (Bti) kills the larvae of certain flies and mosquitoes – before they become biting adults. The main targets for this Bt are the larval stages of mosquitoes, black flies, and fungus gnats; it does not kill larval stages of other flies such as the house fly, stable fly, or blow flies. Aedes and Psorophora are the most susceptible mosquito genera; Anopheles and Culex species require higher than normal rates of Bti. Bti is most effective for mosquito or black fly control when it is used on a community-wide basis by mosquito abatement district personnel. For most homeowners, eliminating sites that periodically serve as sources of standing water (such as tires, birdbaths and empty containers) and controlling weeds around stagnant ponds or drainage lagoons is more effective than applying Bti. It is very useful in rain barrels, water gardens, and other places where standing water is wanted. Bti is not very effective for the control of mosquito larvae in turbid water or waters containing high levels of organic pollutants.Some Bti products are used effectively for the control of fungus gnat larvae in greenhouses and in mushroom production. For these uses, Bti is applied as a drench to potting soils or culture media.

  • Bt Formulations That Kill Beetles. Another group of Bt isolates, including those from Bacillus thuringiensis var. san diego and Bacillus thuringiensis var. tenebrionis, are toxic to certain beetles. Within the order Coleoptera (the beetles), species exhibit great differences in susceptibility to these isolates, presumably because of differences in the receptor sites in the gut wall of the insects where the bacterial toxins must attach. Consequently, the range of susceptible hosts for the beetle-targeted Bt formulations does not include all beetles, or even all of the species within a family or subfamily. Some of the common pests these Bt formulations are effective against include larvae of Colorado potato beetle and adults and larvae of elm leaf beetle and willow leaf beetle. Homeowner products may not be available.

In addition to Bt products used by homeowners, advances in molecular biology and biotechnology led to Bt crop plants, where genes directing the production of Bt toxins are incorporated into plants so that the Bt toxin is produced within the plant. Tobacco was the first modified plant since it is easy to manipulate genetically, but that was never developed as a commercial product. New Leaf potato was the first commercial genetically engineered plant, registered with the EPA in 1995 (but taken off the market in 2001 due to lack of sales), followed by Bt-corn in 1996, and later Bt cotton and soybeans. Dietary risk and allergenicity studies showed no observed adverse effects, and ecological risk assessments have shown that non-target caterpillars, such as monarch butterflies, are not exposed to enough toxin to affect them. However, the season-long, high-level control Bt crops can provide poses a great risk for the development of insect resistance to the Bt toxin, and some insects have developed resistance to Bt crops. The use of Bt crops initially results in dramatically reduced insecticide use, but in many cases more pesticide sprayings are needed over time to control emerging secondary pests. The use of these transgenic crops is still controversial because of these factors, as well as the potential for unintended environmental consequences not recognized yet.

Using Bt Insecticides. Insecticides containing Bt can be very effective for insect control in a variety of situations. Since each Bt insecticide controls only certain types of insects, it is essential to correctly identify the target pest and to confirm that the Bt product label states that the insecticide is effective against that particular pest. Separate stages of insects differ in their susceptibility to Bt; isolates that are effective against larval stages of butterflies, moths, or mosquitoes generally do not kill adults. Bt is most effective when applied to caterpillars during their 1st and 2nd instars, when they are still small. Because susceptible insects must consume Bt to be poisoned, treatments must be directed to the plant parts that the target pest will eat. Poisoned insects normally remain on plants for a day or two after treatment, but they do not continue feeding and will soon die. Where Bt applied to plant surfaces or other sites is exposed to sunlight, it is deactivated rapidly by direct ultraviolet radiation. To maximize the effectiveness of Bt treatments, sprays should thoroughly cover all plant surfaces, including the undersides of leaves. Treating in the late afternoon or evening can be helpful because the insecticide remains effective on foliage overnight before being inactivated by exposure to intense sunlight the following day. Treating on cloudy (but not rainy) days provides a similar result. Production processes that encapsulate Bt spores or toxins in a granular matrix (such as starch) or within killed cells of other bacteria also provide protection from ultraviolet radiation.

