Oat aphids

Agronomist, Ian Cocking (North West Ag Services), says oat aphids (Rhopalosiphum padi) have been found attacking barley crops in the central Mallee, Victoria. The number of aphids observed within and between paddocks is quite variable. Some barley plants have only 1 to 2 aphids, while in other areas around 20 to 50 aphids have been observed per plant. Interestingly, Ian says the aphids have mainly been found in paddocks with higher stubble residues. Agronomist, Tom Lord (Crop Facts), has recently found large numbers of aphids in cereal crops around Wycheproof, also in the Mallee district. Tom says aphids were a problem in the region last season around June-July, however, the conditions were a lot wetter. Alysa Attema (Birchip Cropping Group) has also observed oat aphids attacking cereal crops near Birchip, again in the Mallee district.

Oat aphids (sometimes called ‘cereal aphids’ or ‘wheat aphids’) can be found on all cereals including wheat, barley and oats. They vary in colour from olive-green to black and are characterised by a dark reddish patch on the tip of the abdomen. Adults are pear-shaped and have antennae which extend half the body length.

Oat aphids suck sap, causing yellowing and stunting of plants. A large amount of sugary solution (honeydew) is secreted by aphids when numbers are high. This can lead to black sooty mould which can reduce plant growth. Importantly, oat aphids can spread barley yellow dwarf virus (BYDV). Transmission can occur with very few aphids and must occur early in the life of the plant to cause subsequent yield losses. BYDV, which has been recognized as a problem in Victoria since the early 1960s, causes a disease in barley, wheat and oats which can lead to stunting of plants and subsequent losses in yield and quality. For further information on BYDV refer to: www.agric.wa.gov.au/bydv.

Entomologists are concerned that the overuse of inexpensive synthetic pyrethroid insecticides as ‘insurance sprays’ could be leading to the development of resistance in target and non-target species. Rather than spraying aphids to prevent the transmission of BYDV, it is best to implement sound agronomic practices. Click here to obtain more information on management options for aphid control. Ian says chemical control has been undertaken in some paddocks, although many farmers are delaying spraying as the majority of crops are struggling in the present environment.

Natural enemies are a reliable form of control during the warmer days of spring and when low to moderate numbers of aphids are present. Beneficial insects which attack aphids include parasitoids (tiny wasps) and predators (ladybirds, hover flies and lacewings). These will be building up in crops along with the aphids and they can reduce or contain aphid populations to below threshold levels in some years. A number of aphid ‘mummies’ have been observed in the Mallee, indicating the activity of parasitic wasps. Small female wasps will often insert an egg into an aphid and the developing larvae feed inside the aphid, eventually killing it. The new adult wasp emerges from the mummy by cutting a hole in the skin.