The old favourite carbaryl is still the most effective control and acts as a stomach poison that kills the caterpillar or other cruncher on ingestion.
It is spraying over the foliage and care must be taken not to wash it off too quickly as it is very water soluble. Let them have good feed first.
Carbaryl also has a pleasant clean smell.
The white powder is dissolved in liquids these days to make it safer and easier to measure out.
So how do you know what been eating your precious blooms and leaves.
It all in the signs "Watson". Slugs and snail leave silvery trails,rats and mice leave large wheat grain sized droppings and caterpillars leave smaller dropping where they eat.
I recently found coffee granule sized droppings on the sarcochilus photographed below and knew there had to be a cruncher close by and finding him was easy after following to Poop trail!
It would be lovely if milder pesticides did the job.
Frogs do a great job,always encourage them! 
Dipel, which is often referred to for caterpillar control is useless on all plants except brassicas (cabbage, caulie etc.) as it requires an exact Ph. in the gut of the caterpillar to work. This is only achieved by eating these and only these type of plants.
It is a fungal spore that eats the cruncher inside out. So Dipel is, fungal spores mixed with a spreading powder, but no good for orchids.
One of my special tips, although not very pleasant ,is to keep an old badmitton bat with a long handle handy and go playing badmitton with the cabbage white butterflies.
Sounds afewfull I know, but it sure as hell is satisfying!
