Aphids in lucerne

Agronomist, Greg Parker (Mangoplah Farm Centre), has reported aphids in several lucerne paddocks south of Wagga Wagga, in the South West Slopes of New South Wales. Both spotted alfalfa aphids (Therioaphis trifolii) and bluegreen aphids (Acyrthosiphon kondoi) have been observed in fairly high numbers. Greg says the summer and early autumn rainfall has resulted in a ‘green bridge’ that enabled aphids to survive in high numbers over the summer period. The paddocks containing aphids are established lucerne crops, with some having a history of aphids in previous seasons.

Bluegreen aphids are a large species, measuring up to 3 mm long as adults. Both the winged and wingless forms are a matt bluish-green colour. Annual medics, lucerne, subterranean clover and lupins are susceptible to bluegreen aphids. In lucerne and medics, heavy infestations cause stunted growth, leaf curling and leaf drop. In subterranean clover, leaves wilt before turning brown and dying. Bluegreen aphids favour growing tips of medic and lucerne, while in clover they are widely dispersed under the canopy. Click here for images of bluegreen aphids.

Last year, bluegreen aphids caused more damage to lucerne and pasture seedlings than normally experienced in some parts. Research at South Australian Research and Development Institute (SARDI) indicates this may have been caused by a new virulent biotype. Conventional levels of host-plant resistance appear to be broken down in most pasture legumes including lucerne, annual medics and sub-clover. In some cases, plant death has been observed, which is not common for bluegreen aphids. Growers are encouraged to be on the look-out for unusual damage this year and report directly to SARDI Researcher, Alan Humphries at alan.humphries@sa.gov.au.

Spotted alfalfa aphids are a pest of lucerne, legumes and annual medics. Adults 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. Spotted alfalfa aphids are generally found on the underside of leaves and adults may have wings. Early planted pastures are at risk from spotted alfalfa aphids, which are favoured by warm autumns, particularly following an early break. Click here for images of spotted alfalfa aphids, and refer to PestFacts Issue No. 1 for additional information.

When considering control options for all aphids it is important to remember that in some cases the whole paddock may not require treatment, as infestations often start at crop edges. If most of the infested plants are along the edges of a crop, a perimeter spray is likely to control the majority of aphids. This approach will also preserve some of the beneficial invertebrate species likely to be present.