Bacillus thuringiensis shelf life

Don Barry dbarry at
Thu Jun 25 09:21:28 EDT 1998

Dan & Semjase,

Bt formulations are very susceptible to UV radiation. They breakdown
rapidly and may be ineffective within 24 hours of application. Wide
scale forest treatments are formulated with UV protectants and may last
a little longer, but to suggest that they contaminate areas for years is
flat wrong. 

Bt is effective only when ingested (it is not a contact pesticide) and
only against very young larvae (first or second instar). If the larvae
are larger than 1/2 inch or so, Bt is not going to work very well. For
best results, treat foliage infested with young larvae early in the
morning after things have warmed enough so that they are feeding, say
about 10am. You may have to reapply but make sure to follow the
instructions on the label!

Shelf life varies and is related to both formulation and storage
conditions. Bt breaks down more rapidly when stored in unheated areas
where temperatures repeatedly cycle between freeze and thaw. Even in the
best conditions, Bt will gradually loose effectiveness and breakdown
within a few years.

Don Barry
Univ. of Maine Cooperative Extension
Pest Management Office
491 College Ave.
Orono, ME 04473-1295

Tel: 207-581-3884
Toll free (within Maine) 1-800-287-0279 
Fax: 207-581-3881
Email dbarry at

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