By GRDC Northern Region panellist Kelly Becker, Theodore
Grasshoppers, Rutherglen bug and midge numbers are building up in Central Queensland in late planted sorghum crops, forcing growers to consider control measures in the lead up to harvest.
Instigating control measures can help preserve grain yield as well as minimise the risk of rejection on delivery due to the presence of live or dead insects in the sample and moulds which can be present from pest feeding sites.
Recent integrated pest management workshops supported by the Grains Research and Development Corporation (GRDC) in Emerald and Moura equipped Central Queensland growers and advisors with the latest advice on management and control options for some of the key summer crop pests.
One of the key messages is that any control measures need to be considered not just in terms of their economic benefit but their impact on the farming system as a whole.
If control of grasshoppers is warranted to prevent crop loss, fipronil is an effective option that’s available for use in sorghum at extremely low rates (6.25 mL/ha (200 SC) and 1.5 g/ha (800 WG) applied directly onto hoppers or winged adults.
Applying insecticide to provide a barrier around crops is also an option. However it is critical that growers read and adhere to label recommendations, particularly in relation to water rates for application, the impact of rainfall and withholding periods for harvest/grazing/slaughter. Permits for a number of synthetic pyrethroids are also in place for locust and grasshopper control.
Rutherglen bug (RGB) can have a damaging impact on sorghum yield in later planted crops. Developing grain affected by RGB feeding is light in weight with poor germination. Under wet conditions, fungal and bacterial infections further degrade RGB-damaged grain, infecting through feeding wounds on the seed.
The Queensland Department of Agriculture and Fisheries (QDAF) management recommendations for RGB in sorghum are - flowering — milky dough: control is warranted if more than 20 bugs per head; Soft dough: 20–25 bugs per head and hard dough — physiological maturity: RGB have no impact on yield at this stage.
Growers should calculate the potential yield impact on sorghum crops to determine whether it is economically viable to apply control measures.
As crops come into head and begin grain fill, growers will also need to be on the lookout for sorghum midge.
The online Economic Threshold Calculator for sorghum midge can help growers assess whether they will benefit from applying an insecticide. It considers the hybrid’s level of midge resistance, growing conditions, insecticide costs and commodity prices.
If control is warranted, growers should note that a spray for midge (synthetic pyrethroid) will often be combined with a virus application for helicoverpa.
Whilst the synthetic pyrethroid will have efficacy against midge and RGB, it will devastate the beneficial insect population and as a result, there may be implications for the ongoing control of helicoverpa populations in the crop.
Larvae that survive a spray are more likely to go on and cause damage to maturing grain following a pyrethroid application because predators and parasitoids that might have otherwise killed them will be much less abundant.
Because of the depletion of beneficial insects, another potential downside of midge and RGB control with pyrethroids is the flaring of aphids which may cause stickiness issues at harvest.
Visit The Beatsheet for more information on grasshoppers and locusts (including identification guides), RGB or midge, visit the QDAF blog.
For regionally relevant information and advice, growers should contact their local agronomist or advisor.
Kelly Becker, GRDC Northern Region Panellist, Theodore
0409 974 007
Sarah Jeffrey, Cox Inall Communications
0418 152 859