Bacillus thuringiensis (Bt)

Don’t let the name scare you off. Bacillus thuringiensis (called “Bt” to make things easier) is a naturally occurring, rod-shaped, pest-killing collection of bacteria found in soil.

In addition to occurring naturally in soil, Bt can be found on leaves, in animal feces, and in flour mills. It is even found living in the gut of the caterpillar stage of some moths and butterflies.

Gram stain of Bacillus thuringiensis under 1000X magnification (Dr. Sahay) CC BY-SA 3.0

Being a living thing, Bt doesn’t handle extreme heat very well. Because of this, you will only want to buy as much as you will use in a single growing season, and store it in a cool, shady location. One advantage to being easily killed is that it reduces the likelihood of pests developing a resistance, the way they do for many chemical treatments.

Bt targets

Bt is used against a wide variety of garden pests, including whiteflies, budworms, moths, flies and mosquitoes, aphids, beetles, blackflies, leafhoppers, wasps, and sawflies. Unfortunately, Bt can also negatively impact important beneficial insects, such as honey bees and parasitic wasps. (There are no easy solutions…)

Bt lifecycle
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Bt reproduces using spores. Now, there are spores that generate plants such as mushrooms, moss, and other eukaryotes. This is a different kind of spore, called an endospore. Endospores are not seeds or embryonic offspring. Inside the endospore, a dormant, bare-bones, reduced version of the original bacteria divides within its cell wall. Then one of these divisions swallows the other one! This behavior allows endospores to remain dormant for hundreds (some say millions) of years without food. Endospores can resist ultraviolet radiation, extreme temperatures (even boiling water), and chemical disinfectants. 

Death by bacteria

Bt are 1 µm (micron) wide and 5 µm long. That works out to over 25,000 Bts standing next to each other to cover one inch. If they had feet, that is… Regardless of their diminutive size, MicroWiki describes the [brutal] process of Bt pathology this way:

1. Bt is consumed by larva
2. Digestion by an insect’s highly alkaline digestive juices (unlike our acidic digestives) releases toxic crystalized proteins found within Bt
3. These crystals paralyze and cut holes in the insect’s digestive system, causing the insect to stop eating and starve to death
4. Spores begin to germinate from the crystalized proteins, continuing the Bt lifecycle

Bt varieties

There are hundreds of strains of Bt, and each one attacks a different type of plant (or animal). Bt is related to the same bacteria that cause anthrax (Bacillus anthracis) and food poisoning (Bacillus cereus), but Bt is believed to be harmless to humans and animals. The European Food Safety Authority approved the use of Bt, but pointed out that many safety claims lack adequate scientific proof. The Organic Materials Review Institute (OMRI) has approved Bacillus thuringiensis for use in organic farming with certain restrictions, including the use of crop rotation, proper sanitation, creating habitat for beneficial insects, and minimal use of Bt treatment. There are 5 subspecies of Bt available in the U.S.:

• Bt thuringiensis aizawa (Bt-a) - wax moth caterpillars
• Bt kurstaki (Bt-k) - flies and mosquitoes, fungus gnats, black flies, leaf-eating caterpillars, such as cabbage loopers and tomato hornworms; Mosquito dunks use this form of Bt
• Bt israelensis (Bt-i) attack mosquitoes, many types of flies, and fungus gnats; Mosquito dunks are an example
• Bt morrisoni (Bt-m) - similar to Bt-k 
• Bacillus popilliae - Japanese beetle
• Bt san diego (Bt-sd) - Colorado potato beetle, elm leaf beetles, other leaf-eating beetles that feed on eggplant, potatoes, and tomatoes
• Bt tenebrious (Bt-t) - similar to Bt-sd

There are other Bacillus thuringiensis that claim to attack nematodes, flatworms, and mites. These forms are given mind-numbing names like Cry5B, Cry6A, Cry14A, Cry1A, Cry3A, and Cry4A. Unfortunately, research hasn’t shown them to be nearly as effective as advertisements claim, though they do provide some aid. These bacteria are also being used to genetically modify several food crops, including corn.

Using Bt

Bt is used on a wide variety of crops. The short list includes stone fruits, citrus, cruciferous vegetables, apples, artichokes, melons, berries, tomatoes, lettuce, and grapes, just to name a few. But Bt only works on insects when it is ingested. This normally occurs in the larval stage. Eggs and adult insects are generally not affected, so timing is important. Also, Bt degrades quickly in sunlight, so early morning or evening applications are best. These steps can help you get the most out of Bacillus thuringiensis:

1. Identify the pest you are trying to control.
2. Learn when and where to expect the larval stage to occur.
3. Select the appropriate Bt variety.
4. Apply the product according to package directions.

You can find Bt at most garden centers. It comes in concentrated form or in a ready-to-spray bottle.

IMPORTANT NOTE: NOT ALL Bt PRODUCTS SOLD AS EFFECTIVE INSECTICIDES CAN ACTUALLY DO THE JOB. BE SURE TO LOOK FOR THE SUBSPECIES LISTED ABOVE.