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 http://www.pcca.com/Publications/Commentator/1996/winter/page06-2.jpg |
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: laestes@davidson.edu
or grwatson@davidson.edu