This is the place to discuss not just allotments but all general gardening problems and queries which don't fit into the specific categories below.
(formerly allotments and tips, hints and problems)
Other than stern words with them, what can I do? My fellow allotmenteer said that a solution of water and vinegar is great for slugs, so will that work with caterpillars? I have a sneaky suspicion that I bought a plant with the eggs on, and they hatched. Its the only plant affected out of them all, and they devoured it literally overnight
Apart from picking the buggers off and making a spray from their guts (sorry intestines) and spraying it on the rest, you could try Derris dust or Bacillus Thurignesis in a spray called Dipel here in Aus, may have another name over there..........
I'm going to try covering my brassicas with fleece this year to see if that keeps the buggers off.
There are two types of cabbage caterpillar in the UK; a pale green one, and one with dark stripes. They can both be pretty prolific. The butterfly of the pale one lays single eggs on the underside of the leaves that can be hard to spot, but the other one lays them in a clump - a sort of bright yellow matrix of little eggs. If you see any, crush them with your fingernail before they have a chance to hatch.
Last year I did the hand picking thing, or if you are allowed hosepipes where you are, you can jet then off while you are watering. But be diligent - it's amazing how much they eat.