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Spotted alfalfa aphids

Grower, David Cook, has observed high numbers of aphids present on lucerne stands in some paddocks near Shepparton, in the Northern Country of Victoria. The aphids have been identified as spotted alfalfa aphids (Therioaphis trifolii). The paddocks have recently been sown to canola, which spotted alfalfa aphids will not normally attack. The paddocks will therefore not be sprayed, although the populations will be closely monitored.

Adult spotted alfalfa aphids are pale yellow-green in colour and 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. They are normally found on the underside of leaves and adults may have wings. When disturbed, adult aphids jump in ‘showers’ from plants. Click here for images of the spotted alfalfa aphid.

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.

Reproduction rates of aphids are high, so numbers can increase very rapidly. It is therefore important to monitor all crop stages, although spotted alfalfa aphids are more common in warmer months. If spraying is necessary, the insecticide to be applied should be chosen carefully. Many chemicals have a detrimental effect on beneficial insects and thus increase the likelihood of subsequent pest outbreaks. Many aphid predators are generalists and feed on other pests, such as native budworm and diamond-back moth. Killing natural enemies early in the season may have implications for the control of these pests later in the season. The use of biologically ‘soft’ chemicals, such as pirimicarb, will preserve many beneficial insects.

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