Control and identification of beet leaf miner on sugar beet

Incidences of beet leaf miner (also known as mangold fly) have increased in recent years. Larvae feed within the plant leaves, forming mines and blisters that can reduce the photosynthetic ability of the plant. In severe cases, complete defoliation may occur.

• The first generation is the most damaging, especially in late-sown crops with low leaf area. Although later generations can be numerous, damage is less serious because they inhabit outer leaves and often coincide with periods of rapid plant growth

Scientific name: Pegomya hyoscyami

Eggs are white, patterned and approximately 1 mm long.

Larvae are legless, translucent, whiteish–to–pale green and 6–8 mm long. Not easily visible, the appearance of mines and blisters on the beet leaf reveal their presence. Symptoms of heavy infestations can resemble those of bacterial leaf blight.

Pupae are brown, oval and approximately 5 mm long.

Nov–Mar: Overwinter in the soil as pupae.

Apr–early May: Adults emerge throughout April. After mating, females lay eggs on the underside of beet leaves in groups of between two and ten.

May–early Jun: Eggs hatch after 3–10 days. The larvae enter the leaf and feed between the two leaf layers. The mines join and blister as the larvae grow. After about 10–15 days, the fully grown larvae leave the leaf and fall to the ground, where they pupate.

Jun–Oct: There can be another one or two overlapping generations.

Several generations of this pest can occur in one year, but it is the first generation in spring that is most damaging, particularly to late sown crops.

Non-chemical control

The third generation is most susceptible to predation by natural enemies, such as parasitic flies (for example, Opius nitidulator). The decision of whether to make late applications of insecticide should balance the likelihood of success against the risk of harming these natural enemies.

Monitoring

Eggs can readily be seen with the naked eye on the underside of leaves. If mines are seen in the leaves, they should be checked to see if thresholds have been reached.

Because larvae are hard to control inside the leaf, the optimum spray timing is at egg hatch.

Thresholds

The threshold for treatment is when the number of eggs and larvae exceeds the square of the number of true leaves. For example, a plant with four true leaves would need a population of 16 (4 x 4) or more eggs and larvae to warrant treatment.

Insecticide resistance

None known.

The authorisation status of pesticides can change rapidly. For the most up-to-date information, please talk with your preferred supplier or BASIS-registered adviser.