The bacillus thuringiensis is a biological insecticide widely used in agriculture at a professional level. It is a very useful product, but surprisingly, it is not well known among the general audience of DIY farmers.
When we talk about this product, we refer to a biological insecticide for two reasons: first because it is a bacterium naturally present in the soil, hence of natural origin; secondly, because it has no toxicity towards humans, plants, and animals, but it is effective only against the larvae of certain insect families that are harmful to our organic crops.
For these characteristics, it is one of the very few insecticides allowed in organic farming.
But let’s take a closer look at what it is and how it works.
Bacillus thuringiensis, what it is, and how it works
Bacillus thuringiensis is a naturally occurring spore-forming bacterium found in the soil. It was discovered in the early 1900s by the Japanese scientist Ishiwata, who isolated it from silkworm larvae. It was classified in 1911 by the German scientist Berliner, who isolated it from flour moth larvae found in a warehouse.
Bacteria are microscopic single-celled organisms. Their reproduction usually occurs through cell division, meaning they self-generate infinitely, forming colonies of billions of microorganisms. This happens when environmental conditions are favorable (absence of toxic substances, favorable humidity and temperature, etc.). In unfavorable environmental conditions, however, in some bacterial species, including bacillus thuringiensis, reproduction through cell division stops, and the bacterium transforms into a resistant form, which is the spore.
In this case, the spore is produced inside the mother cell, which transforms into a container called a sporangium. At the same time as the spore, a protein crystal with insecticidal action also forms. Once the spore and crystal are formed, the sporangium dissolves and releases its contents into the environment. Bacillus thuringiensis is made up only of these two elements, which are sprayed on the plants: spore and crystals.
Insecticidal action of bacillus thuringiensis
The larvae of insects that ingest the spores and crystals of bacillus thuringiensis suffer lesions in their intestines. These lesions are caused by the action of crystals, which transform into toxins. The toxins find specific receptors in the larva’s intestine, leading to the paralysis or death of the larva.
Therefore, the insecticidal action occurs through ingestion, not contact like conventional insecticides, or systemic action through the plant’s sap, as with more toxic and dangerous insecticides. Furthermore, the effectiveness of bacillus thuringiensis is limited to the larval stage. In essence, it is not an ovicidal insecticide that destroys eggs or an adulticide that targets adult insects. It works specifically and selectively on larvae, preferably in the early stages of development. Thus, the mechanism of action is highly specific and selective, based on the toxin-receptor combination.
This is why bacillus thuringiensis is an effective insecticide only against certain species of insects. It means that it is entirely harmless to beneficial insects in the garden, such as pollinators (bees and bumblebees) and predatory insects (like ladybugs).
The high degradation rate of spores and crystals under sunlight ensures that their presence in the environment is significantly reduced. It is also entirely harmless to birds, fish, and, of course, humans.
Different strains of bacillus thuringiensis and their specific action
To emphasize the high selectivity of bacillus thuringiensis, we have different strains of the bacterium. Each of these strains acts only against a specific family of insects. Therefore, it is essential to know which existing varieties of bacillus thuringiensis are effective against which insects. Here is a classification:
Bacillus thuringiensis kurstaki (B.t.k.)
The most well-known and widely used variety in organic farming is bacillus thuringiensis kurstaki (B.t.k.). We have already mentioned this type of thuringiensis as a remedy for tomato leaf miner.
Bacillus thuringiensis kurstaki is effective against:
- Vegetables, against larvae of leaf miner (tomato leaf miner), cabbage worm (cabbage moth), corn earworm (tomato fruitworm), Vanessa cardui (painted lady butterfly), and European corn borer
- Pome fruits, against larvae of leafrollers, tortricids, and fall webworm
- Citrus, olive, and grape vines, against larvae of leaf miner
- Potato and eggplant, against Colorado potato beetle larvae
- Forestry and public green areas, against larvae of processionary moth, leafrollers, gypsy moth, fall webworm, and European corn borer
B.t.k. is usually sold as water-dispersible granules, with dosages ranging from 80 to 120 g per 100 liters of water, making it very economical (available here).
Bacillus thuringiensis israelensis (B.t.i)
The second variety we want to present is bacillus thuringiensis israelensis (B.t.i.). This thuringiensis is effective against mosquito, black fly, and midge larvae. Therefore, imagine its potential as a biological remedy against one of the most annoying insects for humans: the mosquito.
B.t.i. is sold as water-soluble tablets. These tablets can be used directly in ponds or stagnant water where mosquito larvae proliferate. It is also possible to dissolve them in water for spraying in the environment or on plants.
Bacillus thuringiensis aizawai (B.t.a.)
The third and final variety we want to mention is bacillus thuringiensis aizawai (B.t.a.). This particular variety of bacillus thuringiensis is used in beehives against wax moth larvae, against boxwood moth, and generally against the same larvae targeted by B.t.k, with a greater efficacy against cutworms.
B.t.a. is also marketed in water-dispersible granules.
Recommendations for the use of bacillus thuringiensis
As we have seen, bacillus thuringiensis is a highly selective biological insecticide, and there are several varieties of it. Therefore, it is necessary to make some recommendations:
The first recommendation is to understand well which insect you are targeting with it. For example, using it against aphids is completely useless.
The advice is, therefore, to carefully read the label of the commercial product and strictly adhere to the indicated dosages. Naturally, do this after conducting a thorough investigation in your garden.
Keep in mind that the bacteria are effective against insect larvae, not against adults. Therefore, try to identify as quickly as possible which pest is attacking your plants. This way, you will be able to target the young larval generations of the parasite.
Spraying on plants should be done during the coolest hours of the day, before it gets dark. This is because, as we have seen, bacillus thuringiensis is easily degraded by sunlight. Spraying at night allows more time for the bacteria to act, delaying the drying of the vegetation.
The waiting period to respect is three days (from the product’s application to the fruit consumption) when treating vegetable crops. Additionally, even though we are talking about a biological insecticide, during spraying (preferably with a backpack sprayer or atomizer), it is advisable to use the appropriate personal protective equipment, such as gloves and a mask with a filter. This is, of course, a recommendation that we always give.
Pay attention to the product’s storage as well. Since bacillus thuringiensis is a living bacteria, it easily loses its efficacy. If you don’t use it all in a single treatment, close the package tightly and store it in a cool, dry place, away from environmental contaminants. And, of course, keep it out of reach of children!
Note: We remind you that for the purchase and use of the different strains of bacillus thuringiensis, no qualification for the use of plant protection products is required. This is because bacillus thuringiensis is not classified as a plant protection product.
- Bacillus thuringiensis (Bt) from National Pesticide Information Center, Oregon State University: This resource discusses Bacillus thuringiensis (Bt), a species of bacteria that lives in soil. It makes proteins that are toxic to some insects when eaten.
- Bt Fact Sheet – National Pesticide Information Center from National Pesticide Information Center, Oregon State University: This fact sheet provides information about Bt, a bacterium found naturally in soils throughout the world. To reproduce, Bt makes spores that grow into new bacteria. Bt spores have proteins that are toxic to some insects.
- Landscape: Bacillus thuringiensis (B.t.) – UMass Extension from University of Massachusetts Amherst: This resource discusses Bt, which is considered non-toxic to plants and to animals other than certain insects. It is highly selective, and only kills certain insects. It does not kill most beneficial insects.