Identification and management of wireworms in field crops

Wireworms are the soil-dwelling larvae of click beetles and can damage crops, including alliums, carrots, cereals, field beans, lettuce, peas, potatoes and sugar beet. They feed for up to five years before pupating and cause damage by burrowing into roots and tubers, leading to significantly reduced marketability, yield loss or seedling death. 

A pre-planting assessment of wireworm risk is essential.

• Crops sown within 2 years of ploughing out permanent pasture are at highest risk
• Rotations dominated by winter cropping, particularly with grass weeds or grassy margins, are at increased risk
• South facing, sloping fields, heavy alluvial soils and minimum tillage cereal crops are associated with higher risk
• Late-lifted potatoes are at greater risk

Scientific name: Agriotes spp.

There are three key species: Agriotes lineatus, A. obscurus and A. sputator. The larvae of the three species are indistinguishable by conventional means.

Adult beetles have a dark brown–to–black elongated body (8–15 mm long and 2–3 mm wide), which is covered in fine whitish–grey hairs. The adults live for about 1 year.

Newly hatched wireworms are transparent, white and 1.3 mm long. They grow to up to 25 mm long and turn shiny golden brown. They have a cylindrical body, tough skin, three pairs of legs at the head end and two dark spots at the tail. The head is dark brown, with powerful biting mouthparts.

Larvae leave ragged holes at the base of the stem in cereals, sugar beet and leeks, moving along rows to attack further shoots. Damage to potato tubers and carrots is visible as small, round holes on the surface leading to narrow tunnels.

Injury on young sugar beet seedlings appears as small wounds that rapidly blacken on the stem below soil level, usually causing the seedling to wilt and die.

Sep–Mar: Adults overwinter below soil surface.

May–Jun: Females lay eggs just below the soil surface.

Jul–Aug: Eggs hatch.

Year–round: Larvae (wireworms) feed for 4–5 years.

Jul–Aug: Larvae pupate.

Non-chemical control

The consolidation of seedbeds helps to restrict pest movement. Control of grass weeds can reduce the availability of food sources. Rolling a thin crop may also encourage tillering. In arable rotations, plough-based cultivation may help to reduce populations. A spring rotation in cereal crops can also be beneficial.

For potatoes, avoid wireworm-infested fields and consider lifting the crop early, especially if damage is expected.

The main natural enemies are fungi and parasitic wasps. Larval stages are attacked by ground beetles and adults are eaten by birds.

No commercial biological control agents are currently available for wireworm control in the UK, although some strains of the insect-pathogenic fungi Metarhizium anisopliae and Metarhizium brunneum have shown encouraging results under experimental conditions.

The use of cruciferous plants as green cover crops or de-fatted mustard meals as soil amendments has been shown to reduce wireworm populations in Italy.

Monitoring

Bait and pheromone traps can be used to determine the presence or absence of wireworms and click beetles, respectively. However, traps do not provide a reliable indication of the level of infestation. Soil samples can be used to assess population size and provide a reasonably good estimation of subsequent damage in potato crops. However, accurate estimations are difficult because populations can be patchy. Additionally, wireworm populations can cause significant damage, even at the lower limit of detection in potato crops (60,000 per hectare).

For cereals, examining field soil cores for larger wireworms, or in the laboratory for smaller ones, is costly and rarely justified.

Thresholds

Potatoes:

A single wireworm in 20 soil cores of 10 cm in diameter (equivalent to approximately 60,000 per hectare) can represent a significant risk in the following crop.

Cereals:

If wireworms exceed the threshold of 750,000 per hectare (as determined by soil extraction), use a seed treatment. Even where a seed treatment is used, damage can still occur under high pest pressure (1.25 million per hectare).

Other crops:

None established.

Insecticide resistance

None known.