How to use the new Russian wheat aphid action threshold calculator
Last year, researchers from SARDI and Cesar Australia developed a predictive Russian wheat aphid calculator to help decide if spraying is economically justified. Here we explain the calculator and how to use it to assist with making control decisions.
Russian wheat aphid
In south eastern Australia, the Russian wheat aphid (RWA, Diuraphis noxia) is now widespread. Its lime green colour, elongated shape, very short antennae, apparent lack of siphuncles and feeding damage symptoms readily distinguish it from other pest aphids found in Australian cereal crops. Contrary to Oat and Corn aphids, RWA is not a major vector of viruses, damage is purely due to aphid feeding and the plants’ reactions.
What the calculator does
The predictive calculator uses economic and agronomic data inputs to calculate the action threshold at which control should be taken in order to prevent an increasing RWA population from reaching the economic injury level.
The calculator can be applied to winter and spring cereal varieties.
Economic Injury Level
The ‘Economic Injury Level’ determines when it becomes cost effective to undertake control measures. The economic injury level is reached when the cost of pest control equals the value of yield loss resulting from feeding. So, when pest densities are higher than the economic injury level, or if they are predicted to exceed the economic injury level, it is more economical to control the pest. When pest densities are lower or are predicted to be lower, than the economic injury level, pest control would be more expensive than the cost of the yield lost.
Because crops are not always inspected immediately before pest control is undertaken, the calculator takes this delay into account and predicts the change in aphid pressure in the lead up to head emergence.
How to use the calculator
The calculator is used at GS30 (start of stem elongation) to determine the action threshold (the maximum percentage of tillers with RWA is usually reached between GS40 (start of booting) – GS50 (start of head emergence).
To use the calculator, you will need to have estimations for cost of control ($/Ha), cereal market price ($/t), yield potential (t/ha), and days until GS50 (start of head emergence). You will also need to input RWA abundance data (percentage of tillers with RWA), collected based on a standardised monitoring technique. Monitoring at GS30 will allow prediction of aphid density leading up to head emergence, and therefore if action is needed during that time period. The main effect of RWA feeding on yield is during flowering and grain filling and that is when aphid numbers are usually highest. Monitoring earlier is not reliable because the number of tillers increases strongly between GS20 and 30 depending on fertility and rainfall.
How to monitor for Russian wheat aphid
Monitoring at GS30 (start of stem elongation)
- At GS30 count the number of tillers in 50 cm row lengths (choose at least 5 areas of the paddock) (e.g. 433 tillers counted).
- In the same row, lengths count the number of tillers with symptoms (e.g. 43 tillers with symptoms, so you have 10% (0.1) tillers with symptoms (43/433).
- In or around each of the five areas check 20 tillers with symptoms for the presence of RWA, total 100 tillers to be checked. (e.g. results = 54 tillers with symptoms and RWA). At this stage, you have 54% (0.54) tillers with symptoms that also have RWA.
- Multiply the percentage of tillers with symptoms with the percentage of symptomatic tillers that are hosting aphids. This will result in the percentage of tillers with RWA. In this example 0.1*0.54 = 0.054 = 5.4% of Tillers with RWA
No results were found
Management considerations
The growth stage at which RWA establishes has a strong effect on yield loss potential. If establishment occurs after GS30 there is low aphid growth potential. Even when aphid establishment occurs at early growth stages, large yield losses can be mitigated through monitoring and timely control.
A control decision should be made based on RWA population densities between GS30 and GS50. Predation and parasitism play an important role in controlling the aphid population growth rate, so keep an eye on natural enemies present in paddocks.
Wetter and cooler summers or early breaks will increase the risk of early season infestation by Russian wheat aphid in mainland grain growing areas. In low green bridge risk years (a dry, hot summer and autumn), infestations reaching economically damaging levels are unlikely.
Acknowledgements
The research initiative that supported the development of the calculator was a GRDC investment that sought to deliver information on RWA management for grain growers. This project was undertaken by the South Australian Research & Development Institute (SARDI) and Cesar Australia. Thank you to Dr Jess Lye for contributing to this article.
Original article written by Lizzy Lowe (Cesar Australia) for PestFacts south-eastern, modified for SA.
