Russian wheat aphid: watching and waiting

Russian wheat aphid has been lurking in volunteer cereals on the sidelines; what can we expect once the efficacy of seed treatments wanes? 

Russian wheat aphid (Sources: cesar)

 

In response to the incursion of Russian wheat aphid (Diuraphis noxia, RWA) in 2016, many cereal crops planted in Victoria and southern NSW have been treated with imidacloprid (see APVMA permit PER82304). Preliminary trials conducted by cesar suggest that those seed treatments currently registered in Australia to control cereal aphids will be effective against early RWA colonisation. This is supported by field reports from advisers and growers last year in numerous parts of southern Australia. Products containing imidacloprid and thiamethoxam should prove effective, although thorough trials are needed to fully understand the role of cereal seed treatments in managing RWA.

So the use of certain seed treatments will help cereals during crop establishment when aphids move from ‘green bridges’, including volunteer cereals and grass weeds, into establishing crops. But knowing we will eventually be outside the efficacy window for the seed treatments, we can look back on learnings from 2016 for assistance in managing RWA.

Observations from 2016:

Climate can impact RWA

Like other aphids, populations of RWA are strongly regulated by environmental conditions. Survival of aphids is affected by exposure to rainfall and drying winds. Rainfall washes aphids from upper leaves, and heavy rainfall may cause mortality of up to half of the population. Populations are generally reduced by cold and wet conditions. This was particularly evident in late winter-early spring in 2016. Monitoring of RWA populations within crops is critical before any management decision is taken. Aphids commonly occur in areas of paddocks where plants are sparse, on sandy rises or adjacent to bare ground, so these areas are best to target monitoring efforts initially.

Beneficials help in the fight against RWA

In 2016, we saw RWA attacked by a range of beneficials (as happens in other parts of the world). Beneficials that commonly occur in Australia include minute parasitoid wasps and generalist predators including ladybird beetles, lacewings, damsel bugs and hoverflies. Entomopathogenic fungal diseases, stimulated by high rainfall, also had a substantial role in the unexpected sharp decline of RWA populations in spring 2016.

The important role of beneficials is a key reason why prophylactic insecticide sprays for managing invading or dispersing RWA should be avoided. These sprays can create secondary pest outbreaks (such as other cereal aphids) by removing these beneficial species. If spraying is warranted, aim to use the softer chemistry to maintain beneficial populations.

Chemical controls are effective

Chemical controls are effective but due to the cryptic feeding habits of RWA, complete coverage and use of an insecticide with fumigant or systemic activity is required for maximum efficacy. Permits have been issued by the APVMA for use of pirimicarb (PER82792) and chlorpyrifos (PER83027) against RWA in winter cereals. Decisions on the need for foliar treatments are based on the proportion of seedlings or tillers infested. Economic thresholds (ET) are yet to be established under Australian conditions but guidelines are available overseas. Guidelines from the United States vary somewhat between regions, but for early season growth, an ET of 20% seedlings infested up to the start of tillering, and 10% seedling infested thereafter, is recommended.

For more information on RWA, including their occurrence, lifecycle, behaviour and management strategies, visit Russian wheat aphid with our PestNote series.

 

Sources of field reports

Daniel Andrews – Rodwells (Northern Country, VIC)

Simon Mock – Clovercrest Consulting (Wimmera, VIC)