Thanks for the ideas.

They are either leopard or tiger slugs. They are revolting and repulsive, and would be harmless if it wasn't for the amount of slime they produce. It's making the frames wet.

We've always had a problem with slugs inthe garden. There's always a heavy dew after even the hottest day, so slugs and snails will happily slither across broken eggshells, sand, coffee grinds and wood ash on their way to a meal. I think they eat vaseline for breakfast.

This hive is on a wooden stand, made of roughsawn timber. None of the others, including the adjacent hive which is sitting on a couple of plastic bread trays, are affected so maybe it's something about the colony or the hive itself that's attracting them. I have no idea what it could be because they're all swienty poly and the bees are local mongrels.

Unless slugs can shred themselves to pass through the varroa mesh they have to be getting in through the entrance. I've bought a strip of copper that's meant to protect flowerpots. I'm going to nail it right round the floor, in an unbroken band, to see if it works.