Sorry to have to show you this horrid beetle photo, but we have an infestation of these nasty things on our allotment. The plot opposite mine has them big time and my poor little spears that are just coming through from the purple asparagus and the little ferny shoots that I'm trialing from Victoriana Nursery have them too. I wasn't sure how to deal with them, so I e-mailed Stephen from Victoriana Nursery and he gave some good advice and a link to his product that will help eradicate them. Here is what he said :----------
The beetles do no damage - its their larvae that nibble the emerging shoots and cause them to grow distorted like shepherds crooks.
The reality is there is nothing that will completely knock out the beetles as they emerge over a long period. Vigilance is the most important thing and pick off any beetles you see and more importantly cut any fern that you see tiny black eggs on (not yet I would think).
Something like http://www.victoriananursery.co.uk/plant_care_and_gardening_tools/biofriend_plant_defence_no1/
will kill some and put off a lot of the rest (we use it on our own beds - do you want some) and birds will help as well as will a hard Winter.
But ultimately you probably need to accept you'll get a few odd shaped spears each year; as you are not Tesco I'm guess that doesn't matter?
They will never kill the bed off.
SO some re-assuring words there from Stephen, which made me feel much better about it. I went up and gave a copy of his advice to my fellow plot holder and he and I went around picking off the nasty beetles and popping them in a bucket of water, there were so many, so hopefully we stopped many eggs being laid.
I hope you don't get them on your asparagus and if you do then I hope Stephens advice and his all natural organic product will help you.