Sorghum Midge Resistance and Management

Sorghum midge is a serious insect pest of grain sorghum in Australia and has the potential to cause significant economic losses. While midge resistance (MR) is present in all commercial hybrids, there are differing levels of resistance available. These levels are indicated by the industry recognised Midge Tested logo and rating number (as seen in Figure 1). The ratings range from 1 (susceptible) to 8+ (highest level of available resistance), with ratings of 3 to 7 being common amongst current commercial hybrids. The rating is a measure of the midge pressure a hybrid can tolerate, with a 5 rated hybrid able to sustain 5 times the pressure of a 1 rated hybrid while incurring the same amount of damage.

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Figure 1: Official MR

Sorghum midge generally overwinter in weedy species such as Johnson Grass and in spring begin to infest crops as they come into head and begin to flower. They are particularly likely to be found in increasing numbers in later planted sorghum crops, as they have had time to increase in numbers throughout the season. Once detected in a crop, mak­ing a deci­sion about whether insecticide control is economically viable becomes increasingly important.  A number of factors such as: number of midge, MR rating, crop value and cost of control, need to be considered. This task can be made easy by using an online economic ‘midge threshold calculator’ developed by the Queensland Department of Agriculture and Fisheries (DAF) – see DAF’s ‘The Beatsheet’ blog.

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Figure 2: Damage levels on different MR rated hybrids

Counting for Midge

Obtaining accurate midge densities is the initial step. The first sign of midge being active is often seen in spi­ders webs in the field. Midge are small red flies, 1–2 mm long (Figure 3) and are attrac­ted to heads at mid-flower.  The best time to look for them is in the morning (9-11am), in the flowering (yellow) part of the head.  Look from slightly above to detect movement of the females (generally walking­ around or bobbing up and down) against the still head. Alternatively, heads can be tapped into a white bucket and the contents examined for midge. Count the number of midge over 10 metres of row in at least 4 dif­fer­ent loca­tions in your crop to obtain a good average density. This aver­age will be required for the economic threshold calculation. More detailed instructions on counting midge are available at ‘The Beatsheet’.

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Figure 3: Female midge laying eggs in a head

Managing Midge

Insec­ti­cides (such as synthetic pyrethroids) only kill adult midge in the crop and do not kill the eggs or hatched lar­vae already present inside the sorghum flo­rets. While adult midge live for only one day, they do most of their egg lay­ing (and sub­se­quent dam­age to the crop) in the morn­ing. It is pos­si­ble to cal­cu­late the­o­ret­i­cal yield loss esti­mates for par­tic­u­lar crop scenarios (see Table 1). Determining whether this poten­tial yield loss is greater than the cost of control is then done using the online cal­cu­la­tor.  Bear in mind that depending on the input values for this calculator (especially midge number, MR rating, crop value, and spray costs) calculating thresholds for the current situation rather than rely­ing on a fixed value from one year to the next can have consequences for the later in the season.

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Table 1: Economic value of midge damage for a range of sorghum midge ratings *Yield loss estimates are based on extensive field trials by DAF that determined the average yield loss/midge/day on different rated midge hybrids. For a susceptible hybrid, one midge is estimated to cause 1.4 g yield loss/day. The estimates assume that spraying results in a 100% kill and that there is no midge damage prior to chemical application. It also assumes that you will receive the same average midge pressures over 4–5 days. In reality research has shown that one well timed insecticide for midge (put on from panicle emergence and before midge even enter the crop) will still only prevent 70–80% damage protection in lower rated sorghum hybrids. In 8 rated hybrids, yield losses can be reduced by over 90% with this spray timing.

Implications of Midge Control for Other Pests

It is important to remember that there are other implications of spraying to consider. Whilst spraying syn­thetic pyrethroids will have effi­cacy against midge and other pests, they can dev­as­tate the beneficial insect pop­u­la­tion, which can have consequences for the ongo­ing con­trol of pests in the crop. Larvae that sur­vive a spray are more likely to go on and cause dam­age to matur­ing grain because preda­tors/par­a­sitoids that might have oth­er­wise killed them will be much less abun­dant. Deple­tion of ben­e­fi­cial insects can then lead to problems with other pests such as aphids, which can in turn lead to issues at harvest. While not eliminating these problems altogether, using the midge threshold calculator and spraying only when warranted can minimise the unintended impacts these sprays can have on the overall biology of the crop.

Acknowledgements

Tracey Shatte (Midge Tested Scheme Project Leader), Queensland Department of Agriculture and Fisheries