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Aphids

Growers and advisers should be aware there are high numbers of several aphid species present across many regions of Victoria and New South Wales. There is a high likelihood that these will infest crops this autumn. Summer rainfall has resulted in a significant green bridge of weeds and crop volunteers, enabling aphids to survive and build up over summer. If possible, green material within a paddock and along fence-lines should be removed and then crops sown after a significant break period. Some species of aphids are attracted to areas of bare earth. Use minimal tillage and sow into retained stubble, ideally inter-row to discourage aphid landings.

Corn aphids (Rhopalosiphum maidis) have been identified for research officer, Geoff Davis (AgriTech), from a summer crop trial site near Young, in the South West Slopes district of New South Wales. The trial site contains several crop types but the aphids were mainly found on sorghum. Corn aphids have an oblong shaped, light green to olive coloured body with two dark areas on the abdomen near the base. Although corn aphids are most commonly found on barley, they can attack all winter cereals as well as maize, sorghum and some grasses. Heavy infestations can result in yield losses.

Aphids have also recently been found in several paddocks containing emerging cereal crops around Forbes in the Central West Slopes and Plains district of New South Wales. Agronomists, Elissa Strong (AGnVET) and Olivia Wright (AGnVET), say many aphids can be found on weeds and volunteer crop plants, and there are low numbers on the emerging crop seedlings. At this stage the species observed are unknown. They are not corn aphids or the oat aphid (Rhopalosiphum padi) which are typically found attacking cereals.

Thresholds for aphids are generally focused on infestations in spring, and relate to potential losses from aphids directly feeding on plants. In autumn, it is just as important to consider the risk of virus transmission by aphids. When transmission occurs early in the season, the potential impact of the virus is greatly increased. Insecticide seed treatments can be used to protect emerging seedlings from aphid attack, however it is important to recognise that this approach may not always protect the crop from virus transmission by aphids. Alternatively, a foliar insecticide can be applied early based on forecast reports of the degree of risk.

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