bacillus thuringiensis shelf life

Don Barry dbarry at
Fri Jun 26 10:26:49 EDT 1998

Dan Jones:
> We've been plagued with squash vine borers 
> both this year and last, and I've been trying 
> to exterminate them using "Safer Caterpillar 
> Killer", a product containing bacillus 
> thuringiensis (B.t.) var. kurstaki. 

I'm embarassed that I didn't see this before but I was caught up in the
shelf life/proper application issue: Bt will not control squash vine
borer. The eggs are laid inside the stem and the caterpillar develops
within, isolated from any topical sprays. I doubt injection will work
because Bt is a stomach poison and an injection would deter feeding.
Control efforts are usually aimed at managing the adult by a series of
weekly residual treatments right on the base of the vines.

> it is unfortunate however that the material 
> is living and can become established in the 
> soil much to easily.  Ever think of Anthrax?

It's true that there are pathogenic species of Bacillus but Bt is not
one of them. It does not infect man and it doesn't really infect
insects. The chemicals involved in producing the spore just happen to be
active against certain insects when ingested. These chemicals are not,
in themselves, toxic and only become active when thay are cleaved by gut
enzymes. The spore forming chemicals of Bt kurstaki are cleaved to toxic
fragments in lepidopterous guts. If another type insect ingests Btk, it
is cleaved to different, non-toxic fragments. In the same way, Bt San
Diego is toxic to beetles but no other groups. Actually most of the
bacteria in Bt formulations are dead or absent. It doesn't matter if
they are there or not because it's the spore forming chemicals (which
crystalize and can be isolated) which are insecticidal. In fact many of
the newer Bt products are isolations of specific chemicals derived from
the spore forming crystals and entirely devoid of any bacteria.
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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