BT: What is it?

As a gardener who works with insects, I get a lot of questions about any news  topics that mention plants and insects in combination. I’ve gotten a lot of questions from guests and friends about the pesticide BT and its uses, applications and presence in genetically modified crops. Here are a few of the questions I have answered recently on this topic.

What is Bt?

Bt is Bacillus thuringiensis, a bacterial disease that affects insects.

From CSU Extension

by W.S. Cranshaw (12/08)

• Bacillus thuringiensis (Bt) is a naturally occurring bacterial disease of insects. These bacteria are the active ingredient in some insecticides.
• Bt insecticides are most commonly used against some leaf- and needle-feeding caterpillars. Recently, strains have been produced that affect certain fly larvae, such as mosquitoes, and larvae of leaf beetles.
• Bt is considered safe to people and nontarget species, such as wildlife. Some formulations can be used on essentially all food Crops.

This bacteria was identified in 1901 in diseased silk moth larvae in Japan and again in 1911 as the cause of disease in larva of flour moths.

What are the uses of Bt?

Bt is used as a pesticide on many food crops. Considered nontoxic to people, wildlife and pets, Bt is applied to crops that will will be consumed by people or may come into close contact with people and wildlife.

Many strains of Bt exist that are used to target specific insect pests:

• The kurstaki strain of Bt is used to control many caterpillar insects on vegetable crops such as the tomato and tobacco hornworm, cabbage looper and European corn borer.
• The israelensis strain of Bt controls larva of the fungus gnat, mosquito larva and black fly.
• The san diego/tenebrionis strain is used to control the elm leaf beetle, Colorado potato beetle and the cottonwood leaf beetle.

BT strains are intended to be specific rather than broad spectrum, meaning that they affect target species and not beneficial insects or other animal species. At least 34 sub-species or varieties of Bt exist and some 800 strains have been isolated.

Bt has been available for commercial use in the United States since 1958.

How does Bt work?

Bacillus thuringiensis bacteria break down to produce a Delta endotoxin that binds to the epithelium or cell lining of the insect gut. Once attached, these Delta endotoxins cause a cell lysis or destruction by creating small pores or holes in the cells of the insect gut. Once the cell wall is breached, molecules of the cell can flow out and surrounding tissues can enter which disrupts crucial reactions inside the cells. This reaction paralyses the insect digestive system and stops the insect from feeding within a few hours. Unable to feed, target insects die of starvation and poisoning within a hours to a few days.

What is a Bt Crop?

Bt crops are vegetable or other crops like cotton that have been genetically modified with short sequences of Bt DNA. This allows the plants to produce the same proteins that break down into Delta endotoxins. This provides the crops with protection from particular insects without the external application of Bt or other pesticides.

From UCSD:
Bt GM crops are protected specifically against European corn borer, southwestern corn borer, tobacco budworm, cotton bollworm, pink bollworm and the Colorado potato beetle.
Other benefits attributed to using Bt include:

• Reduced environmental impacts from pesticides – When the plants are producing the toxins in their tissues there is no need to spray synthetic pesticides or apply Bt mixtures topically.
  

• Increased opportunity for beneficial insects – Bt proteins will not kill beneficial insects.

• Reduced pesticide exposure to farm workers and non-target organisms.

Is Bt safe?

Although Bt is found naturally in soils around the world and has been collected from the guts of wild insects, commercial insecticides provide a higher content of Bt than will be naturally found. Bt is listed as non-toxic to humans and wildlife; however so many pesticides that have been used in the past were found, much later, to have lasting and far reaching effects beyond their intended purpose.

As with all pesticides, we personally recommend that BT should be used responsibly and in the smallest dose necessary to curb insect pests.

Studies are constantly being conducted that measure the presence of Bt in air and water after aerial sprays and treatment of bodies of water. Lab studies have and still are being conducted on laboratory animals to discover dosage levels of Bt that may be harmful or fatal to mammals.

We do not apply Bt or other commercial pesticides in the BioWorks Butterfly Garden with the exception of the occasional treatment of a fire ant mound or wasp nest in a publicly accessible area.

For More Information

• Bacillus thuringiensis Insecticide Fact Sheet at Mindfully.org
• Bacillus thuringiensis Fact Sheet at Colorado State University Extension
• Bacillus thuringiensis Website at University of California, San Diegowire
• EPA Regulations for Bacillus thuringiensis Crops at Environmental Protection Agency
• Bacillus thuringiensis article at Wikipedia