Russian wheat aphid; making control decisions
Heading into spring, rising temperatures are expected to cause an increase in populations of Russian wheat aphid (RWA), Diuraphis noxia, and natural enemies.
The latest cereal aphid monitoring data have shown a recent decline in RW aphid populations in SARDI trial sites monitored fortnightly at Loxton, Bool Lagoon and Roseworthy (SAGIT Funded). During recent warm days (>25°C) RWA has been observed migrating and infesting cereals, included seed-treated crops with symptoms now appearing on flag leaves. Natural enemies (parasitic wasps, lacewings, ladybeetles) are regularly observed. In his recent GRDC updates paper, Russian wheat aphid dynamics in 2017, Maarten van Helden (SARDI) presented new data and provided some tips for assessing RWA population trends.
On Eyre Peninsula, RWA is now reported on Lower Eyre Peninsula, with sightings from Mount Hope, Cummins and Port Neill. Further west, sightings have increased around Kyancutta to Warramboo and circular hotspots are being observed in flowering-heading wheat crops around Cungena to Wirrulla, but overall densities are mostly low across paddocks (<1%) and well below threshold guidelines. Populations have been higher in crops around Kimba, Kelly and Buckleboo that were not seed-treated. In the South East, RWA is widespread but at low densities (<1%) around Naracoorte, Frances and Bool Lagoon.
RWA Intervention Thresholds:
Many growers are currently making control decisions. We recommend using a threshold-based approach. During early booting to the soft dough stage, an intervention threshold of >10% of all tillers infested with aphids can be used. This guideline is based on overseas data and requires validation under Australian conditions. It is not unusual that tillers showing symptoms do not contain aphids, especially if the aphid population is on the decline. Keep an eye open for beneficials that can be present. When assessing aphid densities, verify that plants showing symptoms are infested with aphids.
Control decisions can be challenging; assessing the need for intervention involves predicting the risk of further economic damage (not current damage) if the pest is not controlled. In addition to assessing RWA densities, we recommend taking the following factors into consideration: (i) Crop stage and the time left until soft dough stage. Cereal crops are most susceptible to a yield impact from RWA feeding from early booting to soft dough. (ii) The overall trend in the aphid population (declining, increasing or stable), rather than densities at a single time point. Some tips for assessing population trends are provided above. (iii) Beneficial insect activity. Parasitoids, predators and entomopathogens (insect diseases) attack Russian wheat aphid populations. (iv) Weather conditions. There is good evidence from overseas literature, and local experience in 2016 and 2017, that rainfall events can cause significant mortality. (v) Control cost. This includes the cost of insecticide, application (diesel, machinery), and up to 2% additional crop losses incurred from knocking over grain while driving through the crop.
Spray application should aim to achieve maximum penetration into the crop canopy. Use of at least 100L/ha water with nozzle pressure to produce medium size droplets (2.5-3.0 bar pressure for flat fan nozzles) is recommended.
Pirimicarb has less impact on beneficial populations. While the cost per hectare is typically higher relative to chlorpyrifos, control provided by conserved natural enemies should be factored in, reducing the risk of a secondary spike in Russian wheat aphid or other cereal aphid pests. Adhere to label guidelines regarding the ambient temperatures during application; the fumigant effect of pirimicarb is important in controlling Russian wheat aphid due to their cryptic feeding habits, and can be reduced if temperatures are less than 15-20oC. To help with assessing spray efficacy, we recommend flagging aphid-infested plants at a few locations and counting aphid densities pre- and post-treatment (3 days after application). Symptoms of RW aphid feeding will persist for a period after a treatment or rain event, but new growth will look normal.
A new GRDC best practice management manual, Russian wheat aphid: Future tactics for integrated control, has been developed by Ag Communicators together with entomologists from cesar, SARDI and CSIRO. The manual provides a comprehensive review of research information available in the international literature to guide grain growers and advisors in the southern region on integrated control of Russian wheat aphid.
Sources of reports: Amy Gutschke (Lincoln Rural Supplies Pty Ltd), Leigh Davis (Landmark), Nigel Myers (Landmark), Troy Stoeckel (Landmark), Adam Hancock (Elders), Troy Maitland (EP Ag n Fert).
For more information on Russian wheat aphid biology and management, refer to Russian wheat aphid ().