Recently I have noticed several Euphorbia have been damaged by web forming caterpillars. They seem to be particularly attracted to Euphorbia rigida, and have destroyed around 50% of those I planted earlier this summer. They have also inhabited a Euphorbia umbellata 'Ruby' which is in a pot. The advice gleaned on-line is that one should pick off the web, but I have found this to be ineffective. By the time the web becomes noticeable the damage is done, and some eggs always remain and develop into more caterpillars. I have tried washing them off with a jet of water, but I suspect this just creates favourable conditions for the previously invisible caterpillars which have already burrowed into the stem. I suspect the occasional watering of young plants may have encouraged the pest too. Any ideas?