ONLY A decade ago many farmers wonld not plant sorghum after November for fear of invasion of the small but devastating sorghum midge in flowering grain sorghum crops. However, due to ongoing research to increase levels of natural host plant resistance, this insect pest is now largely under control.
Grain sorghum hybrids are rated according to their level of midge resistance under the highly successful 'Midge Tested Scheme'. The current scheme - a collaboration between Queensland 's Department of Primary Industries and commercial seed companies - has a top rating of 7.
This does not do justice to the increased levels of resistance in some new cultivars and so the system of rating is being modified to include levels of resistance of up to 8+.
Hybrids are rated 1-8+ against 'standard' hybrids for midge damage. A susceptible (rated 1) hybrid will sustain eight times greater yield loss than a hybrid rated 8, when exposed to the same number of midge at flowering.
Three years of research, supported by growers and the Federal Government through the GRDC, have confirmed that a nnmber of current commercial hybrids and lines bred by Queensland's Department of Primary Industries have levels of resistance at least eight times that present in susceptible hybrids.
At present two 8+ hybrids are commercially available, while several others may be released in the next few years. The new 8+ hybrids contain levels of resistance that mean spraying is not of economic benefit in most situations. However, when midge pressures remain over 6 midge per panicle, there may be benefit in spraying an 8+ hybrid to control midge. Pressure at this level is rare however.
Program 2 Contact: Mr Adam Hardy, OOPI 0746881200 email: Adam.Hardy@dpi.qld.gov.au
Economics of midge resistant hybrids
| ||midge damage*|
|Damage over 4 days of flowering which is the residual life of a spray||t/ha||$/ha|
|1 midge per head per day (still common in some regions)|
|Rating 1 (susceptible hybrids)||0.15||25|
|Rating 7 (old top rating)||0.02||4|
|Rating 8+ (new top rating)||0.02||3|
|3 midge per head par day (seen occasionally late season)|
|Rating 1 (susceptible hybrids)||0.45||72|
|Rating 7 (old top rating)||0.06||10|
|Rating 8+ (new top rating)||0.05||8|
|6 midge per head per day (rare)|
|Rating 1 (susceptible hybrids)||0.90||144|
|Rating 7 (old top rating)||0.13||21|
|Rating 8+ (new top rating)||0.10||16|
* assuming an establishment density of 90,000 heads/ha and approximately 1/3 crop flowering at assessment, at a price of $160/t