Asparagus beetle

I picked / squished and sprayed last year, but they're back in force this year  many of the fern tops are now stripped completely.  This years crop was lower than last with 4-5 spears per plant max and most of those either thin or malformed.

Is control a realistic option? anyone else having/had the same problems - If it's going to be a slow 3-4 season demise I might as well dig the bed up now. 

I use a pyrethrum based spray and it does knock them down, but you have to spray regularly.  I had loads of the little beasties earlier on in the season, but not seen many about lately.  Hopefully I got to them before they laid eggs and I'm benefiting from that now.

Asparagus does like a good soil so thin spears may be due to that.  Mine get a heavy mulch each winter of compost and/or manure.  Malformed ones are often where they have been slug nibbled on one side, just as they are emerging.

The bed was mulched with chicken manure compost in spring, I can't imagine it's a lack of food.

I've been spraying Bayers greenfly + but I swear they've been taking a bath in it.