The new pest Russian wheat aphid on our radar

Current extent of the known distribution in Australia

The Russian wheat aphid, Diuraphis noxia, (RWA) was detected for the first time in Australia in May 2016 in cereal crops in the Tarlee district of the Mid North of South Australia. Subsequent surveillance to delimit the extent of the RWA incursion has found the species across much of the eastern half of South Australia and western and central Victoria. At the time of publishing this article RWA had not yet been found in the states of Western Australia, New South Wales, Queensland or Tasmania. A map of the known distribution of RWA is being regularly updated on the Plant Health Australia website.

Landholders and agronomists in regions where RWA has not yet been detected are requested to check emerging cereal crops and report any suspect aphids or unusual damage to their state’s Exotic Plant Pest Hotline.

The Russian wheat aphid

RWA is one the world's most economically important pests of wheat, barley and other cereal grains. It is native to southern Russia, the Middle East and Central Asia, but since the late 1970s and early 1980s has rapidly spread to other major grain producing regions in Europe, Africa, North America and South America. Unlike other cereal aphids that damage plants by removing nutrients, RWA also injects salivary toxins during feeding that cause rapid, systemic phytotoxic effects on plants, resulting in acute plant symptoms and potentially significant yield losses.

Occurrence and potential spread in Australia

Around the world, the distribution of RWA is primarily associated with cereal production regions characterised by warmer, drier climates. It is less prevalent or nonexistent in higher rainfall areas. This species has a wide range of Graminaceous host plants, including cultivated and wild grasses which occur widely throughout Australia. RWA has a history of successfully invading new regions, partly due to its widely available host plants and capacity for rapid population growth. Aphids could potentially disperse to new areas on contaminated plant material, on machinery and other equipment, or via dispersal of winged adults over large distances by wind assisted flight.

Biology

Host range

The host range of RWA includes more than 140 species of cultivated and wild plants within the Graminae (grasses) family. These include wheat, barley, triticale, rye, oats, pasture grasses and wild genera including Poa, Bromus, Hordeum, Lolium, Phalaris and others. Wheat and barley are most susceptible, while triticale, rye and oats are less susceptible. In South Africa, native grasses apparently do not host RWA.

Lifecycle

In its native range, the annual lifecycle of RWA includes sexual and asexual phases. However, like most other introduced aphid pests in Australia, invasive populations of RWA reproduce asexually, with females giving birth to live female offspring.

In autumn, aphids may infest wheat seedlings soon after emergence, usually from wingless aphids walking off nearby senescing hosts. Aphids require actively growing plants for development; populations start to increase from tillering and stem elongation. Aphids regularly move by walking among leaves, tillers and plants, so that the percentage of infested plants increases during the crop cycle. Population growth becomes most rapid from booting onwards. Early in the crop cycle, the vast majority of aphids are wingless. Later in the crop cycle as aphid population density increases, the proportion of winged aphids increases and may reach high levels prior to ripening; at this stage, aphids emigrate in search of alternative summer hosts. Alate RWAs (winged aphids) are weak flyers, but are thought to travel on wind currents efficiently enough for some aphids to locate isolated host plants.

RWA is able to survive under a wide range of temperatures and may perform better at lower temperatures than the other cereal aphids. The optimum temperature range is considered to be around 18-21°C. RWA does not do well under higher temperatures (>25°C). Under laboratory conditions, generation time ranges from approximately 20 days at 10°C, and 9 days at 20°C.

Plant damage

Aphids feed in dense colonies, typically at the base and sheath of younger leaves and within leaves curled by their feeding. Aphids prefer the newest leaves of plants, and are often found on the last two leaves unfurled. At high densities they can be found on any foliar parts.

Even a few aphids can cause symptoms to appear as early as seven days after infestation. Damage symptoms are characterised by (Figures 1-6) longitudinal rolling of leaves, forming a hollow tube inside which aphids shelter; whitish, yellowish to pink-purple chlorotic streaks along the length of leaves. Heavily infested plants are often stunted and may appear flattened, with tillers lying almost parallel to the ground. Viewed from a distance, damage may appear as a general loss of colouration across the affected crop area. Later in the crop cycle, wheat awns may become trapped by rolled leaves, resulting in hook-shaped head growth and bleaching, and reduced yield. Economic damage is mainly caused by direct feeding. The virus transmission status of RWA is unclear.

