Wheat blossom midges

Wheat blossom midge larvae feed on developing seeds, reducing yield. They can also promote sprouting in the ear and fungal attacks.

Orange wheat blossom midge (OWBM)

Pest

Sitodiplosis mosellana

Crops affected

Wheat, barley, oats, triticale and rye

Identification and symptoms

Adult midges are orange and about 3 mm long. Males have long, feathery antennae. Females have a short ovipositor and lay cylindrical eggs inside florets. Larvae, which are also orange and up to 3 mm long, feed on the developing grain and can cause grain to become small and shriveled.

Life cycle

OWBM larvae hibernate in the soil inside cocoons. In mild winters, larvae remain in this state and can survive for over ten years. Following a sufficient period of cold, larvae emerge and move to the soil surface. If the soil is warm (over 13°C) and moist, larvae pupate. If conditions are unsuitable, larvae return to the cocoon stage.

Adults emerge in late spring and mate on the soil surface. For flight, the midge prefers still, warm conditions – air temperatures above 15°C (although flight has been observed at 10°C). Low light conditions are also preferred, so midges tend to fly later in the evening on sunny days.

Eggs are laid inside the florets of emerging ears. Females lay most of their eggs on the first evening of flight. Depending on temperature, larvae hatch within 4–10 days. After hatching, larvae crawl down to the developing grain and begin to feed. Larvae that hatch after flowering do not develop properly and cause little damage. Larvae feed on developing grain for 2-3 weeks before dropping to the ground, usually after rain has moistened the soil surface. They then burrow into soil and hibernate within cocoons.

Importance

When larvae feed, it can result in small, shrivelled grains with poor germination. The outer layer of the grain (pericarp) can also become damaged. This allows water to enter and can lead to sprouting and secondary attack by other fungi (fusarium and septoria). This affects both the yield and quality of grain. Wheat and rye are the most susceptible cereals. Adults may also lay their eggs on barley, in the absence of a suitable wheat crop, but larval survival is generally lower.

Risk factors

Midge abundance is usually highest in fields where susceptible wheat has been grown recently (in the last four years).

Susceptible crops are at the highest risk when adult midge emergence coincides with ear emergence. Ears are most at risk during the period between one quarter and full emergence above flag leaf ligules (GS53–59).

Management

Monitoring and treatment thresholds

The monitoring of susceptible varieties should take place shortly before and during ear emergence. Priority should be given to milling crops, seed crops and sheltered feed crops. Monitoring should also take place in nearby fields that have had a recent history of OWBM. Larvae are not controlled effectively once they have moved down into the ear, so spray timing is essential. Treatment thresholds depend on the monitoring method used.

Pheromone traps

Pheromone traps provide the earliest and most reliable warning of midge activity

  • Attach traps to stakes at crop height when the flag leaf sheath is swollen (GS45)
  • Place a minimum of two traps in each field. Traps should cover discrete blocks that represent different soil types, rotations, rainfall levels and soil temperatures
  • Traps should be checked daily until the start of flowering (GS61)
  • If 30–120 midges are caught per day, there is a general risk to crops in the following week. The visual crop inspection method should be used to detect females
  • If over 120 midges are caught per day, there is a very high risk to crops in the monitored and surrounding fields. Susceptible wheat crops (at GS53–59) should be treated as soon as possible

Yellow sticky traps

  • Use at least two yellow sticky traps, hung at crop height, in each field at ear emergence
  • Both sexes are caught, as well as many other insects, so correct identification is essential
  • If about 10 midges/trap are caught per day during ear emergence (GS53–59), this indicates an increased risk. Monitor crops for female midges using the visual crop inspection method

Visual crop inspection

  • Inspect susceptible crops at ear emergence (GS53–59) from mid-evening (as light levels fall)
  • Walk about 30 metres into the field and examine up to 100 ears
  • For susceptible feed crops, the treatment threshold is one midge for every three ears*
  • For susceptible milling and seed crops, the treatment threshold is one midge for every six ears*
  • *All susceptible crops should be treated if a cloud of midges is observed when parting the crop

Cultural control

Resistant varieties

Resistant varieties do not require monitoring or insecticide treatment. Wound plugs form on attacked grains and prevent larvae from feeding. Varieties that are resistant to OWBM can be found on the AHDB Recommended List. Resistant varieties are, however, susceptible to yellow wheat blossom midge, although this species is less common.

Rotation

Rotation has little effect on overall risk of OWBM. Where crops are grown in blocks, this may help monitoring and control and may also reduce future risk.

Cultivation

Cultivation methods that are more aggressive can cause the greatest reductions in OWBM numbers but may also have a negative effect on natural enemies that also overwinter in the soil. Burying the larvae deeper in the soil can also lead to a longer period of emergence. The presence of straw residues can reduce the incidence of OWBM but may encourage slugs.

Natural enemies

Any insecticide applied may also harm non-target organisms, including all natural enemies of OWBM. Only apply insecticides after populations have breached thresholds.

Parasitoid wasps Macroglens penetrans and Platygaster tuberosula, lay their eggs within midge eggs.  When OWBM larvae hatch, so do the wasp larvae. The wasp larvae consume the OWBM larvae. As levels of parasitism can reach 80%, such wasps are the most significant natural enemy of OWBM.

Dance flies (Platypalpus spp.) feed on adult midges during flight. When present in large numbers, they can significantly reduce midge populations. Spiders' webs can also trap many adult wheat blossom midges.

Ground beetles (Carabidae) and rove beetles (Staphylinidae) eat midge larvae as they drop to the ground in late summer. If the soil is dry, larvae are unable to burrow into the ground as quickly, making them easier prey.

OWBM life cycle

1. Larva hibernates inside a cocoon

2. Larva emerges

3. Larva pupates

4. Adult emerges and lays eggs inside florets

5. Eggs hatch, larvae feed and drop to the ground

OWBM damage

Pheromone trap

Yellow sticky trap

Dance flies feed on midges

Yellow (or lemon) wheat blossom midge (YWBM)

Pest

Contarinia tritici

About

Yellow (or lemon) wheat blossom midge is less common than OWBM and is rarely damaging. It follows a similar life cycle but is less persistent, with larvae pupating within three years of entering the soil.

Adults tend to emerge at a similar time as OWBM but prefer to lay their eggs at an earlier growth stage - as the boot splits to reveal the ear. They do not lay eggs in florets once they are clear of the boot.

The larvae feed on the stigma, preventing pollination and development of the grain, and then on the anthers, which are retained within the floret.

The larvae are smaller and more yellow, compared OWBM larvae. After feeding for 2-3 weeks, they return to the soil. Some larvae do not form cocoons, but pupate in September forming a secondary generation that feeds on couch grass.

Further information