Woke up this morning, looked out of the window at the back, and one of the plants out there (about 5ft tall and about as wide), that was full of leaves, suddenly all the leaves are skeletons. Didn't have a chance to look more closely till now, but have now found that it's covered with little green caterpillars that have eaten the leaves. So far it's only that plant that's affected (it's a weed that I've let grow far too big), but are they going to go on and munch the rest of the garden? What do I do, please? Anyone know?