Bt is a microbe naturally found in soil. It makes proteins that are toxic to immature insects (larvae). There are many types of Bt. Each targets different insect groups. Target insects include beetles, mosquitoes, black flies, caterpillars, and moths.
With Bt pesticides, routine testing is required to ensure that unwanted toxins and microbes are not present. Bt has been registered for use in pesticides by the US Environmental Protection Agency (EPA) since 1961.
Currently, Bt strains are found in over 180 registered pesticide products. Bt products are used on crops and ornamental plants. Others are used in and around buildings, in aquatic settings, and in aerial applications. These products are commonly sprays, dusts, granules, and pellets. Some of these products are approved for use in organic agriculture.
Some crops have been engineered to make the Bt toxin. These plant-incorporated protectants include corn, cotton, and soybeans.
Always follow label instructions and take steps to avoid exposure. If any exposures occur, be sure to follow the First Aid instructions on the product label carefully. For additional treatment advice, contact the Poison Control Center at 1-800-222-1222. If you wish to discuss a pesticide problem, please call 1-800-858-7378.
Bt makes toxins that target insect larvae when eaten. In their gut, the toxins are activated. The activated toxin breaks down their gut, and the insects die of infection and starvation. Death can occur within a few hours or weeks.
The different types of Bt create toxins that can only be activated by the target insect larvae. In contrast, when people eat the same toxins, the toxins are not activated and no harm occurs.
Each type of Bt toxin is highly specific to the target insect. For example, the ‘kurstaki’ type targets caterpillars. The ‘isrealensis’ type targets immature flies and mosquitoes. Little to no direct toxicity to non-target insects has been observed.
People are most commonly exposed to Bt through their diet, at very low levels. Exposure can also occur if you breathe it in or get it on your skin or eyes. For example, this can occur while applying sprays or dusts during windy conditions. You may also be exposed after using a product if you don’t wash your hands before eating or smoking. Since Bt is commonly found in soils, exposures not related to pesticides are also possible.
Pets might be exposed to this product in treated birdbaths or water fountains. You can limit your exposure and reduce the risk by carefully following the label instructions.
Bt is low in toxicity to people and other mammals. Several studies have found no evidence of sickness or infection as a result of exposure. However, some products with Bt have caused eye and skin irritation. In one study, rats breathed in very high doses of concentrated Bt. Some had runny noses, crusty eyes, and goose bumps. Others were less active or lost weight.
In another study, people were surveyed before and after aerial applications of Bt. Most people were not affected. However, some people with hay fever reported certain symptoms. These included difficulty with sleep and concentration, stomach upset, and nose/throat irritation. Seasonal factors, such as pollen, may have contributed to some of the effects.
Scientists also evaluated whether Bt can cause allergic reactions. Researchers found that farmworkers exposed for one to four months did not experience any problems related to their airways, nose, or skin. However, further exposure showed evidence of an immune response and the potential for skin allergies to develop.
When eaten, Bt is confined to the gut. It does not reproduce, and the toxin is broken down like other proteins in the diet. Bt leaves the body within 2 to 3 days.
If breathed in, Bt can move to the lungs, blood, lymph, and kidneys. Bt is then attacked by the immune system. Levels of Bt decrease quickly one day after exposure.
No data were found on the carcinogenic effects of Bt in humans. However, in one animal study, rats were fed very high doses of Bt for 2 years. No evidence of cancer was observed.
In a 2-year study, rats were fed high doses of Bt daily. Female rats had lower body weights. However, no evidence of an infection was found.
Bt is only activated in the alkaline environment of the insect gut, compared to the acidic environment of human stomachs. In human stomachs, it is easily digested. As such, no adverse effects are expected after long-term dietary exposure to Bt, whether its proteins are sprayed on plants or grown within plant tissues.
Children may be especially sensitive to pesticides compared to adults. However, there are currently no data showing that children have increased sensitivity specifically to Bt.
Toxins created by Bt are rapidly broken down by sunlight and in acidic soil. Other microbes in soil can also break it down. Bt does not readily leach in soil. It typically remains in the top several inches of soil. Bt remains dormant in most natural soil conditions. However, there has been some reproduction in nutrient rich soils. On the soil surface, dormant Bt cells last only a few days. However, below the soil surface, they can last for months or years. The half-life in unfavorable soil is about 4 months. Bt toxins break down much faster. In one study, 12% remained after 15 days.
In water, Bt does not readily reproduce. A study found Bt toxins in the air were broken down rapidly by sunlight. Forty-one percent (41%) of the toxin remained after 24 hours. On plant surfaces, sunlight breaks down Bt; the half-life of Bt toxins is 1-4 days.
Bt is practically non-toxic and non-pathogenic to birds, fish, and shrimp. No adverse effect or infection was found in rats given large doses of two different Bt strains. There is no evidence that Bt can cause a disease outbreak among wild animals.
Little to no direct toxicity to non-target insects and other shelled invertebrates has been observed. Bt does not seem to hurt earthworms. However, the aizawai strain is highly toxic to honeybees. Other strains have minimal toxicity to honeybees.
Water fleas exposed to the kurstaki and israelensis strains showed moderate toxicity. The aizawai strains are highly toxic to water fleas. However, evidence suggests that toxicity to these non-targets may be related to impurities from the production of Bt.
For more detailed information about Bacillus thuringiensis (Bt) please visit the list of referenced resources or call the National Pesticide Information Center, Monday - Friday, between 8:00am - 12:00pm Pacific Time (11:00am - 3:00pm Eastern Time) at 1-800-858-7378 or visit us on the web at http://npic.orst.edu. NPIC provides objective, science-based answers to questions about pesticides.
Please cite as: Perez, J.; Bond, C.; Buhl, K.; Stone, D. 2015. Bacillus thuringiensis (Bt) General Fact Sheet; National Pesticide Information Center, Oregon State University Extension Services. http://npic.orst.edu/factsheets/btgen.html.
NPIC fact sheets are designed to answer questions that are commonly asked by the general public about pesticides that are regulated by the U.S. Environmental Protection Agency (U.S. EPA). This document is intended to be educational in nature and helpful to consumers for making decisions about pesticide use.