   Bacillus thuringiensis: An Insecticidal Bacteria - September 29, 2004 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County Most farmers and home gardeners are aware of the biological insecticide Bacillus thuringiensis: Bt for short. Bt is often used to control plant damaging insect larvae and is preferred because it has no effect on many beneficial insects and is relatively harmless to humans, pets, wildlife, and other non-target organisms. Bt products are available at garden centers and can also be ordered and shipped from catalogs and Internet suppliers. Bt is a naturally occurring soil bacterium and was discovered in diseased flour moth caterpillars in 1915. Work began on it during the 1950s and it was first sold commercially in 1961. Some Bt strains kill insects kill insects with toxins called insecticidal crystal proteins or delta endotoxins. After feeding on these Bt-proteins, the digestive system becomes paralyzed and the infected insect stops feeding within hours. Bt-affected insects generally die from starvation, which can take several days. The most commonly used strain of Bt (kurstaki strain) will kill only leaf and needle-feeding caterpillars. Some of the specific crop pests controlled are European corn borer, cabbage looper, tomato hornworm, alfalfa caterpillar, tent caterpillar, and leafrollers. In the past decade, Bt strains have been developed that control certain types of fly larvae (israelensis strain or Bti) and are used to control larvae of mosquitoes, black flies and fungus gnats. Bti has become one of the standard mosquito control tools in the battle against West Nile Virus. More recently, strains have been developed with activity against some leaf beetle larvae, such as the Colorado potato beetle and elm leaf beetle (san diego strain and tenebrionis strain). Among the various Bt strains, insecticidal activity is quite specific (i.e., Bt strains developed for mosquito larvae do not affect caterpillars). Bt is susceptible to degradation by sunlight and most formulations persist on foliage less than a week following application. Even shorter persistence occurs with some of the newer strains developed for leaf beetle control, which may become ineffective in about 24 hours. As with any pesticide, users should always read and follow label directions. Commercial agriculture has also embraced Bt in other ways. Production of delta endotoxin is controlled by a single gene in the bacteria. This has lead to Bt’s use in genetically modified food crops. By inserting the delta endotoxin gene into plant chromosomes, it is produced within the plant providing it with natural resistance to susceptible insects. Genetic modification with Bt has been used in corn, cotton, and other crops for several years now. Some readers may remember popular press reports about monarch butterflies being negatively affected by Bt corn pollen. Subsequent research has proven that Bt-modified crops were much less harmful to monarch butterflies under actual field conditions. When used in field crops, such as corn and cotton, there are valid concerns that insects can become resistant to the Bt toxin. If an insect receives a low dose of the toxin and survives, then its offspring could become increasingly tolerant of the Bt toxin. There are two strategies that growers use to address this issue. The high dose strategy tries to make sure that the Bt dosage is high enough to kill the insect so that it cannot reproduce. The refuge strategy reserves an area that is planted to non-Bt varieties then any insects coming out of these areas will be susceptible to Bt. They will mate with any survivors from the Bt corn and preserve the genetic susceptibility of the overall population. Bt is a wonderful tool for managing susceptible insect species. It has also reduced inputs of chemical insecticides in our environment, made food safer for consumers, and reduced the risks of pesticide exposure for farm workers and subsequent food processors. The University of Arizona Cooperative Extension has publications and information on gardening and pest management. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at mgardener@verdeonline.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/. |