Any pesticide, including BT should be used when every other
possible solution (including wait and see) has been exhausted.
BT has been implicated in the decline of many species of
butterflies. Rememeber many of these caterpillars turn into
butterflies and even many of the moth species are important
pollenators and plants have established a kind of balanced
give and take with most insects (sure, eat my leaves, but
pollenate me, too).

A plant's goal is to reproduce for the next generation.
A few holes in a few leaves, spots on the apples... these are
asthetics issues. Yes, the plant may appear more rubust
or produce a larger crop, but chances are the flowers will
get dead-headed or the fruit will be harvested without
seed dispersal. From a reproductive success point of view,
the plant is better off with a few caterpillars....


--
Mike Patterson
Astoria, OR
celata at pacifier.com

Hey, Farmer Farmer, put away that DDT now
Give me spots on my apples, but leave me the birds and the bees
Please....
- Joni Mitchell

http://www.pacifier.com/~mpatters/bird/bird.html