Question
What is the Bt you recommend for use against the caterpillar's on the datura?

Answer
Hi Abbey,
Thanx for your question.  Bt is a natural insecticide, rather it is a parasitic organism (Bacillus thuringiensis).  It comes in liquid and powder form.  When applied to plants, the caterpillars ingest it.  The bacteria attack the cells of the caterpillar's gut and kind of punch holes in it. Within a few hours the insects stop eating and they die unable to feed themselves.  It is an organic method of controlling caterpillars because the Bt is harmless to beneficial insects like lady bugs.  I hope this helps.
Tom