Russian wheat aphid detected
An exotic insect pest, the Russian wheat aphid, Diuraphis noxia, (RWA) has been detected in cereal crops in the Tarlee and adjacent districts in the Mid North of South Australia. The initial detection was made in a paddock of Beaufort wheat near Tarlee on 13 May, in which 30 hectares showed patches of heavy aphid infestation and significant damage symptoms. Targeted surveillance around the initial detection sites by PIRSA Biosecurity SA and SARDI entomologists has since identified numerous infested sites (see map of the affected area ()). PIRSA is conducting surveillance and assisting affected growers with immediate control in an effort to locally contain infestations while the extent of distribution is being assessed. This species has not been previously detected in Australia.
Landholders and agronomists are requested to check emerging cereal crops and report any suspect aphids or unusual damage to the Exotic Plant Pest Hotline on 1800 084 881.
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 charcterised 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.
The host range of RWA includes more than 140 species of cultivated and wild plants within the family Graminae (grasses). 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.
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 RWA 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. However does not do well under higher temperatures (>25°C). Under laboratory conditions, generation time ranges from approx. 20 days at 10°C, and 9 days at 20°C.
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 few aphids can cause symptoms to appear as early as 7 days after infestation. Damage symptoms are charcterised by (also see images) 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.
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, 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.
See chemical control for economic thresholds (ET).
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 0.5 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.
More information on distinguishing characteristics of RWA appear in the GRDC I SPY manual (Page 39)
At this stage, while prospects for local containment are being assessed, growers or agronomists with suspect aphids should contact the Exotic Plant Pest Hotline on 1800 084 881. Treatment advice can be provided - if RWA is detected in a small area, it may be controlled with a suitable insecticide. 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.
An APVMA permit (PER81133) has been issued for the use of products containing 500 g/L chlorpyrifos (rate: 1.2 L/ha), with a LI700 surfactant (rate: 240 ml/ha), and products containing 500 g/kg pirimicarb (rate: 200-250 g/ha) to control RWA in cereals. We advise use of a high water volume (100-120 L/ha) at 7 bar pressure to maximize coverage.
General management strategies
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 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 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. Seed treatments offer some early season protection, as indicated by preliminary results collected by SARDI and Mick Faulkner 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% seedlings infested up to the start of tillering, and 10% seedling infested thereafter. We will provide more information on thresholds in later editions.
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 reduces their impact on the crop.
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% mortality. Populations are generally reduced by cold and wet conditions.
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. ), lacewings (Chrysopa spp.), damsel bug (Nabis sp.), hoverflies (Syrphus spp.), and also entomopathogenic fungi. SARDI entomologists have already observed mummified and fungus diseased RWA.
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).
Hughes RD, Maywald GF. Forecasting the favourableness of the Australian environment for the Russian wheat aphid, Diuraphis noxia (Homoptera: Aphididae), and its potential impact on Australian wheat yields. Bulletin of Entomological Research. 1990 Jun 1;80(02):165-75.