Bt is a bacterium which kills insects of the Lepidoptera order which includes butterflies and moths when their larvae (caterpillars) consume it. It kills by releasing a toxin when in their gut. It does not harm other beneficial insects such as bees which do not consume foliage. It requires alkaline digestive tracks like those of insects to be effective. It does not harm humans or animals that have acid digestive systems which destroy the harmful effects of the toxin. There is, however, some controversy as to whether Bt may cause allergic reactions in some people.
Bt is available commercially mixed with other safe compounds for spraying.
Because Bt must be eaten by the larvae in order to kill them, it is important
to spray their food supply when they are actively eating on the surface.
Once they burrow into the leaves or fruit Bt is no longer effective.
Also, Bt is not very long lasting being only effective for 2 to 4 days once it is exposed to ultrviolet rays in sunlight. When used for controlling insects like Obliquebanded leafroller which lays eggs for a 30 day period, it could require as many as 6 sprays. Mixing a small amount of milk into the spray will help protect against the UV rays and may encourage some lepidopteras to feed on it. Research is being done to extend the effective time of Bt.
Bt has been bioengineered to have many strains, each one specific to a particular pest e.g. Bt / berlinerkurstake for caterpillars and Bt / San Diego for Colorado potato beetles. Bt / kurstake (BtK) however is one strain that will kill both beneficials and pests. Some insects such as the leafrollers are becoming resistant to Bt.
Bt comes in liquid form and a wettable powder or dust. It is sold under names such as DIPEL DF, THURICIDE, COSTAR and BIO-TROL. A fairly new product, BIOPROTEC 3P, is in the form of a dustless powder that mixes well with water. This new product will stay effective in the field for a long period and is more resistant to UV, rain, and dew compared to conventional Bt products.
See suppliers list.
1. Excellent article on Bt from the University of Illinois.
2. Another informative article from University of Illinois, Co-operative Extensions Service.
2. Detailed scientific information about Bt, Biotechnology Information Centre of the USDA.
3. Use of Bt, Canada Department of Agriculture.
4. Biological Control, Bt, NYSAES, Cornell University.
5. List developed by an International Committee of over 50 pesticidal toxic genes isolated from Bt.
6. Proceedings of the 2nd Canberra Meeting on Bacillus thuringiensis by RJ Akhurst (Ed), CSIRO Division of Entomology.
7. Commercial size trials in California. Use of Bacillus thuringiensis (Bt) for Oblique-banded Leafroller Control.
8. BioProtec Bt (new product manufactured by AEF Global)