Bt is Bacillus thuringiensis, a bacterium naturally found in the soil used to control pests, is the most widely used biopesticide worldwide (1). A biopesticide is a pesticide derived from a biologically occurring, natural source, such as from an animal, plant, bacteria, or mineral. Although the use of Bt can be traced back to the early 1900s the first commercial use was in France in 1938 (1, 2) and was first registered for use in the United States by the Environmental Protection Agency in 1961 (3). B. thuringiensis is a rod-shaped bacterium that switches between normal vegetative growth and a sporulation state in which round spores are formed (4). When spores are formed, the bacterium also produces crystalline proteins (Cry family of proteins) which are toxic to certain insects (5). When the insect ingests the Bt, for example, while eating a leaf sprayed with the Bt-containing pesticide, the crystalline proteins bind to specific receptors on the epithelial (skin) cells of the insect’s gut, the cry proteins then form a pore in the cell, causing them to lyse (rupture) due to osmotic shock (6). Other organisms, such as humans, other mammals, birds, earthworms, and most other insects do not have the receptor necessary for binding and are therefore not harmed (2, 3).
Different subspecies of Bt are specific for different types of insects. For example, Bt kurstaki (Btk), the most common and easiest to find Bt, is used to control Lepidoptera (butterfly and moth) larvae such as cabbage worms, cabbage loopers, gypsy moth, tobacco hornworm, and more (2). It has no effect on beneficial insects like honeybees or ladybugs (3) but should only be applied to plants that butterflies, such as monarchs, are unlikely to visit, for example, brassicas.
Bt israelensis (Bti) is used to control mosquitos and fungus gnats (Dipterans or flies) by targeting the larvae of those insects (2) and is commercially available as Mosquito Bits or Mosquito Dunks. Mosquito Bits are in pellet form and can be sprinkled in standing water to kill mosquito larvae. I have also mixed the pellets into my plant water when I have a fungus gnat outbreak in the soil of my house plants. Simply watering your plants soaks the soil with the Bti allowing the pesticide to control the larval stage. Mosquito Dunks are larger and float on water and slowly release the Bt as they dissolve over time and can also be used in standing water.
Bt aizawai (Bta) is used to control certain moth larvae species, especially those that eat grains (6). This formulation appears available for commercial farms but not for the average homeowner to purchase.
Bt tenebrionis and Bt san diego are used to control beetles such as the Colorado potato beetle (6). This formulation also appears available for commercial farms but not for the average homeowner.
Bt is a very safe pesticide (2, 3) especially when compared to other non-organic synthetic pesticides and even some organic pesticides. Bt is also very effective against specific pests (6), but its mechanism of action does have some disadvantages (see below). Bt is also a very host-specific pesticide (2, 3, 6), particularly when used according to the label instructions, which reduces off-target effects.
Because the larvae need to eat the Bt for it to be active, plants will sustain more damage until the toxin takes effect, usually within a few days. The Bt spores are also sensitive to UV light (sunlight will break them down) (7) and may wash off the leaves, particularly after a hard rain. The typical half-life of Bt on foilage in field conditions is 1-2 days (7), therefore it is recommended that plants be sprayed weekly or following a hard rain or overhead irrigation. There is also the possibility of off-target effects however, Bt is much more host-specific than many other pesticides on the market, even other biopesticides. When used carefully on specific plants off-target effects are minimized.
One last use for Bt has been introducing the gene into the plant itself, making a transgenic crop. This eliminates the need for spraying, instead, the plant cell itself produces the insecticidal protein. The use of Bt in transgenic crops is beyond the scope of this blog post and I will likely address the topic in the future. However, in the United States, the most common Bt crops are corn and cotton, but potato and tobacco have also been modified. To my knowledge these crops are not available to the average home gardener but only to commercial farms.
In 2016, my family and I moved from the New York City area to small town Wisconsin. Our move, this website and blog (and our previous Etsy store) is the result of our desire over the past several years to simplify our lives, increase our quality of life, reconnect with nature, and enjoy a more self-sufficient life. I grew up as a country kid in central Pennsylvania working on my grandfather's fruit farm and as a corn "de-tassler" at a local seed farm. My background is in biology where my love of nature originated. I am a former research scientist and professor and have now transitioned to a part-time stay-at-home mom, self-employed tutor, and small business owner. Thank you for taking the time to check out my site.