There are two species of asparagus beetles.
The common asparagus beetle, Crioceris asparagi, and the spotted asparagus beetle, Crioceris duodecimpunctata.
Common Asparagus beetle:
Damage is caused by both the larvae and the adult beetles.
The grubs feed on the tender young tips and foliage.
When the adults emerge they begin to feed on the tender growing tips of newly sprouted asparagus, and chew holes in the plant which causes a brownish discoloration to the tissue.
This damage can seriously affect plant growth causing a decrease in the size and quality of the crop.
The adult beetles are 6-8mm (1/4”-3/8”) long have blueish-black heads, legs and antennae and reddish thorax.
The wing markings are quite distinctive with three pairs of yellow squares on black wings with reddish margins.
The adults overwinter in any detritus that has accumulated in the bed.
Clearing such debris from the area will help to reduce numbers of overwintering pupae.
On emergence in May, the adult beetles begin feeding and laying elongated black eggs on the young shoots.
These eggs can be laid either singly or in rows of two to eight, and they hatch in three to eight days from laying.
Depending up on the climate, there can be two or three generations of larvae in a season which lasts from May to September.
When the 10mm long the dark gray to olive-green grubs mature, they drop to the ground and construct a small earthen cell where they transform into pale yellowish pupae.
The adult beetles then emerge from this pupae.
The best form of control is to remove the larvae and adult beetles by hand.
They can also be controlled during the cutting season by cutting the shoots off just below ground level and this will remove the eggs before the larvae can establish themselves.
When necessary, spray the young larvae with suitable pesticide, but please note these sprays have little or no effect on the adult beetle.
Apply as frequently as needed but do not repeat applications within three days.
*Many insecticides are unsuitable for use on culinary asparagus.
If used, be sure to check the number of days between harvesting and application to avoid ingestion.