As we near the Spring months, conditions warm up and pest populations will increase along with the temperatures. Russian wheat aphid (RWA), first found in Australia in 2016, is a relatively new pest for Australian cropping systems.
Although the behaviour and populations of RWA in Australia are not yet fully understood, it is known that autumn infestation is important for building RWA numbers. Plants are ‘primed’ to host RWA populations through biochemical changes made in the plants as a result of aphid feeding at this time and aphid numbers begin to build over Autumn and Winter and into Spring.
RWA require green plant material for survival so controlling the green bridge over the summer months is important to reduce populations that may survive long enough to infest crops in autumn.
The South Australian Research and Development Institute (SARDI) conducted a survey of RWA numbers in road side grasses in January and February of this year including TAS, VIC, SA and NSW with very low RWA populations found. Additional Autumn sampling was done around the Birchip region following floods in December where some RWA were found in low numbers on volunteer barley and barley grasses.
Since March 2019, RWA has been detected on the Eyre Peninsula, in the south east of South Australia, and throughout Tasmania (interactive map found here). There have been two reports in New South Wales and some detection’s have been made in the northern Wimmera, southern Mallee area. It seems that RWA pressure is again very low this season.Â
Seed treatment with Imidacloprid at label rates will offer protection for crops for 4-6 weeks. However, observations across paddocks so far reveal that little or no RWA colonisation occurred this Autumn, so aphids have neither been detected in seed treated crops nor in untreated crops.
Economically significant yield impacts are more likely from infestations that occur before stem elongation, but only RWA persist during the later (heading and flowering) growth stages.
Crops beyond growth stage 40 (Z40) do not appear to be attractive to migrating RWA, therefore colonisation of such advanced crops during Spring is unlikely.
Look for leaf streaking, which can appear white or purple.
At late tillering and during stem elongation you may notice a flat habit and leaf rolling. will often uncover the aphid.
Currently, economic spray threshold recommendations are based on US research, which is the best information available. This research recommends control at >20% of all seedlings infested up to GS30 and >10% of tillers infested from GS30.
It is important to consider the effect of insecticide treatments on other pests that may also be present in an infested paddock, as cross resistance may occur, and beneficial species can be impacted. Surveys by the SARDI project team have revealed there are a variety of natural enemies preying on RWA, including ladybird beetles and parasitoid wasps.
BCG is currently undertaking trials as part of the GRDC funded Russian Wheat Aphid risk assessment and regional thresholds project (GRDC 9176535) with project partners SARDI and cesar, aiming to better understand RWA in Australian cropping systems.
Written by Maarten Van Helden (SARDI) and Jess Lye (cesar)