Asparagus beetle

Asparagus beetle is the most serious pest of asparagus. It only attacks edible asparagus – it doesn’t affect ornamental asparagus plants.

Both the distinctive adult beetles – black with six yellowish blotches and a reddish body, measuring 6mm (¼in) long – and the greyish-black larvae up to 1cm (3/8in) long feed on asparagus plants.

The adult beetles emerge from where they have overwintered from May to June, mate and the female adults then lay small black eggs on both asparagus spears and foliage. The resulting larvae feed on the plants until fully grown. They then drop to the ground and pupate in the soil.

There are usually two generations of adults and larvae per year between May and September.

In autumn, the second generation of adult beetles, move away from the plants to find a sheltered place to overwinter.

• Edible asparagus plants

Both the adult beetles and the larvae devour the leaves, almost stripping plants completely of their foliage. They can also chew and damage the stems, resulting in the growth above drying up and turning yellow and then brown.

Heavy infestations that strip plants of their foliage weaken the plants and so reduce crops the following year. Several years of infestation can reduce plant vigour so significantly that yields are severely reduced.

Keep a close eye on asparagus plants between May and September and deal with any adult beetles or larvae as soon as you see them.

Remove any adults and larvae you see and deal with them.

Cutting down and burning the stems in autumn once they have died back, may kill overwintering beetles.

Very careful hoeing around the plants, so as not to cause any damage to the roots, may kill pupae, or expose them to foraging birds and other animals.

There are numerous chemical insecticides available that will kill soft-bodied insect pests, such as asparagus beetle larvae, and are approved for use on edible crops. Just bear in mind that the majority of these are what is known as “contact” insecticides. That is, you have to spray the actual insect (make contact with it) to kill it. If you miss some, these will not be affected. Also, most are based on “organic” or “naturally-occurring” materials and have very little persistence once sprayed on to the plant.

These may not have any effect on the adult beetles.

Use plant protection products safely. Always read the label and product information before use. Plants in flower should not be sprayed due to the danger to pollinating insects. Either spray early in the morning or late in the evening when pollinating insects are less likely to be active.

There is very little you can do to prevent asparagus beetle attacking asparagus plants. Totally covering plants with horticultural fleece or fine-mesh, insect-proof netting may give some degree of control, providing it is put in place early in the year before the adult beetles emerge.

• Insecticides
• Hoes