Mosquito Dunks are one brand of slow-release Bti.

Bt products are available as sprays, dusts, granules, and pellets. Dunks (right) are small donuts or briquettes containing slow-release formulas of Bti that float on the water surface, slowly dissolving to release the insecticide for up to 30 days. Users are advised to handle all microbial insecticides cautiously even though this bacterium is non-toxic to humans. Bacterial spores, mold spores, and virus particles become “foreign proteins” if they are inhaled or rubbed into the skin and can cause allergic reactions. The dusts or liquids used to dilute and carry these microorganisms also can act as allergens or irritants. These problems do not prevent the safe use of microbial insecticides, but users should not breathe dusts or mists of microbial insecticides. Users should wear gloves, long sleeves, and long trousers during application and wash thoroughly afterwards. These are common-sense precautions that will help prevent unexpected reactions and minimize any effects from unknown toxicity.

In addition to B. thuringiensis, there are about 100 species of Bacillus reported to kill insects. Several have been used for insect control, including:

  • One brand of milky spore powder.

    Bacillus popilliae var. popilliae and B. lentimorbus cause milky disease which kills Japanese beetle larvae (Popillia japonica) that infest lawns in much of the Midwest. It is not effective against the closely related annual white grubs (masked chafers). The bacteria spread naturally when infected beetle larvae die, releasing billions of new spores into the soil as the insect decomposes, so only a single application is usually necessary (although it may take a long time for the population to build up to sufficient levels in the soil. This was the first microbial insecticide to be developed commercially in the US.

  • Bacillus sphaericus is especially active against larvae of mosquitoes in the genera Culex, Psorophora, and Culiseta, remaining effective in stagnant or turbid water. One strain of B. sphaericus was registered by the U.S. EPA in 1991.

– Susan Mahr, University of Wisconsin – Madison

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What is Bt?

Bt is the abbreviation for bacteria that were named Bacillus thuringiensis in 1911 after the discovery that they could kill Mediterranean flour moths in the German city of Thuringia.

These common soil bacteria have been used as microbial insecticides for the last century. Farmers in Europe were the first to recognize their potential in 1920, and US farmers started using them in 1958. The Environmental Protection Agency (EPA) registered them as a pesticide in 1961.

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 inexpensive 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 that you purchase a strain that will kill that particular species.

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

We at Gardener’s Path will cover the strains of Bt that are widely used. With thousands of strains out there, we can’t go over all of them!

We will list the strains that you are likely to encounter and describe how to use them.

What Kinds of Insects Does Bt Kill?

Each strain of Bt will kill different kinds of insects. Some strains are very specific for a few species, while others will kill across a large class of insects. The most commonly used types are shown below:

Bt Type: Controls:
Bacillus thuringiensis kurstaki (Btk) Most caterpillars
Bacillus thuringiensis israelensis (Bti) Mosquitoes, flies, fungus gnats
Bacillus thuringiensis San Diego Specific beetles
Bacillus thuringiensis tenebrionis Specific beetles
Bacillus thuringiensis aizawai (Bta) Some caterpillars

Since the insect has to eat the toxin to be affected by it, that means that insects that don’t feed on the surface of plants are not likely to come into contact with the toxin and therefore will not be affected by it.

For example, soil insects are not likely to eat Bt. Insects that quickly bore inside plant tissue, like corn earworm, are not exposed to enough Bt to be affected by it. And beneficial insects – busy grazing on other insects and not plants – are likely to be unaffected by the bacteria.

How Does Bt Kill Insects?

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

Once eaten, the toxin affects a specific switch in the stomach – like a lock and a key. This switch (a receptor) activates a reaction that makes holes in the stomach. The contents, including the toxin and normal gut bacteria, spill out into the rest of the insect’s body, and it dies of infection and starvation.

It can take the insect hours or days to die. Typically, the insect remains on the plant as it is dying, so don’t be alarmed if there are still insects on your plants even after treatment. They are probably on their deathbeds.

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

How to Kill Caterpillars With Bt

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

This means that you can control most caterpillars with Btk. This includes many types of caterpillars more commonly known as “worms” that feed on broccoli, cabbage, and other crucifers.

