Diuraphis noxia (Kurdjumov)
Common name: Russian wheat aphid (RWA).
Geographical distribution: Cosmopolitan.
Host plants: Various Poaceae, especially wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.).
Morphology: The body is 1.5-3.0 mm in length, green-yellow, covered by waxy white powder. Antennae very short, siphunculi short and truncated, cauda short, with a similar protuberance, the supracauda, being located above it. Alatae are slightly darker than the aptera and have dark patches on the thorax.
Life history: The pest generally prefers plants that grow on dry, poorly managed sites and are widely spaced and/or weak. Between seasons it survives on stubble and nearby weed grasses. RWA is multivoltine, raising a generation in 10-12 days at 20°C. The threshold of development is around 4.0°C and it requires 140 day degrees (DD) to complete the development of the apterae and about 160 DD for the alatae. It may produce 1-3 nymphs/day and can live for almost 3 months in the laboratory under conditions of 15-21°C. Several biotypes of RWA have been identified, differing in their ability to overcome resistant wheat cultivars.
Economic importance: Diuraphis noxia is a major pest of wheat and barley in many parts of the world, especially in warm climates. It also attacks oats (Avena sativa L.), rye (Secale cereale L.) and sorghum (Sorghum bicolour (L.) Moench). Damage is due to sucking out cell contents, injecting toxins and to transmitting several viral diseases. Infested plants lose their chlorophyll and show whitish, yellow or red leaf streaking, rolled leaves and may be stunted. Late season damage includes flag leaf curling, which traps the wheat head, resulting in incomplete grain fill. Heavy infestations may kill the plants. Losses can reach 35-60% of the yield. The economic injury level can be 1.0-4.0 aphids/7 plants, depending on the season.
Horticultural control RWA populations were three times higher on late sown (January) than early-sown (October) wheat, showing that sowing date is a crucial cultural control method for RWA.
Plant resistance: Several resistant wheat cultivars are known, and their planting is an increasingly major component of RWA management. However, certain RWA biotypes can overcome these resistant wheat cultivars.
Biological control: Several parasitoids, predators and entomopathogenic fungi attack the pest in different regions. The more important parasitoids are Aphelinus albipodus Hayat and Fatima, and Aphelinus varipes (Foerster) (both Aphelinidae), and the Braconidae Aphidius colemani Viereck and Diaeretiella rapae. The predators consist of Coccinellidae, especially Hippodamia variegata Goeze, and Syrphidae, which can feed on RWA within curled leaves. The effect of these enemies is variable, due to the slow buildup of their populations, in contrast to the rapid development of RWA. Thus in some cases control is effective, whereas in others the enemies are unable to prevent or reduce the damage.
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