Bt microbial insecticide

The microbial insecticide Bt is a concentrated formulation bacterium that kills the caterpillar stage of moths and butterflies. The active ingredients are spores and toxic crystals produced by the naturally occurring bacteria, Bacillus thuringiensis. As early as 1960, Bt was produced and sold as an insecticide. Currently, it is registered and sold for use as an insecticide in over 50 countries for use against leaf-eating caterpillars.

Why use Bt?
Bt products are considered nontoxic to humans, animals, fish, plants, other insects and microorganisms. Bt is highly selective; it only kills caterpillars of moths and butterflies. Bt will not contaminate groundwater because it degrades rapidly and becomes a natural component of the soil. And it is effective. When a caterpillar eats Bt, it will stop eating within hours and will die in two to five days.


In general, Bt is very safe for use. The U.S. Environmental Protection has found no hazards to human health associated with the use of Bt. The EPA considers Bt sufficiently safe that it has exempted it from food residue tolerances, groundwater restrictions, endangered species labeling and special review requirements. Bt has no known effect on nontarget organisms in aquatic and terrestrial habitats, including bees, fish, birds, and wildlife. It is the pesticide of choice for use near lakes, rivers, and dwellings.

In some instances, killing of nontarget caterpillars could be of concern, especially if they are rare or beneficial species that are in the caterpillar stage at the same time and location as the target species.

Mode of action

Bt is very selective. It is active against only one group of insects, the caterpillar stages of moths, skippers, and butterflies. Bt is more effective when eaten by a small, young caterpillars than when eaten by a large, older caterpillars.

To work, Bt must be sprayed onto foliage and then consumed by the caterpillars along with the plant foliage. Unlike many chemical insecticides, Bt cannot be absorbed through the caterpillar's skin. The active ingredients of Bt consists of spores and crystals. When eaten, the crystals dissolve and paralyze the insect gut, causing the caterpillar to stop feeding. The insect gut lining begins to break apart, which permits spore germination and production of the bacterial spores in the body cavity. This causes a general infection, which leads to death of the caterpillar.

Insecticide use

In Minnesota, Bt is registered for use in caterpillar control on forest and shade trees to control about two dozen insects. Major insects affected included tent caterpillars, gypsy moth, and jack pine and spruce budworms. In addition, bagworm, spanworm, red-humped caterpillar, cankerworms, and saddled prominent can be controlled with Bt. It does not control sawfly larvae, bees, grubs, maggots, or any adult insects.

Unlike chemical insecticides, the active ingredients in a Bt formulation are measured in BIUs (billion international units). It is measured in this way because Bt is a formulation of living organisms and its potency varies from strain to strain. Application rates vary for different Bt formulations and the target caterpillar, but are normally between 8 and 24 BIUs per acre. In some spray programs, a single application of 24 BIU has been as effective as 2 applications of 12 BIUs. Most formulations also list recommended application rates in ounces or pounds per acre.

Bt formulations can be applied to foliage in much the same way synthetic pesticides are applied. Water-based flowable formulations of Bt have recently gained wide acceptance in forest spray programs. These are mixed with water or applied undiluted with ultra low volume application (ULV) equipment. ULV applications are more efficient since the aircraft spends more time spraying the product and less time spraying water. Spreaders/ stickers are sometimes added to the Bt improve the coverage, rain fastness and resistance to wash-off.

Timing of the application and good coverage of the foliage is essential. Forest tent caterpillars, for example, should be treated when the caterpillars are 3/8-inch to 1/2-inch long or the tree leaves are one half of their full size. Bt should be applied to dry foliage and have a droplet size of 75–150 microns.

Registered formulations

In addition to forest and shade tree registrations for Bt, it is registered for use on a wide variety of food and crop plants including corn, soybeans, small grains, fruit (including strawberry and apple), lettuce, spinach, squash, onions, and many other crops. Trade names for Bt products registered in Minnesota include Dipel, Foray, Novodor, Biobit and MVP. They are produced in the U.S. by Abbott Laboratories (North Chicago) and Mycogen ( San Diego).

Note: While Bt products are very safe, they are pesticides. Like any other pesticide, its use is controlled by Federal and State laws. Pesticide users must read and follow the label directions and precautions on the container. Misuse of a pesticide is a violation of state and federal law.