Insect pest risk
|High risk||Reduced risk||Low risk|
|Low MR rated variety and high midge pressure.|
Spread of flowering in the crop (> 3 weeks).
|Corn earworm (Helicoverpa armigera)|
|Large population generated in chickpea or other winter crops.|
|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.|
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. 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.
- 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
- 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