It is a powder that you can buy from the garden centre, you dilute it with water (it is toxic to breathe it in so avoid the spray and veg cannot be eaten for 24hrs after spraying). However it is allowed by the soil association, so is not like some of the more potent insect killers. Basically it gets into the spiricles of the insect - so they cannot respire (breathe) and they die. It doesn't work on eggs, so you have to look for caterpillars - but doesn;t sound like you have eggs anymore! We tried picking them off and other organic techniques, but Derris did the trick.
We have them too in our alotment, DH gets the trio to pick them off, they love it....yuk! Trouble is they brought about 50 home and have them now as pets and now some have turned into chrysalis`s, if thats how you spell it!
sorry but derris is horribly potent and kills absolutely everything - you spray at twilight so it has less impact on nice things like bees. It's one of those 'traditional' insecticides hence theoretically allowed in commercial organic farming but really is not something natural or organic to use. Maybe use once and then after that...
pick pick pick, and then keep an eye out for eggs and squish them as soon as you see them.
the lurid green and yellow caterpillers taste foul which is why birds won't eat them. They are certainly not poisionous to touch! if you see a very hairy caterpiller it's best not to touch as some have irritants on their hairs.