PestFacts June (no. 2) 2021
Russian wheat aphid reported in seed treated cereals
Russian wheat aphid (RWA, Diuraphis noxia) has been reported in low numbers on:
- 5 leaf wheat at Port Neill
- seed treated cereals in the Riverland.
Finding RWA in a seed treated crop is unusual but it has been reported elsewhere this year (see Russian wheat aphid in seed treated crops).
RWA on seed treated cereals
Several reports of RWA have been on dry sown, seed treated cereals, 6-8 weeks before rain started germination. It is unlikely this prolonged time affects the effectiveness of the seed treatment (either neonicotinoids or a mix of neonicotinoids and synthetic pyrethroids), as soil moisture and good bacterial activity are required to mobilise and breakdown neonicotinoids. In many cases, seed treatments are applied months in advance on the seeds and they are stable when dry.
For seed treatments to be effective, plants need to take up the insecticide and this process is, in fact, not very efficient. When rain arrives, the seed treatment dissolves into the soil moisture and is taken up through the root tips. The uptake depends on the number of roots in the area where the insecticide is present. Overall, the efficiency of uptake of neonicotinoids seed treatments is low with estimated values of 1-10% (Bonmatin et al 2021). The rest of this insecticide stays in the soil, where it is mostly associated with organic soil matter. The insecticide then either:
- breaks down slowly under the influence of microbial activity
- leaches down
- moves by runoff or wind erosion.
It is not impossible that RWA could be becoming more tolerant to some of the seed treatments, however, we suspect it is more of an insecticide exposure issue in this case.
RWA may be migrating into crops depending on how well they built up on green bridge hosts over summer and early autumn. Consider the possible grass hosts of the green bridge in your area, especially around cereal paddocks. Such migrating (winged) aphids might produce a few offspring on any grass plant. Reports of RWA on rice seedling (rice is not a host for RWA) have been received over the last few years and these aphids simply disappeared after a few weeks. There may be some delay between aphids colonising seed treated plants and insect death as sufficient volumes of insecticide need to be taken up by the aphids.
There are many possible reasons for why RWA is colonising cereals with seed treatments in some areas but it is not cause for concern. Research shows plants tend to resist early RWA feeding well. You should keep observing but keep in mind that spraying will often not be needed.
RWA action threshold calculator
The RWA action threshold calculator helps decide if spraying is economically justified by considering dynamic factors including:
- cost of control
- cereal market price
- aphid numbers.
When using the calculator, monitoring at growth stage 30 (GS30) stem elongation is recommended. For more information on using the calculator, see How to use the new Russian wheat aphid action threshold calculator.
Economic justification for sprays before GS30 cannot be calculated with available data as a lot can happen between now and GS30, including:
- cold, wet weather conditions
- the presence of beneficials.
Overseas advice on thresholds in these early growth stages (Kieckhefer and Gellner 1992) state that >10 aphids per plant are needed at this stage to obtain yield loss, whereas Archer et al (1998) suggest a threshold of 20% of plants with aphids (not just symptoms). Plants at these early growth stages will be able to compensate, just as they can compensate for grazing or herbicide damage.
What actions should be taken?
RWA is not expected to significantly reduce plant growth at this stage. The likely cold and wet coming months should limit RWA population development. Plants will develop tillers, which will reduce the percentage of tillers with aphids by a simple dilution effect.
A range of natural enemies feed on RWA, including parasitoid wasps and generalist predators such as ladybugs, hoverflies and lacewings. Potentially, these beneficial insects can build up in crop on small populations of aphids and this means they are present when aphids are more problematic in spring.
Entomopathogenic fungi is also very effective at controlling aphid populations.
Continue keeping an eye on your cereal problems, particularly areas where RWA has already been seen. The key monitoring time to determine thresholds is GS30. If something unusual is occurring or you have specific questions please contact the PestFacts SA team.
Please keep reporting all RWA sightings to:
Maarten van Helden – SARDI Entomology, PestFacts SA
Phone: 0481 544 429
Email: maarten.vanhelden@sa.gov.au
Lizzy Lowe — Cesar Australia, PestFacts South Eastern
Email: llowe@cesaraustralia.com
Or through the PestFacts Map online report form.
References
Bonmatin, JM., Giorio, C., Sánchez-Bayo, F. et al. An update of the Worldwide Integrated Assessment (WIA) on systemic insecticides. Environ Sci Pollut Res 28, 11709–11715 (2021). https://doi.org/10.1007/s11356-021-12853-6
Kieckhefer RW and Gellner JL 1992. Yield losses in winter wheat caused by low density cereal aphid populations. Agron. J. 84, 180–183.
Archer T. L., Peairs F. B., Pike K. S., Johnson G. D., Kroening M.. 1998. Economic injury levels for the Russian wheat aphid (Homoptera: Aphididae) on winter wheat in several climate zones. J. Econ. Entomol. 91: 741–747.
RLEM resistance testing available
Cesar Australia is offering a screening service to test for insecticide resistance in RLEM in South Australia, Victoria, NSW and Tasmania. The screening is at no-cost for Australian grain growers and advisers, thanks to funding from GRDC.
Cesar Australia are particularly interested in hearing from growers or advisors who have experienced recent or past chemical control failures or have paddocks that are frequently impacted by the mites, and often require spraying.
Understanding the dynamics of RLEM populations in your paddocks including resistance will help identify the best control options for growers.
How to send samples for screening
Please contact Dr Aston Arthur if you wish to send in mites for screening. If required, a collection kit can also be provided containing necessary equipment to send mites in. Otherwise, check the sending in samples to Cesar Australia factsheet.
Avoiding the evolution of insecticide resistance
The GRDC RLEM Best Practice Management Guide and RLEM insecticide resistance management strategy are available to guide decisions around the management of RLEM. Monitoring is key to understanding what’s going on in your paddocks. Importantly, use of chemical controls should be judicious and only when thresholds are reached.
In a current SARDI research project, SARDI entomologists have found that summer egg populations are a good predicator for autumn egg populations in the soil – but also that many paddocks tested found no eggs or mites. Observations are the first step in any plan for tackling invertebrate pests.
Remember to inspect paddocks post-spraying and report any suspected spray failures.
Contact
For more information or to access the testing service, contact:
Dr Aston Arthur
Phone: 0427 875 040
Email: aarthur@cesaraustralia.com
