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David Hare-Scott[_2_] · 14-03-2009, 07:45 AM posted to rec.gardens.edible

wrote:
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.

OK, but I'm told that borers can live in the soil, and emerge the
following season. That's why floating row covers are not necessarily a
good idea for borer control. But following the logic here, if Bt
washes off the plant and gets into the soil, wherein it is tilled in
to make sure sunlight doesn't hit it, why doesn't it just kill those
emerging borers?

Because it isn't a poison as such, it is a microorganism. Grubs and
caterpillars need to get it into their gut by eating it for it to be
effective, which is why you put it on to leaves and other parts of the plant
that makes up their diet, they don't eat dirt.

David