Bt (Btk) is the shortened form of writing or saying Bacillus thuringiensis. Bt is a natural soil-dwelling bacteria. It is used as an organic pesticide. It is totally organic and approved for certified organic food and plants. There are different types of Bt that are deadly to different types of critters. On this page, we are discussing the type of Bt that kills caterpillars, not the types that kill other critters.

Bt is incorporated into pesticides to kill caterpillars. On many pesticide labels, butterfly and moth caterpillars are incorrectly called ‘worms’. Bt is totally organic simply because it is natural in nature. In pesticides, Bt can be incorporated as the living bacteria or as crystals from the bacteria. Either way, it kills caterpillars. (Some plants have been modified genetically, inserting the crystal from Bt into the plant itself. More about this at the end of this article.)

Bt must be ingested to kill caterpillars. After a caterpillar eats Bt, it soon stops eating. Bt proteins form holes in the gut wall. Soon, the gut contents spill into the blood/hemolymph of the caterpillar. This causes the caterpillar to die.

This process takes about three days. Caterpillars stop eating on day one. Instinctively, they often continue to crawl or stay on their host plant. These caterpillars slowly become soft, move slower, and may turn a bit darker in color. They may have diarrhea. These caterpillars usually die on day three. Although it takes three days for them to die, they stop eating on day one.

When raising butterfly or moth caterpillars, it is wise to test any new host plant with one or two caterpillars for three days before feeding it to other caterpillars. This is a method to determine whether or not the plant has not been treated with Bt.

Note: mosquito dunks with Bt are not dangerous to caterpillars. The type of Bt that kills mosquito larvae will not harm caterpillars.

Sunlight quickly degrades Bt. Between sunlight and irrigation or rain, Bt application is only effective to control ‘worms’ for less than a week. In field or garden use, it is recommended that Bt be applied again one week after the first application. Many people use Bt on their tomato, corn, bean, squash, and other vegetable plants. Corn earworms, tomato and tobacco hornworms, cabbage worms, squash borers, and other caterpillars are not considered a good diet by most of us.

Many organic and non-organic gardeners prefer to use Bt on parts of their garden, especially in their vegetable garden. With Bt to control caterpillars, other pesticides can often be avoided.

The USDA considers Bt safe to humans. The acid pH of a human body is too low for it to affect them. The basic high pH of a caterpillar’s gut activates Bt.

Bt bacteria have been used since 1958 as a form of caterpillar control on crops. For farmers and many nursery owners, butterfly and moth caterpillars are simple plant pests that damage or destroy their crops. Some Bt pesticides (powder form) include the living bacteria. Other Bt pesticides are made with crystals from the bacteria, not the living bacteria itself.

The advantage of incorporating Bt into plants is simple. Farmers who grow these crops rarely use a wide-spectrum pesticide in their fields. Without Bt GMO crops, tractors and airplanes sprayed pesticides that contacted every plant in the field as well as plants along the sides of the field and would also drift with the wind to other fields. Instead of killing only the target pest, caterpillars, these pesticides kill almost all insects in the area it is used. With GMO crops, Bt is normally the only pesticide used and it is incorporated into the plant. Many tests have been conducted with pollen from Bt corn. The damage done to other species than the target pest (corn earworms) is negligible. At the time of the research, there were a few varieties of Bt corn that did cause more caterpillar deaths. Those varieties have been discontinued for years and are not available to or grown by farmers. Corn releases pollen only for a short time. The negligible damage that is done by the pollen is only for that short period of time. Broad spectrum pesticides would be used at least once, normally more than once on some crops before GMO Bt crops were developed. We are not saying that GMO crops are best. We are simply pointing out advantages of GMO Bt crops. (Please understand there is a difference between GMO Bt and GMO Round-up Ready crops. Round-Up Ready GMO is a totally different subject.)