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Spotted alfalfa aphid

Spotted alfalfa aphids (Therioaphis trifolii) have been identified for agronomist, Allan Edis (AGnVET Services), from a lucerne paddock near Henty in the Riverina district of New South Wales. Very high numbers of aphids were found across the paddock and their feeding has visibly affected many plants. Allan says plants are showing signs of yellowing and wilting, and in some cases plant death is likely to occur if aphids are not controlled. The paddock is currently in its third year of lucerne, and has not previously experienced problems with aphids.

Spotted alfalfa aphids are a pest of lucerne, legumes and annual medics. Adults and nymphs suck sap and inject a toxin into the plant. Initial symptoms are a yellowing or whitening of the leaf veins. Leaves may then turn yellow, wilt and fall. Damage proceeds from the base of the plant upwards, until only stems remain standing. Stems can become sticky with honeydew exuded by the aphids. Early planted pastures are at risk from spotted alfalfa aphids, which are favoured by warm autumns, particularly following an early break.

Adult spotted alfalfa aphids are pale yellow-green in colour and can be up to 2 mm long. They have at least 6 rows of tiny black spots on their back, which are just visible to the naked eye. Nymphs are similar but smaller in size. A good magnifying glass or a microscope will make this species readily identifiable. Spotted alfalfa aphids are generally found on the underside of leaves and adults may have wings. When disturbed, adult aphids jump in ‘showers’ from plants. Allan says when checking the affected paddock, aphids could be found all over plants as well as on the ground. Click here for images of the spotted alfalfa aphid.

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