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David Hare-Scott[_2_] · 12-03-2009, 06:38 AM posted to rec.gardens.edible

wrote:
I understand that Bacillus thuringiensis (Bt) breaks down naturally,
but I'd like to know how. I'm going to be applying Bt powder, and the
instructions say to do it roughly weekly, and especially after it
rains.

Now, does this mean that Bt powder loses potency when wet, or just
that it gets washed off the plant? Hard to imagine the former since
you can keep a spray bottle of the stuff dissolved in water. I know
that sunlight breaks it down, but does that mean that if cultivated
into the soil it stays there for longer, such that it kills grubs that
are just hatching there? I've also read that dry spores have a shelf
life of several years (implying that wet spores do not).

Yes, I know you want it on the plant, so I know that you want to
reapply if it gets washed off. But what if it just drizzles?

The problem is it washes off fairly easily and as it is not absorbed by the
plant this means it is not available for the grubs to ingest. This is
inconvenient but also one of the reasons why it is so safe, it works by the
bacteria attacking the gut lining of grubs and they must eat it it for this
to happen. But this means it doesn't effect anything else. So no it
doesn't effect anything in the soil.

The instructions to apply it regularly are intended to give full coverage
and of course to sell more BT. If you can deal with the odd hole in your
cabbage it is fine to just apply it when grubs or caterpillars are evident.
You don't need it at all when the season is too cold for the insects to be
actively breeding and laying eggs on your leafy veges.

David

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