If you are looking for a safe way to control insect pests, and would like to stay away from using synthetic pesticides, it is worth giving Bt a try. Bt stands for Bacillus thuringiensis, a bacterium that exists naturally in soil. While Bt was discovered over a hundred years ago, and is well known in organic agriculture, many home gardeners are less familiar with it.
There are many kinds of Bt. All produce toxins that can kill a target insect at a certain stage in its life cycle, while remaining harmless to other creatures. It is primarily used to control caterpillars feeding on vegetables, though other types work on flies, beetles, gnats and mosquitoes.
Bt is a natural pesticide. It does not harm humans, pets or wildlife, including fish and birds, even if they ingest insects that have eaten Bt. Bt does not harm insects other than the target insect, another reason why Bt is considered so safe. (There is one variation used to control moths, Bt aizawai, that can be toxic to honeybees).
If it's a caterpillar and it eats leaves (like the tomato hornworm, above), there is probably a form of Bt that will control it. If ingested when in larval form, Bt acts as a stomach toxin by releasing a protein that ruptures an insects' gut, making it unable to feed. Bt only works in the larval stage; if it is applied earlier or later, it will be ineffective.
The Organic Materials Research Institute lists Bt as an accepted pesticide in organic agriculture. In conventional agriculture it has been genetically engineered into corn, soybeans, potatoes and cotton to make them insect resistant.
Variations of Bt target specific pests: Bt kurstaki (Bt-k) targets caterpillars that eat leaves, including gypsy moth, armyworms, and cabbage looper. Bt is fatal to pretty much any caterpillar that attacks cole crops such as broccoli, kale, cabbage and cauliflower. Once these pest insects are out of the larval stage, it's no longer necessary to use it.
Bt should be applied to both sides of leaves, or in the case of corn earworms, squirted as a liquid into the tips of forming ears. It needs to be reapplied every seven to ten days, or if the leaves get wet from overhead irrigation or rain. Once a caterpillar eats the Bt covered leaves, it will stop feeding, but can take a few days to die. Apply Bt-K in the late afternoon or on cloudy days because sunlight breaks down Bt, making it ineffective.
Bt San Diego and Bt tenebrionis work on beetle larvae. One, the Colorado potato beetle, is resistant to many other kinds of pesticides. A related bacteria, Bacillus popilliae, better known as milky spore, kills Japanese beetle grubs in lawns.
Bt isrealensis (Bt-i) targets mosquitoes, black fly and fungus gnats in their immature stages. It is one of the safest ways to control the spread of mosquitoes, and by extension, control diseases such as West Nile Virus, malaria, and dengue fever. Bt mosquito dunks are either formed into disks or sold as granules that are distributed over the surface of water. Added to standing water, Bt kills almost 100% of mosquito larva within 24 hours.
Most variations of Bt are sold as a powder that is mixed with water to create a spray, or as a liquid concentrate, granules, or a dust. When stored properly, powdered Bt should last up to five years, and liquid forms two to three years. If it is stored in a place where temperatures regularly go over 90 degrees F or under 20 degrees F, it is best to replace the product each year.
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By Joan Casanova, Bonnie Plants,
Photographs courtesy of Bonnie Plants, Inc.
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