I have tried to grow different, head-forming cabbages in the last couple of years in my allotment (this is in London, UK), and while they grow well enough, I have some unusual problems; or at least I haven't seen them before. They are not infested with butterfly larvae or any of the usual things, but when I harvest them, they are full of slugs and earth-worms inside. I didn't think of taking pictures, but I have been finding small earth-worms between the tightly packed leaves.
I am wondering if it is because I have added a lot of well-rotted horse-manure and compost; the worms definitely like that, but I'd rather they stayed away from my cabbages. Any ideas about what to do about this? I'm not in favour of using pesticides.