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Posted 15 April 2008. PMN Crop News.

'Bt' Spells Bad News for Caterpillars

Oklahoma State University.

Stillwater, Oklahoma-(April 4, 2008)-Caterpillars will start to negatively affect various crops, shrubs, trees and other green plant material as the weather warms up, leading many homeowners and agricultural producers to use "Bt" to control the pests.


“There has been tremendous renewed interest in Bacillus thuringiensis, or ‘Bt’ as it is often called, an insecticide that is created from bacterium that occur naturally in soils; several strains can infect and kill insects,” said Phil Mulder, interim head of Oklahoma State University’s department of entomology and plant pathology.

Mulder said “Bt” formulations typically represent an excellent choice of a biological control. Affected insects stop feeding within hours of exposure, eventually dying of starvation because the “Bt” essentially paralyzes their digestive system.

The most commonly used strain – kurstaki – will kill only leaf- and needle-feeding caterpillars. Other strains have been developed that control certain types of fly larvae. These are widely used against larvae of mosquitoes, black flies and fungus gnats. More recently, strains have been developed with activity against some leaf beetles.

“It’s important to remember that insecticidal activity is specific,” Mulder said. “Strains developed for mosquito larvae don’t affect caterpillars, for example.”

But that is a good thing. A person needs only to choose the correct formulation for the job at hand.

“Since ‘Bt’ formulations don’t have a broad spectrum of activity, they don’t kill beneficial insects,” Mulder said. “This includes the natural enemies of insects as well as beneficial pollinators such as honeybees. This means ‘Bt’ works well with other natural controls.”

Perhaps the major advantage is that “Bt” formulations are essentially nontoxic to people, pets and wildlife.

“This is a key safety benefit for food crops and other sensitive sites where pesticide use can cause adverse effects,” Mulder said.

Unfortunately, ‘Bt’ formulations do not have a tremendous residual capacity because they break down in ultraviolet light and, like many insecticides, are susceptible to wash off in the rain.

In addition, “Bt” formulations must be eaten to be effective, and application coverage must be thorough. This can limit effectiveness against pests that are susceptible to “Bt” formulations but rarely have the opportunity to eat them in the field, such as the codling moth or corn earworm that tunnel into plants.

“Since ‘Bt’ doesn’t kill rapidly, a person might incorrectly assume it is ineffective a day or two after treatment,” Mulder said. “However, this often is just a perceptual problem because affected insects eat little or nothing before they die.”

Many but not all formulations are exempt from pesticide tolerance restrictions and may be used right up to harvest time on a wide variety of crops.

“This makes ‘Bt’ formulations useful in applications where pesticide drift into gardens is likely to occur, such as when a homeowner treats trees and shrubs,” Mulder said. “The exceptional safety of ‘Bt’ products also makes them useful where exposure to pesticides is likely during the mixing and application phases.”

The most widespread use of “Bt” involves the kurstaki strain used as a spray to control caterpillars on vegetable crops. Such formulations also are used widely in agriculture as a liquid that is applied through overhead irrigation systems or in a granular form.

Additional information on “Bt” formulations and their recommended uses is available through the OSU department of entomology and plant pathology’s Plant Disease and Insect Advisory at on the Internet.

Donald Stotts