Bayer CropScience - Pestspotter - Wireworm on cereals

Pest: Wireworm on cereals

Identification
Wireworms are the larvae of the click beetle and they are readily recognized by growers and agronomists. They can grow up to 25 mm in length and are orangey-brown in colour with a narrow, segmented body, biting jaws and three 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. 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. There are more than 60 species of click beetles in the UK, but three species A. lineatus, A sputator and A. obscurus are responsible for the majority of damage to crops.

Symptoms
The wireworm larvae feed on the seeds, emerging shoots as well young developing cereal plants which can result in severe plant loss especially where wireworm populations are high. Larger wireworms feed on both live and dead plant material at two main times of the year ; March to May and September to November.

Life-cycle
The adult females lay eggs in grassy ground in the spring. These hatch one 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
Established grassland swards with undisturbed conditions allow wireworm numbers to build up. Crop damage caused by wireworms used to be associated with the traditional grassland areas of the country. More recently crops grown in the traditional cereal growing areas have also been encountering levels of wireworms causing economic damage particularly in high value crops such as potatoes.

Threshold
Any field which has been down to permanent grass within the past four years is likely to harbour a population of wireworms. Where wireworm populations are present economic losses from growing cereals is likely unless control measures, such as insecticidal seed treatments, are used. Seed treatments can help to reduce the damage caused by wireworms but if pest pressure is high then other protection measures should also be used.

ADAS threshold advises a seed treatment if more tha 750,000 wireworms /ha are present. If numbers exceed 1.25 million / ha some damage is likely even in the presence of a seed treatment.