Wireworm

Identification

Wireworms are the larvae of the click beetle. The larvae grow up to 25 mm in length and are orangey/brown in colour with a narrow, segmented body, biting jaws and 3 pairs of short legs behind the head. The adult beetles are dark brown or black and are approximately 2 cm long and live in hedges and grassland areas. When alarmed the adults can leap into the air by flexing the joint between the abdomen and the thorax creating tension like a coiled spring which when released catapults the insect into the air making an audible click and hence their name. It has been estimated that there are more than 60 species of click beetles in the UK.

Symptoms

Wireworms living in the soil can eat their way through the tap root of young beet seedlings leading to complete plant loss. The main feeding period, relevant to beet, is March to May when crops will be at greatest risk.

Life-cycle

The adult females lay eggs in grassy ground in the spring. These hatch approximately 1 month later and the young larvae start to feed on organic matter in the soil. This first stage (first instar) is white and 1-2 mm long. At this stage very little economic damage is done. The larvae continue to grow by moulting at the end of each instar and the full life-cycle can take up to 5 years to complete. The mature wireworms pupate underground for about 4 weeks and the new adults emerge in July or August and hibernate until the following spring when they lay their eggs. At any one time a field can contain every stage of the wireworm and click beetle life-cycle.

Importance

The incidence of wireworm damage in arable rotation is increasing but crops at greatest risk would be those following grass grown within the past 4 years.

Threshold

None established.

Modern seed treatment insecticides can help to protect beet crops from damage caused by wireworms. Where insecticidal seed treatments have not been used and wireworm activity threatens to affect successful crop establishment the application of contact acting insecticides should be considered.