CBA's all round today I was going to do chicken and leek pie but I have a stinking headache and don't feel like eating let alone making a pie. Probably do a cheese sandwich later after a restorative nap.
Rats love to live somewhere they can always get water, so allotments are popular because of all the taps and waterbutts around. I worked in garden centres for quite a few years and you always get them there because of the same reason.
Our site has no chickens on it, but we still get rats around. Luckily we are surrounded by houses and there a lot of cats that patrol the site, so they never get too out of hand.
I’d agree with John that breaking the tunnel up where you can get to it without disturbing the asparagus may well work. You will need to keep doing though until they get the message.