Insect pest risk
|High risk||Reduced risk||Low risk|
|Corn earworm (Helicoverpa armigera)|
|Large population generated in chickpea or other winter crops.||
|Control of midge with synthetic pyrethroids can flare Helicoverpa, Rutherglen bug and corn aphids (SPs kill beneficials that may otherwise keep these species below threshold).||Cutworm are favoured by weedy fallows and crop edges.||
Fall armyworm (FAW; Spodoptera frugiperda) arrived in Australia in early 2020. While it is listed as a potential pest of sorghum, damage to grain and forage sorghum has so far been minimal compared to damage to maize and sweet corn crops. Further investigation into this pest under Australian conditions is required to assess appropriate threshold levels.
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.
Sorghum is susceptible to insect pests from emergence to late grain fill.
|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
- 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.
- Nucleopolyhedrovirus (NPV) is selective for control of helicoverpa (most 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.
- 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 makes control decisions simpler.
- Seed dressings may be the most effective control for some pests, as well as the least disruptive to natural enemies.
- Where pests invade from adjacent fields, consider spraying only borders and not the whole field.