Monterey Btk Liquid via Arbico Organics

Other caterpillars commonly controlled by Btk include the European corn borer, tent caterpillars, gypsy moths, and other forest caterpillars. There are even formulations designed to control Indian meal moth larvae in stored grain.

Several Btk formulations are available from Arbico Organics.

Another strain of Bt kills the larvae of wax moths in honeycombs, although there can be problems with its use. Bacillus thuringiensis aizawai (Bta) is toxic to honeybees.

How to Kill Mosquitoes and Flies With Bt

Fortunately, there is a strain of Bt that controls the larvae of many biting flies and mosquitoes! Bacillus thuringiensis israelensis (Bti) is commonly 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 rather than use chemical pesticides.

AQUABAC® 200G Granular Bti Mosquito Control

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

Mosquito Bits & Dunks Combo Kit

Bti is very effective against Aedes and Psorophora mosquitoes. However, if you are trying to kill Anopheles or Culex, you will need to use greater amounts of this insecticide.

As far as flies go, you can use Bti to kill black flies but not houseflies, blow flies, or stable flies.

You can also use some strains to control the larvae of fungus gnats in greenhouses or mushroom production. In this case, you would apply the Bti as a drench to the soil (for plants) or culture media (for mushrooms).

How to Kill Beetles With Bt

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

However, the Bt strains available only affect a few types of beetles. Some of them are major pests, though. The bacteria to use to control beetle larvae are either 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 should also make sure that the insect will be in its larval stage when you apply the Bt.

You will have better control if the larvae are small. There will be less damage to the plants because the little insects won’t eat as much. If you treat larger larvae, they will eat more of the plants and cause greater damage. Also, they may morph into the reproductive phase and become insensitive to the toxin.

And avoid high pH water! A pH greater than 8 is what activates the toxin in the insect’s gut, and you don’t want it to be activated ahead of time and then be 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. Make sure that you are spraying both the top and bottom surfaces of the leaves.

While it can survive for years in the ground if adsorbed to soil particles, Bt is rapidly inactivated by the UV radiation in sunlight. Many people spray their plants in the evening, so the toxin can work overnight before being inactivated by the sun the next day.

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

Precautions to Take

Bt has tested extremely well when examined for toxicity or the ability to cause cancer in animals – even at extremely high doses.

This microbe does appear to be truly safe for humans – even when eaten. Human stomachs are acidic, not alkaline, so they digest the Bt toxin.

However, some products are used in the form of a dust, and there have been a few instances of allergic reaction to the dust when the product was applied.

Just to be safe, you should wear gloves, long sleeves, and trousers when you apply the microbial insecticide. And then wash thoroughly afterwards.

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 primary example of an insect that has developed resistance in the field.

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

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

Another is to rotate your crops. Since different kinds of insects feed on different crops, you would be using a different type of Bt.

The large variety of Bacillus thuringiensis strains available enables gardeners, farmers, and mosquito control experts to control an array of insect pests.

Since the strains are highly specific to the insects targeted, residual effects against other organisms are not a concern (with the exception of Bta on honeybees).

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


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© Ask the Experts, LLC. ALL RIGHTS RESERVED. See our TOS for more details. Product photos via Monterey, Becker Microbial Products, and Summit. Uncredited photos: .

About Helga George, PhD

One of Helga George’s greatest childhood joys was reading about rare and greenhouse plants that would not grow in Delaware. Now that she lives near Santa Barbara, California, she is delighted that many of these grow right outside! Fascinated by the knowledge that plants make chemicals to defend themselves, Helga embarked on further academic study and obtained two degrees, studying plant diseases as a plant pathology major. She holds a BS in agriculture from Cornell University, and an MS from the University of Massachusetts Amherst. Helga then returned to Cornell to obtain a PhD, studying one of the model systems of plant defense. She transitioned to full-time writing in 2009.

