Bt Crops
*This web page was created by Lane Estes and Graham Watson for Biology 361, an undergraduate course at Davidson College*


Image courtesy of


What is Bt?
Bacillus thuringiensis (Bt) is a common soil bacterium that produces crystals containing proteins that are toxic to certain insects. Discovered in Japan in 1901 from dead silkworm larvae and first used commercially in the United States in 1958, this bacterium with insecticidal properties captured 95 percent of the biopesticide market as of 1989. At least 34 subspecies and 800 different strains of this bacterium are used today in a variety of spays and dusts to control both beetle larvae and moth and butterfly caterpillars which feed on fruits, vegetables, and other cash crops including corn, potatoes, and cotton. Bt is also effective against mosquito and blackfly larvae, mites, flatworms, and nematodes (Swadener, 1994).

How Does Bt work?
After Bt is ingested by the larvae of certain insect species, digestive enzymes specific to those insects dissolve the Bt protein crystal and activate the bacterium’s toxic component, called delta-endotoxin. This endotoxin binds to certain receptors on the intestinal linings of these insects, which causes pores to form in the membrane of the intestine. By disrupting the ion balance of the intestine, this proliferation of pores in the insect’s intestinal membrane causes the insect larvae to stop feeding and eventually starve (Levidow, 1999). Although highly toxic to certain insects, Bt is relatively harmless to humans because the human body lacks the digestive enzymes needed to dissolve Bt protein crystals into their active form (Swadener, 1994).

What are Bt Crops?
One of the first genetically modified organisms were transgenic crops containing a gene from Bacillus thuringiensis that coded for the protein crystals in Bt which are toxic to some insect larvae. The Bt gene was successfully inserted into the genome of several cash crops via a gene gun. The insertion of the Bt gene directly into the genome of crops allowed the crops to perpetually produce this Bt toxin in their own cells. Theoretically, this conferred constant insect resistance to transgenic Bt crops without the application of any pesticide dusts or sprays. Also, since Bt crops constantly produce Bt protein crystals in all tissues of the plant, Bt crops, unlike traditional Bt sprays and dusts, remain effective once the larvae have bored into the plant stalk (Levidow, 1999). Today, versions of Bt cotton, Bt corn, and Bt potatoes are being grown in the United States, Canada, Argentina, South Africa, France, and Spain.

© Copyright 2002 Department of Biology, Davidson College, Davidson, NC 28035
Send comments, questions, and suggestions to: or