Image of Wheat showing damage caused by Russian Wheat Aphid

Figure 1: RWA feeding damage symptoms to wheat.

Image of Russian Wheat Aphid on wheat leaf displaying distinctive striping

Figure 2: RWA on wheat leaf with distinctive striping attributable to aphid feeding.

Image of wheat showing white and purple chlorotic streaks

Figure 3: White and purple chlorotic streaks along the lengths of leaves.

Image of wheat and colony of Russian Wheat Aphids

Figure 4: RWA aphid colony.

Image of wheat showing stunting, flattened growth and white and purple streaks.

Figure 5: Stunting, flattened growth, white and purple streaks on leaves.

Image of wheat crop showing stress, stunted growth and loss of green colouration

Figure 6: Stress, stunted growth and loss of green colouration caused by heavy infestation at Tarlee, May 2016.

Yield impact

The salivary toxins injected by RWA during feeding damages plant chloroplasts, resulting in reduced photosynthetic ability, delayed leaf initiation and tillering, reduced numbers of fertile tillers, shoot and root biomass, and reduced grains per ear and grain weight. Yield impacts are determined by the percentage of infested tillers and plants and crop development stage. Heavy infestations during early growth can cause serious damage (under USA conditions). From early booting to soft dough stage, feeding on upper leaves, in the leaf sheath and next to the developing head, can cause direct yield losses. In wheat and barley, damaged leaf tissue does not recover. If aphids are controlled, new growth proceeds normally (new root and shoots are unaffected) and plants may recover unless excessively stressed. After soft dough stage, further impact is minimal.

Description and identification

RWA are small, pale green in colour, elongate and spindle-shaped, often with a fine dusting of whitish wax. Apterous (wingless) adults have very short antennae, about 0.33 to 0.5mm body length, and very short siphuncles ('exhaust pipes') which initially appear absent to the naked eye. The alate (winged) adults are small, up to 2mm in length, with body-length antennae and are generally darker in colour with dusky colouration on parts of the body, particularly the thorax. They have two 'caudal processes' (short tails) on the end of the abdomen, one above the other, giving the appearance of a 'double tail'. Their general small size, green colour, elongate shape, very short antennae and apparent lack of cornicles are characteristics that can readily distinguish RWA from other aphids found in Australian cereal crops (Figures 7 to 12).

Image of Russian Wheat Aphid showing scale

Figure 7: Russian wheat aphid.

Image showing scale of Russian Wheat Aphid with double tails

Figure 8: Rear end of RWA with distinctive double tails.

Image showing rear end of Russian Wheat Aphid with indistinct short siphuncles

Figure 9: Rear end of RWA showing indistinct, short siphuncles.

Image of Russian Wheat Aphid skin moult

Figure 10: RWA with skins (moults) on leaf with typical symptoms.

Image of Russian Wheat Aphids at different life stages with scale

Figure 11: RWA at different stages. Note the very short siphuncles on largest aphid which appear as two dark spots.

Image of Russian Wheat Aphid colony inside sheaf

Figure 12: RWA colony inside sheaf at base of leaves.

Recommendations

At this stage, growers or agronomists in areas where RWA has not yet been confirmed are asked to submit suspect aphids to their State Exotic Plant Pest Hotline. Adopt best-practice farm hygiene procedures to retard the spread of the pest between fields and adjacent properties. Keep traffic out of affected areas and minimise movement in adjacent areas.

Management options

An APVMA permit (PER81133) has been issued for the use of products containing 500g/L chlorpyrifos (rate: 1.2L/ha), with a LI700 surfactant (rate: 240 ml/ha), and products containing 500g/kg pirimicarb (rate: 200-250g/ha) to control RWA in cereals. We advise use of a high water volume (100-120L/ha) at seven bar pressure to maximise coverage.

