Green caterpillars on the thyme

Urgle! Two days ago we had a healthy common thyme in a terracotta pot. Yesterday evening we were astonished to find it had been stripped bare and was covered in fat bright green caterpillars! :eek: How on earth did that happen? We're 150 feet up in Central London and growing everything on a terrace in containers!

I've checked everything else and can't see the critters on any other plants. Bought this one in Hombease last October, but I can't think that has anything to do with the problem. Help!

 

There are very many different sorts of caterpillars and not only of butterflies and moths. There are those of sawflies too and although they are smaller, they can strip a plant very quickly. Thyme is usually pest free, but if there is not much else to eat in Central London, then your thyme was a tasty meal. If the eggs were not on the plant when you bought it, then they would have been laid by a high flying moth, butterfly or sawfly . Never mind, console yourself with the thought that you may have increased the butterfly population of Central London

 

If it had turned out to be something exotic I wouldn't have minded so much, but I just Googled it a an image and it's the caterpillar of the small cabbage white butterfly - described as 'probably the most destructive larva of all'. You're telling me!