Taking stock of Russian wheat aphid

This spring, can we expect to see the same Russian wheat aphid narrative that played out in 2016? 

 

Our understanding of Russian wheat aphid (Diuraphis noxia, RWA) in Australia is continually evolving. One year has not been sufficient to appreciate the full complexity of how RWA can behave under different growing conditions. The current information available to growers and agronomists is drawn from decades of research and experience with RWA overseas, and has been adapted based on our own learnings as an industry from 2016. This has served as valuable guide navigating RWA management in autumn and winter this year. But as we enter spring and reflect on the growing conditions so far in 2017, we must prepare for the fact that the impact of RWA could be different to last year. We are bound to learn more about this newcomer in the following months. 

The wet conditions in late winter and spring in 2016 is thought to have played a key role in reducing RWA numbers. Rainfall not only can physically displace aphids from leaf shafts, it produces conditions conducive for the development of entomopathic fungi, which contributed to the sharp decline in aphid numbers last year. However, as we enter spring this year, we must stay vigilant. Ultimately, we don’t know how the situation will play out in the event of a dry spring, or even a spring with average or slightly above average rainfall for that matter. 

As winter draws to a close, field reports indicate that RWA numbers are increasing. RWA activity has been reported in the Victorian Mallee, Wimmera, Northern Country, as well as the NSW Riverina. These are all areas in which RWA has been previously reported. This spring, it is important that crops are monitored more frequently than usual. 

If you are experiencing RWA aphid infestations in your cereal paddocks, here are the key points to consider based on the best available information:

Chemical controls are available

Chemical controls can be effective but due to the cryptic feeding habits of RWA, good spray 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. However, prophylactically spraying broad-spectrum insecticides for managing invading or dispersing RWA should be avoided. If spraying is warranted, aim to use pirimicarb to maintain beneficial populations. This is particularly important at this stage of the season, given broad-spectrums will kill many beneficial insects and can lead to RWA ‘flare-ups’ in spring and/or secondary pest outbreaks (such as other cereal aphids).

Decisions on the need for foliar treatments should be 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% tillers infested thereafter, is recommended.

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). These are just a few of the beneficial groups that commonly occur in Australia:

Parasitic wasps

Adults vary in size (1-20 mm long) and colour ranging from bright orange to completely shiny black. They have two sets of wings that are clear or dark coloured. Female wasps often lay their eggs into their host's body or eggs. The developing wasp larva feeds inside the host, usually aphids or pest caterpillars. Aphid ‘mummies’ (bronze-coloured, bloated/enlarged aphids) indicate the activity of aphid parasitic wasps that are small, usually dark in colour and difficult to detect.

Beneficial wasp parasitising an oat aphid (left) and an aphid ‘mummy’ (right) (Source: cesar).

 

Hoverflies

Adult hoverflies are 4-7 mm long, have dark-coloured flattened bodies with black and yellow markings, and have only one set of wings (typical of flies). As the name suggests they ‘hover’ over objects and look similar to bees or wasps. Larvae are legless, green in colour, 8-10 mm long and appear grub-like. They are often mistaken for pest caterpillars such as diamondback moth but lack the typical head capsule of caterpillars. Hoverfly larvae attack a range of soft-bodied insects, but prefer aphids. 

Hoverfly adult (left) and larva (right) (Source: cesar)

 

Ladybird beetles

There are numerous types, but three species commonly found are the white collared ladybird, the common spotted ladybird and the transverse ladybird. Adults are round to oval shaped, with black spots on red, orange or yellow shells. Larvae have grey/black elongated bodies with orange markings and may be covered in spines or white fluffy wax material. Egg to adult stage takes 3-4 weeks, while adults can live for several months. Ladybird adults and larvae are predatory, feeding on aphids, leafhoppers, thrips, mites, moth eggs and small caterpillars.

Ladybird beetle adult feeding on aphids in canola (left); larvae are more slender and are wingless (Source: cesar).

 

Lacewings

Adult brown lacewings are mottled brown in colour and 6-10 mm long, while adult green lacewings are 15-20 mm long and pale to bright green in colour. Both have prominent eyes and long antennae. Their clear membranous wings are typically folded in an upside-down v-shape, and are large with numerous veins giving a lacy appearance. Larvae lack wings, have protruding sickle-shaped mouthparts and a body that is long and varies from thin to stout in shape. Lacewings attack aphids, thrips, mites, caterpillars and moth eggs. Brown lacewings adults and larvae are both predatory, while only green lacewing larvae are predatory.

Lacewing larva feeding on an aphid (top) and a brown lacewing adult (bottom) (Source: cesar).

 

Damsel bugs

Adults are 8-12 mm long and move quickly when disturbed. They have a slender light-brown body with long antennae and large protruding eyes. Juveniles are similar but smaller in size. They have a long curved ‘snout’ that is carried under the body when not feeding. Damsel bugs feed on a range of soft-bodied prey items including small caterpillars, moth eggs, aphids, and mites. 

Damsel bug (Source: cesar).

 

So how many beneficials is enough to keep RWA numbers below damaging levels? Unfortunately, there is no simple answer. There are currently no predator:prey ratios that would guide management decisions. However, there are some guiding principles to assist:

-       Most beneficial species are highly mobile and will move from crop to crop if left unsprayed.

-       There is often a ‘lag’ time between the growth of pest populations, and increases in abundance of beneficials. This is particularly so in southern cropping systems in spring.

-       Monitor crops regularly enough so you can measure whether the relative rate of increase in beneficial insects (per sweep, per m, etc.) is faster (or slower) than that of pest populations. In the former case, the beneficials are winning!

 

Field reports

Bruce Adriaans - Landmark (Northern Country, VIC)

James Challis - Rodwells (Northern Country, VIC)

Barry Haskins - Ag Grow Agronomy and Research Pty Ltd (Riverina, NSW)

Luke Maher - AGRIvision (Mallee, VIC)

Andrew McMahen - Landmark (Mallee, VIC)

Simon Mock - Clovercrest Consulting (Wimmera, VIC)

Mark Thompson -  Advanced Ag (Northern Country, VIC)

Justin Whittakers -  Landmark (Riverina, NSW)