I have a community pot of small opuntias that were completely trashed by caterpillars, my crested one just had a big piece fall off so I am assuming they got right in
I'm not much of a pesticide expert, any tips on how to avoid it? The garden here (we're staying with the parentals at the moment) has a lot of them and dad sprayed something toxic for them recently but i am still spotting them

Carbaryl was the recommended caterpillar spray a few years ago, but it's nasty stuff. I have no idea of its safety on cacti, though. I suspect it's no longer available due to toxicity concerns. Malathion was another nasty one, but honestly organo-phosphate poisons give me the heebie-jeebies. Dimethoate (Rogor) is one that I have used, but it's still pretty nasty. Confidor seems to be a popular systemic insecticide, but I have no idea about its effectiveness against caterpillars specifically.

I personally favour squishing 'em manually.

On a related note, I reckon Opuntias seem to be snacked on by more pests than pretty much any other cacti that I've grown. Snails and slugs seem to love them, and *something* - I'm yet to work out what - has been going the munch on my feral O. monacantha. That said, if it distracts them from my valuable plants, I'm okay with that...

I think I might try coffee grounds for the snails, I'll see if the smell puts them off. I might google natural remdies too, I;m going to try the copper strips for snails, they had an article in Burkes Backyeard where they actually tried out all the 'natural' snail remedies and they claimed that one worked