From the middle of April it is essential to watch out for asparagus beetle - well, if you grow asparagus that is - I believe they are specific.
The beetles themselves are approx, quarter inch or half cm in length. They are quite colourful - black head, thorax and abdomen with bright orange/red wing cases decorated with 6 bright, white dots( these are the common beetles - the only ones I've seen on Horfield allotments - However, the spotted variety have more white dots and arrive later).
The beetles will lay hundreds of tiny blackish eggs( 1mm in length), which are attached to the undersides of the asparagus fronds.
Within about a week the eggs wil hatch into larvae. It is the larvae which will do the damage to your asparagus and really, they will do massive damage if not stopped. Left to their own devices they will strip the ferns of their foliage. They will then die back, causing your crowns to fail the following year.
All is not lost however, Unless you are totally swamped by the little pests, just pick them off as you see them and, if you can bear to do it, crush them in your hands - they are quite hard so will require some force. Otherwise go equipped with a container of soapy water and drop them into it.
For the larvae - when they do appear, I have been crushing them - a most unpleasant job. It has been suggested that using a soft brush to brush them off the ferns is quite effective - that they will die before they can get back onto the ferns.