Cereal aphids
Crop stage
Tillering, flowering to maturity. BYDV transmission.
Description
Corn aphid (Rhopalosiphum maidis) and wheat/oat aphid (Rhopalosiphum padi) cause most yield loss. Wingless females are about 0.2-2.5mm long. Rice root aphid (Rhopalosiphum rufiabdominalis) and grain aphid (Sitobion miscanthi) are also found in cereals and may be important as virus vectors. Corn aphids are dark blue- green to grey-green, often with a fine white powdery dust over the body. Colonies develop within the furled tip of tillers, starting any time from seedling stage to head emergence. Few farmers see them because they are hidden in the furled leaves. Barley is most likely to be affected. Corn aphid probably kills tillers, resulting in fewer heads. Wheat/oat aphids vary from mottled yellow-green through olive-green and dusky brown, to a blackish green.
Colonies develop on the outside of tillers from the base upwards, on stems, nodes and backs of mature leaves, starting any time between late tillering and grain filling. Heavy infestations can blacken heads and flag leaves, and are the aphids most commonly reported by farmers. Wheat/oat aphids are more mobile than corn aphids, and can drop to the soil and crawl to other plants. They cause yield losses probably by reducing grain weight and grains per head. They may also be important in spreading BYDV. Rice root aphids are like wheat/oat aphids, but can also infest plant roots. They have a reddish patch in the middle of the back, and are most likely to be found in drier agricultural areas. Grain aphids are dusky green with yellow-green tinges, usually found in spring, without developing large colonies. Rose-grain aphids are potentially serious pests, but are yet to enter Western Australia from the eastern states where they are widespread. They are green spindle-shaped aphids attacking wheat and barley during grain filling.
Life cycle
Winged aphids fly into crops from pasture grasses or other crops, and start colonies of wingless aphids. Reproduction is rapid when weather conditions are favourable, leading to population outbreaks. Plants can become sticky with honey- dew excreted by the aphids. When plants become unsuitable or overcrowding occurs, winged aphids redevelop and migrate to other plants or crops. They can carry viruses in saliva or on their feeding tubes. BYDV damage is most serious after plant infection early in the season.
Damage
Cereal aphid damage in barley, wheat and oats has no obvious signs or symptoms. Research in medium and high rainfall zones has shown aphids can cause losses up to 30% where crop yield potential is three tonnes per hectare (t/ha) and higher. Experimentally, losses of 400-1800kg/ha have been measured in barley yielding about 5t/ha. As this work is new, some questions remain unanswered. Damaging populations may develop in three out of five years. Aphids affect cereals by direct feeding on plants, and/or by transmitting barley yellow dwarf virus (BYDV). BYDV affects wheat, oats and barley and is spread by flying aphids. Direct damage occurs when colonies of 10-100 aphids develop on stems, leaves and heads, from seedling stage through to head filling. The degree of damage depends on the percentage of tillers infested, aphids per tiller, and the duration of the infestation.
Control
In southern areas, barley crops should be checked from late tillering onwards for corn aphids in the furled growing tips, and for wheat/oat aphids on stems, backs of leaves and in the crown. Crops expected to yield 3t/ha or more are most at risk. Spraying is worthwhile if 50% of tillers have 10-20 or more aphids. Mixed infestations of both aphids may cause more damage than either species on their own. Crops sprayed before Zadoks stage 30 may need respraying at Zadoks stage 50 or later, if aphid numbers build up again. Parasitic wasps, ladybeetles, lacewings and hoverflies can provide useful biological control, mainly by preventing secondary outbreaks. The use of "soft" insecticides that only kill aphids is advocated.