Cereal aphids

Aphid populations continue to increase in many regions and are now being found across a large number of cereal crops. In the majority of paddocks the species present are the oat aphid (Rhopalosiphum padi). Heavy infestations of oat aphids will cause the crop to turn yellow, become stunted and generally appear unthrifty. Consultant, Simon Mock (Clovercrest Consulting), has reported aphids in high numbers across many cereal paddocks west of Horsham, in the Wimmera district of Victoria. Simon cannot recall observing cereal aphids in such high numbers previously. Contracts services manager, Stuart McColl (BCG), says cereal aphids are very prevalent around Birchip, in the Mallee district of Victoria.

In New South Wales, consultant, Sandy Biddulph (Biddulph Rural Consulting), has reported aphids within several wheat and barley crops near Grenfell, in the South West Slops district. Senior farm consultant, Tim Condon (Delta Agribusiness), has reported high levels of cereal aphids in many crops around Harden, in the South West Slopes district. Tim says the numbers of aphids are the highest he has observed for over a decade. Consultant, Glen Smith (3D-Ag), has also reported high numbers of aphids in wheat crops around Wagga Wagga, also in the South West Slopes district. Glen says aphids have recently been observed infesting the heads of some wheat plants.

One of the challenges in managing cereal aphids at this time of year is the lack of local knowledge about the likely impact of infestations on yield and quality. Research trials in SE Queensland, for example, show that there is very little impact on yield after grain has filled and is maturing. Infestations that occur during booting to milky dough, particularly where aphids are colonising the flag leaf, stem and ear, can result in yield loss. Aphid infestations in the heads of cereal plants are quite unusual in Australia (but commonly observed overseas). Crops should be monitored closely in paddocks where aphids are moving into plant heads; there is likely to be a higher probability of yield loss in these instances.

There is little information and therefore uncertainty regarding the impact of rainfall on aphid populations in spring. Heavy rainfall events will directly knock some aphids off plants and this can help keep populations in check. However, the overall reduction in numbers is likely to be quite limited in most situations, particularly in cereals crops where aphids are somewhat sheltered/protected.

Beneficial insects have the largest impact on aphid populations; these include parasitic wasps, ladybird beetles, hoverflies and lacewings. Recent reports suggest the number of beneficial insects within cereal crops has started to increase in the last few weeks. However, the weather conditions expected over the next month may also encourage the continued build-up of aphid populations. Remember, it is the week-to-week population trends (up, down or steady) of both aphid and beneficial populations that provide the best indication of the need to spray.

Refer to PestFacts Issue No. 9 for further information on aphid management in cereals.