The message
from "Jack" contains these words:

This is probably a daft question, but if you use specific plants to attract
butterflies to the garden, won't the caterpillars then eat everything in
sight? Are there some caterpillars that are not good to have in the garden?
I'm thinking of cabbage white, for instance -- don't they ruin your veggies?
How do you tell the difference between "good" caterpillars and "bad" ones
(no doubt that's politically incorrect)?

Caterpillars are very plant-specific. So while cabbage white
caterpillars eat anything cabbagey + nasturtiums, red admirals'
caterpillars feed on nettles.
Encouraging butterflies means more caterpillars, which means more
wasps and birds feeding on the caterpillars (so long as you don't poison
them all). That natural food-chain balance protects veg, far more than
it harms them. The more variety of insects and birds you attract to the
garden, the less damage you will get to plants.

Janet