Adult sorghum midge on a sorghum head.
Major pests of sorghum
Sorghum midge - Stenodiplosis sorghicola
Sorghum midge is a serious pest of sorghum that requires several repeat insecticide applications during flowering, particularly for late planted crops. Management is now centred on growing midge resistant hybrids that not only simplify management and decreases cost, but provide greater midge control.
Damage
- Midge eggs hatch into transparent white larvae that feed on immature seed.
- Feeding creates a depression in the developing seed and prevents seed kernel development.
- High populations of midge can completely destroy the crop.
- The progeny of each egg-laying adult can destroy 1.4 gm of grain.
- Sorghum midge damage is evident by white pupal cases that stick out of the tips of glumes.
The midge lifecycle varies from 2-4 weeks, depending on temperatures. This allows many generations to occur in season and accounts for the rapid build-up of extremely high midge densities especially where the flowering period sorghum is extended by successive plantings
Monitoring
Count adult midge on flowering heads at mid morning. Repeat daily.
Thresholds
Thresholds vary with the resistance levels of the hybrids as well as commodity prices and the cost of insecticides. Threshold levels can be calculated using the factor of 1.4 gm of grain destroyed per one egg-laying adult. On susceptible hybrids this level is usually about one adult per head.
Larvae destroy the developing seed. A small, silvery pupal case can be seen protruding from florets aborted due to midge damage.
Control
The most common means of controlling sorghum midge is through the use of resistant hybrids. Since 1993 all commercial sorghum hybrids have been assigned official midge resistant (MR) ratings from 1-7.
A 7-rated hybrid, when exposed to the same midge density as the susceptible hybrid (rated 1), sustains 7 times less damage. In 2002 the rating system was extended to a new ´open-ended´ rating of 8+. Trials have shown that some 8+ hybrids contain levels of resistance that approach ´practical field immunity´. It is worth noting that for 8+ varieties, some are just a little better than 7 while others are ´practically immune´
Today, over 99% of grain sorghum in Australia has some level of midge resistance with most commercial hybrids rating from 4-6. The high level of adoption of MR cultivars and the elimination of low rated MR hybrids means that spraying for midge is now very rare with less than 5% of crops treated, in contrast to the mid 1990s when 30-40% of the crops were sprayed. The use of resistant hybrids also means that natural enemies are conserved.
It is recommended that producers use resistant sorghum hybrids to combat sorghum midge. Insecticides are still available for the control of midge, but these chemicals will severely disrupt natural enemies. For current chemical control options consult Pest Genie or APVMA.
Natural enemies
Three small black wasp parasitoids play a role in the control of sorghum midge. They are: Eupelmus sp., Tetrastichus sp. and Aprostocetus sp. Their presence in sorghum crops may be recognised by their small round emergence holes the spikelet.
Photographs
Sourced from: http://www.dpi.qld.gov.au/26_8157.htm
