bacillus thuringiensis shelf life

Chris Conlan conlan at
Mon Jun 29 22:32:51 EDT 1998

>The toxin is produced by the living bacteria.  The spores germinate in the gut,
>produce the toxin (thurien I believe) which causes paralysis of the gut, then
>the bacteria proliferate in the gut destroying the larvae.

Close, but not quite.  The insect ingests the bacteria with the protein
toxin crystal.  The crystal is then broken down in the alkaline environment
of the insect gut and activated.  There it binds to the gut wall causing it
to basically ulcerate.  The gut contents then leak into the main body
cavity and raise the pH to lethal levels.  This process can take days.

>A pure toxin prep would be limited much more in its effect and by deteriorate
>quickly.  I do not at the moment have the information on how long it will last.

Some formulations contain a mix of spores and toxin crystals while others
contain only toxin crystals and there is little difference in efficacy
between them.  The spores can enhance and speed up the rate of kill
somewhat but it's not all that dramatic.  It's the toxin already in the
formulation that is responsible for most of the mortality.  I have been
involved in the testing of numerous formulations and strains of B.t. and
witnessed this first hand.  Feeding the insects pure spores does not cause
anywhere near the mortality that the pure toxin prep does.  While it is
true that the spores may persist in the soil for a long time, they just
don't cause the kind of widescale mortality you are describing.  If B.t.
was the persistant killer you seem to think then why do farmers and
gardeners constantly need to re-apply it to maintain control of their
caterpillar problems (even in areas with no resistance problems)?  Heck,
the agricultural community would be dancing in the isles if you only had to
spray something once in a lifetime to control your Lep pest problems
(something legal anyway!).

I'd like to point out that I'm not advocating everybody go out and spray
B.t. all over the place.  It's just another tool for insect control that,
when used properly, is much less harmful to the environment than many of
the other options that exist.

Sorry for the lengthy posting.  I'm outta here.

conlan at

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