A mosquito-like orange fly, 1.5 to 2 mm long, with very long antennae. Females are larger than males and have a slender ovipositor on the rear of the abdomen.
The sorghum midge is on all continents where sorghum is grown. It has been recorded in eastern Australia and Northern Territory, but not Western Australia.
|Pest status||Major, widespread, regular occurrence.|
|Crops attacked||Grain sorghum, forage sorghum, Johnson grass, Columbus grass, but not Australian native Sorghum spp.|
|Life cycle on sorghum|
Adult midge emerge from overwintering diapause in spring and, after one to two generations on Johnson grass, attack grain sorghum for five or six generations. Eggs are laid into the developing flower spikelets and larvae feed on the developing ovary, preventing normal seed development.
The life cycle takes about three weeks. Under optimum conditions, eggs hatch within three days and larval development is completed within 9-11 days. Adults emerge from the pupae within three days and rarely live for more than a day. Female sorghum midges may lay up to 120 eggs and up to 20 eggs in an individual flower spikelet. After harvest, larvae enter an overwintering diapause in cocoons within damaged or trashed florets.
A proportion of overwintering larvae enter diapause and may remain in diapause for up to five years. After temperatures increase in spring, rainfall triggers the start of development with adult midges emerging about 2-3 weeks after rain.
Risk period: During flowering from December to March.
Damage: Midge larvae destroy developing seed. One larva is enough to prevent seed development. On midge-susceptible hybrids, the progeny of each egg-laying adult on a head destroys 1.4 g of grain. High populations on susceptible hybrids can completely destroy the crop. Other causes of head damage include heat and moisture stress, but sorghum midge damage may be distinguished by the empty white pupal cases protruding from the glumes and emergence holes through the tips of glumes.
|Monitoring and action level|
Monitoring: Count adult midge on flowering heads at about mid-morning. Repeat at 3-5 day intervals.
Action level: The level varies with resistance level of the hybrid and other factors such as commodity price and cost of insecticide. The level can be calculated using the factor of 1.4 gm of grain destroyed for each egg-laying adult. On susceptible hybrids the level is usually about one adult per head.
Cultural control: Early planting, removal of alternative hosts, cultural practices to ensure even flowering and resistant hybrids. Conservation of natural enemies: with the use of cultural controls (particularly resistant hybrids) insecticide use is rare and natural enemies are conserved. Natural enemies are three small black wasp parasitoids: Eupelmus sp. Eupelmidae, Tetrastichus sp. and Aprostocetus sp. Eulophidae, whose presence may be recognised by their small round emergence holes in the spikelet.
Transport of grain containing diapausing larvae believed to be the main method of spread and movement of sorghum into Western Australia is presently restricted.
Resistant hybrids are available and widely used.
- Pests of field crops and pastures: identification and control, Editor: P.T. Bailey.