Damage caused by cutworms (noctuid moths) on field crops

The term ‘cutworm’ applies to various moth larvae, the majority being species of noctuid moths and the turnip moth (Agrotis segetum) in particular. Although sporadic pests, damage can be severe, leading to the loss of plants and reductions in quality. The most susceptible crops are lettuce, leeks, and beetroot.

• The most susceptible crops are lettuce, leeks, and red beet
• Moderately susceptible crops include brassicas, carrot, celery, parsnip, and sugar beet
• The least susceptible crops are onion, potato, swede, and turnip
• Damage is most severe in light, sandy soils – especially in hot, dry years

Scientific name: e.g., Agrotis segetum

Adult turnip moths have a wingspan of about 40 mm. The forewings are pale greyish-brown with dark brown markings that include rings and lines.

Eggs are globular, about 5 mm in diameter and white, later turning cream with reddish-yellow markings and an orange band.

Fully grown caterpillars are greyish-brown and about 40 mm long, curling into a ‘C’ shape when disturbed.

Adults and larvae are generally nocturnal.

May–Jun: Adult turnip moths lay eggs on plants or on plant matter in the soil.

Jun: Eggs hatch in around 8–24 days, depending on temperature, and larvae feed on the aerial parts of plants.

Jun–Jul: Following a further 10–20 days, the larvae go through a second moult and begin feeding on roots below ground.

Aug–Sep: A second generation of turnip moths may emerge in late summer, but the caterpillars of this generation do not appear to be damaging.

Oct–Apr: The pest overwinters as the larvae of the second generation, with pupae forming in April–May.

The life cycles of other species may vary from this pattern. For example, Euxoa nigricans (garden dart moth) lays eggs in the late summer, which hatch in the following spring. The larvae feed on beet seedlings, during April and May, before pupating.

Cutworm damage can kill seedlings and young plants. Larvae may move along the rows of crops, such as lettuce or leek and cut plants off, one after another. Like slugs, cutworms make cavities in stems, rhizomes, tubers, and roots of large plants. Damage to root crops may not be evident until harvest.

The potato stem borer attacks sugar beet, potatoes, and other crops. In sugar beet, it tunnels inside the crown and upper part of the root of young plants, causing roots to blacken.

Non-chemical control

Irrigation can be an effective method of control for young larvae while feeding aboveground. To date, biological control with predators or parasitoids has not been investigated in the UK. Pesticides based on microbial control agents (for example, Bacillus thuringiensis, Bt) may be effective.

Monitoring

Catch male moths in pheromone traps.

Use weather-based models to predict the rate of development of turnip moth eggs, the level of raininduced mortality among young larvae and target dates to apply irrigation or insecticides.

Thresholds

None established.

Insecticide resistance

None known.