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Sorghum midge resistance and management

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Sorghum midge is a serious insect pest of grain sorghum in Australia and has the potential to cause significant economic losses.

midge-rating-logo.jpgWhile 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 Rating logo and rating number. The ratings range from 1 (susceptible) to 8+ (highest level of available resistance, with rating 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 five times the pressure of a 1 rated hybrid while incurring the same amount of damage.

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 through 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,
  • Midge rating (MR)
  • 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.

midge-damage.jpg
Damage levels to different MR rated hybrids under
equivalent midge pressures.

Female-midge.jpg
Female midge laying eggs in a head

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, 1-2mm 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 details instruction counting midge are available at The Beatsheet website.

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 only for one day, they do most of the 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 (especially midge number midge rating (MR), 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 later in the season.

Table 1: 

Economics of midge damage at different grain prices without chemical control
Midge rating of hybrid Yield loss* Damage ($/ha) @ selling price of:
  (t/ha) $160/t $200/t $250/t $300/t
1 midge per panicle          
susceptible (rating 1) 0.52 42 104 130 156
3 Rating 0.18 14 36 45 54
5 Rating 0.10 8 20 25 30
7 Rating 0.08 6 16 20 24
8+ Rating 0.05 4 10 13 15
3 midge per panicle          
susceptible (rating 1) 1.58 252 315 394 473
3 Rating 0.53 84 105 131 158
5 Rating 0.32 50 63 79 95
7 Rating 0.23 36 45 56 68
8+ Rating 0.12 25 24 30 36

*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 1.4g yield loss/day. The estimates assume that spraying results in 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 hybrids. In Midge Rated 8 hybrids, yield losses can be reduced by over 905 with this spray timing.

Implications for 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 again 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 otherwise kill 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.

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