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.