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Spotted alfalfa aphids

Consultant, Ian Cocking (AGRIvision Consultants), reports that spotted alfalfa aphids (Therioaphis trifolii) have been found in high numbers in a dryland lucerne paddock near Hopetoun, in the Mallee district of Victoria. Ian says the entire paddock is infested, with feeding damage leading to yellowing of the plant leaves. In severe cases, the leaves will wilt and plant death can occur. Due to the extent of the problem, this paddock, which has been sown to lucerne for the past 3 seasons, will be sprayed with an organophosphorus insecticide. Tolerant lucerne varieties could be a viable control option next season that could eliminate the need for chemicals altogether.

Adult spotted alfalfa aphids are pale yellow-green in colour and about 2 mm long. They have 6 or more rows of tiny black spots on their back, which are just visible to the naked eye. They are normally found on the underside of leaves and may have wings. When disturbed, adult aphids jump in “showers” from plants. Nymphs are similar but smaller in size. Adults and nymphs suck sap and inject a toxin into the plant. Initial symptoms are a yellowing or whitening of the leaf veins. Damage proceeds from the base of the plant upwards, until only stems remain standing.

The spotted alfalfa aphid is a pest of lucerne and annual medics. All aphids are female and reproduce without mating. Reproduction rates are high, so numbers can increase very rapidly. It is therefore important to monitor all crop stages, although this pest is more common in warmer months.

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