Wheat bulb fly

Pest

Delia coarctata

Crops affected

This pest affects all cereals, with the exception of oats.

Identification and symptoms

The larvae of wheat bulb fly are white and legless. They have no distinct head. However, their front ends are pointed and their hind ends are blunt. Larvae bore into the base of the stem to feed. This can cause tillers to die back and display 'deadheart' symptoms. Adults are slightly smaller than, but similar in appearance to, house flies.

Other larvae

Yellow cereal fly larvae are present at a similar time to wheat bulb fly. However, they are thinner and enter the crop through the top of the shoot. This results in a characteristic brown incision that spirals down the tiller. Unlike wheat bulb fly, each larva only attacks a single tiller. Frit fly larvae are present earlier, with damage seen in September to January.

Life cycle*

Jan–Mar: Eggs hatch in the soil and larvae bore into crops

Mar–Apr: Larvae feed and can attack several (3–5) shoots

Apr–May: Larvae pupate at the base of plants

Jun–Aug: Adult flies emerge and feed on saprophytic fungi on host plants

Aug: Adults lay eggs on bare soil

*Approximate timings shown

Importance

Wheat bulb fly is one of the most serious pests of wheat in the east of the UK, although it is not abundant every year. Yield loss depends on tiller density at the time of attack. Crops with a single tiller during egg hatch are the most vulnerable.

Risk factors

The following factors are associated with an increased risk:

  • Late-sown winter cereal crops (November or later)
  • Early sown spring cereal crops (before April)
  • Crops sown in higher-risk regions (especially eastern England)
  • Late cereal harvest (more time for flies to feed and mature their eggs)
  • Bare, especially freshly cultivated, soil during egg laying

Management

Cultural control

Avoid bare ground prior to drilling cereals. Delay cultivation until after egg laying. Drill as early as practical. Avoid deep drilling. Increase seed rates to compensate for attack, especially in late-drilled crops. Top dress early to promote crop recovery.

Thresholds

Chemical control is limited to seed treatments, with thresholds as follows:

  • Early-sown winter wheat crops (before November) are unlikely to benefit from seed treatment, as they have more time to tiller and are better able to withstand attack. Seed treatments also lack persistence to fully protect such crops
  • For late-sown winter wheat crops (November to December), consider seed treatments where populations exceed 100 eggs/m2
  • For late-winter/spring-sown crops (January to March), consider seed treatments irrespective of the population size (unless no eggs are present)

Drill treated seed at the recommended depth of 2.44 cm in a firm, even seedbed.

Further information

AHDB’s wheat bulb fly survey can help indicate whether a seed treatment is justified. Conducted by ADAS, the survey involves taking soil samples in September from 30 fields prone to attack (split equally across eastern and northern England) and calculating the number of wheat bulb fly eggs/m².

Wheat bulb fly survey results

Full annual reports (includes 2019)

Low wheat bulb fly pressure trend continues in 2019 (20 September 2019)

Wheat bulb fly pressure low in 2018 (9 October 2018)

Wheat bulb fly adults feed on fungi on cereal ears

Wheat bulb fly larvae are white and legless with no distinct head