GardenLine | Common Saskatchewan Pests | Bt (Bacillus thuringiensis) The Natural Insecticide

Bonnie Willie

Bacillus thuringiensis (Bt) is a naturally occurring, soil borne organism that has gained recent popularity for its ability to control certain insect pests in a natural, environmentally friendly manner.

Bacterial agents, like Bt, are effective in controlling insects in the larva stage only. The larva stage in an insects's life cycle is the stage during which most of the feeding occurs. Since, Bt must be ingested to work, the insect must be controlled during the larval stage. The larval stage is the stage in which the insect appears worm or caterpillar-like.

The Bt is applied to the foliage of plants infested with a leaf or needle eating larva. If possible, apply the Bt to the underside of the leaf surface for two reasons:

i) Most larva feed from the underside of the leaf surface.

ii) Bt is broken down faster in sunlight. Application to the undersides of the leaf surfaces will prolong Bt's activity. High temperatures do not encourage Bt's breakdown.

After Bt spores are ingested by larvae, they grow and reproduce, meanwhile producing crystalline toxins. The crystalline toxins paralyse the digestive tract of the larvae causing it to cease eating. Death will follow, but time of death can range anywhere from 12 hours to 5 days after ingestion This depends on the amount of Bt ingested, the size and variety of the larvae and variety of Bt used for control.

There are different strains or varieties of Bt available that have been selected for the control of specific insects. Bt variety kurstaki (BTK) controls the European corn borer, tomato hornworms, fruitworms, cabbageworm, cabbage looper, spring and fall cankerworm, spruce budworm, and other caterpillar-like larvae. Bt variety san diego (BTSD) controls early larvae of the Colorado potato beetle. Bt variety israelensis (BTI) controls mosquitoes, black flies and fungus gnats.

The Bt has gained well-earned popularity because of its distinct advantages over other pesticides:

1) Hazards to humans are negligible although inhalation or contact with eyes or open wounds should be avoided.

2) Bt can be used right up until harvest, which allows for a longer term control as compared to other insecticides requiring a waiting period from time of application to time of harvest.

3) There is no waiting period from time of application before re-entering the field.

4) The different strains of Bt are class-specific, meaning beneficial or non-target insects are not harmed.

5) The insects that ingest the Bt and later die from it, are not considered dangerous to birds or other animals that may feed the dead insect.

6) Bt is not known to cause injury to plants on which it has been applied and is not considered harmful to the environment.

7) Little or no resistance has been reported to date, but one still should be cautioned against the over use of Bt. Relying on any one pesticide can lead to the build up of resistance in the pest population Use of other insecticides and cultural control methods should be used to slow down or eliminate the possibility of resistance developing.

REFERENCES:

"Bio control '93, Beating Bugs with Bts"; by Kris Firth and Perry Klassen; American Vegetable Grower; March 1993.

Rodale's All-New Encyclopedia of Organic Gardening; Edited by Fern Marshall Bradley and Barbara W. Ellis; 1992.

"The Swat Team"; by Jennifer Bennett Harrowsmith; June 1993.

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Sustainable horticultural information, offered free of charge to the public with the support of the University of Saskatchewan Extension Division, the Department of Plant Sciences and the Provincial Government.