Sorghum

Insect pest risk

High risk Reduced risk Low risk
Sorghum midge
  • Wet winter/spring with abundant alternative host (Johnson grass).
  • Late season crop that flowers when midge pressure is high
  • Crops where flowering is staggered – heads are susceptible for longer
Low MR rated variety and high midge pressure.

Spread of flowering in the crop (> 3 weeks).

  • Dry winter/spring allows few alternate hosts (Johnson grass)
  • Use of resistant (MR rated) hybrids, particularly for late season plantings
  • Early planting that avoids high midge pressure
  • Uniform flowering
Corn earworm (Helicoverpa armigera)
Large population generated in chickpea or other winter crops.
  • Midge spraying reduces beneficial insect populations.
  • Spread of flowering times and larval sizes in crop.
  • Uniform flowering.
  • High beneficial activity (egg and larval parasitism and predation).
Other pests
Control of midge with SPs can flare Helicoverpa, Rutherglen bug and corn aphids as SPs also kill beneficials, that may otherwise keep these species below threshold. Cutworm are favoured by weedy fallows and crop edges.
  • Rapid seedling emergence reduces the impact of soil insects.
  • Open head sorghum hybrids are less infested by Yellow Peach Moth and Sorghum Head Caterpillar.

Midge resistant sorghum

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.

Pest incidence

Sorghum is susceptible to insect pests from emergence to late grain fill.

Pest Crop stage
Emergence Vegetative Flowering Grainfill
False wireworm Damaging
Cutworm Damaging
Northern armyworm Damaging
Midge Present Damaging Damaging
Helicoverpa armigera Damaging Damaging Damaging
Rutherglen bug Present Damaging Damaging
Corn aphid Damaging Present
Sorghum head caterpillar Damaging Damaging
Yellow peach moth Damaging Damaging
Present Present in crop but gen­er­ally not dam­ag­ing
Dam­ag­ing Crop sus­cep­ti­ble to dam­age and loss.

IPM management strategies

Plant host resistance

  • Open-headed type sorghum hybrids deter aphids and Rutherglen bugs, who have a preference for compact or closed panicle types on which they are hard to control due to the difficulty of achieving spray penetration. May also deter sorghum head caterpillar.
  • Resistant hybrids have been developed for the control of midge. A variety with a midge rating of 6 – has 6 times the resistance to midge when compared to a variety with a rating of 1.

Selective insecticides

  • Nucleopolyhedrovirus (NPV) is selective for control of helicoverpa (more effective on small larvae, less than 7 mm long)
  • Use aphid-selective products e.g. pirimicarb to preserve the important beneficial insects thus potentially reducing the need for follow-up applications
  • Controll of aphids with broad spectrum insecticides in the vegetative stage can cause bigger problems late on with aphids and helicoverpa infesting the sorghum heads

Consider cultural control or biological control methods

  • Pupae busting after previous crop (e.g. chickpeas) can reduce helicoverpa populations in sorghum
  • Also consider pupae busting after sorghum to reduce helicoverpa in following crops
  • Crop uniformity will make control decisions simpler
  • Seed dressings may be the most effective control as well as the least disruptive to natural enemies
  • Where pests invade from adjacent fields consider spraying only borders and not the whole field

 Further information

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