Sometimes scientists get it wrong and have to revise the conventional wisdom they helped create. A bacterium known as Bacillus thuringiensis produces a toxic crystal that has been identified as a killer of garden pests since at least 1911. Researched extensively, the so-called Bt toxin was turned into a commercial product starting in the 1950s, and more recently, plants have been engineered to produce it on their own. Yet, through all these years of research, the mechanism by which it kills bugs has simply been assumed: the toxin eats holes in the insect’s gut and they either starve or experience septicemia–a deadly bacterial infection in the blood–from the Bt itself. “If you don’t have a good gut wall, you can’t take up nutrients,” explains microbiologist Jo Handelsman of the University of Wisconsin-Madison.

But Wisconsin graduate student Nichole Broderick discovered that nearly a century’s worth of consensus was wrong when she cleared gypsy moth caterpillars of the bacteria in their gut and fed them Bt toxin. “Initially, I was testing the hypothesis that the gut bacteria were actually protecting the moth,” she says. “I found that once they did not have a gut community I could no longer kill them with Bt.” In fact, large numbers of the nascent moths did just fine–even when fed as much as 10 international units of the toxin, 10 times more than ordinarily needed–as long as they were also fed antibiotics that cleared their systems of the seven to 20 bacteria species their guts normally host. “I think it was by the third fairly large, replicated experiment that we looked at each other and said, ‘this is real,'” Handelsman recalls. The Bt toxin could not kill on its own.

Reintroducing bacterium one-by-one revealed that Enterobacter species NAB3 reinstated Bt’s deadly impact. Further tests with insect blood showed that this Enterobacter thrived in the hemolymph, multiplying as much as when raised in rich laboratory broth, whereas B. thuringiensis was cleared from the blood within six hours. And introducing Escherichia coli capable of producing Bt toxin quickly killed larvae, unless the E. coli was heat-killed and introduced into caterpillars without any other gut bacteria. “The gut wall will heal and regenerate itself if there are no gut bacteria there,” Handelsman says.

Of course, none of this answers the basic question: how does Bt toxin, in partnership or alone, kill insects? “Is the Enterobacter moving into the blood? At this point, that’s the hypothesis we’re pursuing,” Broderick notes. “I think it’s probably more complex than that. The simple model doesn’t always turn out to be the case.” As her prior work proves once again. The paper presenting the finding publishes online in Proceedings of the National Academy of Sciences USA on September 26.

OMRI listed. A liquid microbial insecticide containing Bacillus Thuringiensis. Controls cabbage looper, hornworm, fruit tree roller, gypsy moth and other leaf eating worms. Nontoxic to humans, pets, birds, and bees. Apply MONTEREY B.t. when worms or caterpillars are first noticed, then repeat at five (5) to seven (7) day intervals while they are active. Apply more frequently to control heavy infestations. Apply thoroughly to top and bottom of foliage. Reapply after heavy rains. MONTEREY B.t. must be eaten by worms or caterpillars to be effective. After ingesting the insecticide, they immediately stop feeding, though they may otherwise appear to be unaffected for several days. Best results are obtained by treatments when worms are small; they must be actively feeding on treated, exposed foliage. You may apply MONTEREY B.t. up to and on the day of harvest.

HOW TO APPLY Always shake or stir MONTEREY B.t. thoroughly before use. Partially fill sprayer with water before adding the application rate amount of product, then mix in product thoroughly and add remaining amount of water. Spray leaf surfaces thoroughly, top and bottom, for complete control. Agitate regularly while spraying. Use all of spray mixture within 24 hours.

FRUITS AND VEGETABLES: Mix 1.5 fluid ounces per 3 gallons of water (1 tablespoon per gallon) for a hand sprayer and apply to all plant foliage to cover an area of 1,000 square feet. Apply at first sign of infestation and repeat at weekly intervals when needed to maintain control of the following insects: Cabbage Looper on broccoli, cauliflower, collards, kale, mustard greens, turnip greens, cabbage, celery, lettuce,melons, and tomatoes. Imported Cabbage Worm on broccoli, cabbage, cauliflower, collards, kale, mustard greens, and turnip greens. Tomato Hornworm on tomatoes. NOP compliant, check with your certifier for use in organic production.

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