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