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Sorghum midge

Sorghum midge is a major pest of sorghum. High populations on non-resistant varieties can completely destroy the crop.

Scientific name

Stenodiplosis sorghicola

Description

  • An orange, mosquito-like fly.
  • 1.5–2mm long.
  • Long antennae.
  • Females are larger than males.
  • Females have a slender ovipositor on the rear of the abdomen.

Distribution and habitat

Present in all states where sorghum is grown except Western Australia. Transport of grain containing diapausing (hibernating) larvae is believed to be the main method of spread so movement of sorghum into Western Australia is restricted.

Hosts

Grain sorghum, forage sorghum, Johnson grass, Columbus grass. Not Australian native Sorghum spp.

Damage

  • Eggs are laid into the developing flower spikelets and larvae feed on the developing ovary, which prevents seeds developing normally.
  • On susceptible hybrids, high populations can completely destroy the crop.
  • Can be distinguished from other causes of head damage (heat and moisture stress) by the empty, white pupal cases protruding from the glumes (leaf-like structures), and emergence holes through the glumes' tips.

Life cycle

Adult midge emerge from overwintering diapause in spring (usually 2–3 weeks after rain). After 1 to 2 generations on Johnson grass, they attack grain sorghum for 5 or 6 generations.

Female sorghum midges may lay up to 120 eggs, and up to 20 eggs in an individual flower spikelet. Under optimum conditions:

  • eggs hatch within 3 days
  • larvae complete development within 9–11 days
  • adults emerge from the pupae within 3 days
  • adults rarely live for more than a day.

After harvest, larvae enter an overwintering diapause in cocoons within damaged or trashed florets. Some may remain in diapause for up to 5 years.

Monitoring and thresholds

The risk period for sorghum is during flowering from December to March.

Count adult midge on flowering heads at about mid-morning. Repeat at 3–5 day intervals.

The threshold varies with resistance level of the hybrid and other factors such as commodity price and cost of insecticide. On susceptible hybrids 1.4gm of grain is destroyed for each egg-laying adult and the threshold is usually about 1 adult per head.

An interactive online threshold calculator is available at the Beatsheet.

Control

Resistant hybrids are available and widely used.

Chemical control may be cost-effective, but more than one application of insecticide may be needed on susceptible hybrids.

Further information