General management strategies

Monitoring/sampling

Aphids may infest crops during any stage of crop development, from early establishment to maturating flag leaf. Check crops regularly following seedling emergence. RWA are often difficult to find when at low numbers. Check for the characteristic leaf streaking and rolling. Infestations often begin along crop edges, usually on the windward side or adjacent to infested grasses. RWA also commonly occurs in areas of paddocks where plants are sparse or adjacent to bare ground. After initial infestation, aphids can rapidly spread across a paddock.

SARDI entomologists have observed that weather conditions may affect distribution of aphids on plants. During inclement weather RWA on volunteer cereals (GS5 to 8) were only found on lower leaves and in their leaf sheaths, but were more broadly distributed over plants during fine weather.

Chemical control

Chemical control of RWA is effective. Due to the cryptic feeding habits of RWA, complete coverage and use of an insecticide with fumigant or systemic activity is required. In many regions, organophosphate insecticides are commonly recommended.

The latest foliar spray recommendations, based on the findings of winter insecticide trials conducted in SA and VIC, will be presented in the GRDC Update presentation.

Seed treatments offer some early season protection, as indicated by preliminary results collected by SARDI and Agrilink staff from an Agrilink early season wheat trial infested with RWA.

Decisions on the need for foliar treatments are based on the proportion of seedlings or tillers infested. Threshold guidelines (ET) recommended in the USA vary somewhat between regions, but for early season growth we currently recommend an ET of 20 per cent seedlings infested up to the start of tillering, and 10 per cent seedling infested thereafter. Local research will be required to test, and if required, to modify these thresholds for Australian crop conditions.

Cultural control

In certain regions around the world, wheat cultivars with resistance to RWA are deployed. In some regions, ‘virulent’ aphid biotypes have developed that have overcome host resistance genes.

Cultural controls include eliminating refuge volunteer cereals and grasses in fallows and other areas during summer and autumn; later planting of winter cereals to delay and reduce early aphid infestation; agronomic practices to promote crop vigour and dense canopy growth which inhibit RWA populations and reduce their impact on the crop.

Abiotic control

Like other aphids, populations of RWA are strongly regulated by environmental conditions. Survival of aphids outside the shelter of leaf rolls is affected by exposure to rainfall, drying winds, and predators and parasitoids. Rainfall washes aphids from upper leaves, and heavy rainfall may cause 50 per cent mortality. Populations are generally reduced by cold and wet conditions.

Biological control

RWA is attacked by a range of natural enemies in other parts of the world, many of which also attack other aphids. Of these, groups that commonly occur in Australia include the parasitoid wasps Aphidius colemani, A. ervi, Diaeretiella rapae and generalist predators including ladybird beetles (e.g. Coccinella spp., Hippodamia spp. [12]), lacewings (Chrysopa spp.), damsel bug (Nabis sp.), hoverflies (Syrphus spp.), and also entomopathogenic fungi. SARDI entomologists have already observed mummified and fungus diseased RWA (Figure 13).

Image of Russian Wheat Aphid affected by fungus disease

Figure 13: The two aphids appearing 'yellow' have been effected by a fungus disease.

Plant resistance

RWA resistant wheat and barley germplasm is available overseas, and some has already been introgressed into certain Australian cereal lines. GRDC and commercial breeding companies are initiating screening and breeding programs to provide industry with resistant/tolerant cultivars in the future (anticipated to take 5-6 years).

Further / recommended reading

Hughes RD. A synopsis of information on the Russian wheat aphid, Diuraphis noxia (Mordwilko).(Revised edition). CSIRO Australia Division of Entomology Technical Paper. 1996(34).

Pike KS, Allison D. Russian wheat aphid. Biology, damage and management. Pacific Northwest Cooperative Extension Publication. 1991(PNW371).

Monitor RWA numbers closely over winter (This Paddock Practices article contains useful answers to a range of Frequently Asked Questions about RWA).

Contact details

Greg Baker
greg.baker@sa.gov.au