Oat Aphid (Rhopalosiphum padi) and Corn Leaf Aphid (Rhopalosiphum maidis)
Adult corn leaf aphid:
Body length: approximately 2.1 mm long
Colour: varies from bluish, greyish, bottle green to a yellowish green
Abdomen: Often covered with a fine white powdery dust.
Winged and wingless forms
Adult oat aphid:
Body length: approximately 1.8 mm long
Colour: varies from olive green to almost black
Abdomen: Has a characteristic rusty red patch around its base.
Winged and wingless forms
To distinguish between the two aphids using colour is often difficult. A good guide to identification is where the aphids are located on the plant. The corn leaf aphid is normally found on the lower portion of the plant feeding on the back of leaves, base of tillers and on stems. The oat aphid is found on the upper portion of the plant, particularly in the tightly furled emerging leaves.
Cereal aphids feed only on grasses and cereals. Over the summer they persist in very low numbers on perennial grasses and self sown cereal crops that have grown following summer rains. Their severity from year to year is dictated by the seasons. Ideally a mild, wet summer followed by an early break and mild autumn provides favourable conditions for numbers to greatly increase before winter cold sets in. The rate of development of aphid populations is governed by temperature. Under optimum conditions that normally occur in spring they are capable of multiplying at very rapid rates.
Habitats:Origin and History:
The two main species of aphids causing damage to wheat and barley crops are the corn leaf aphid (Rhopalosiphum maidis), mainly found on barley, and the oat aphid (Rhopalosiphum padi), which attacks wheat, barley and oats.
Heavy infestations of cereal aphids in wheat and barley crops with a yield potential of greater than 2.5 t/ha can result in yield losses of up to 30%. Recent research across the South Coast of Western Australia has recorded yield losses in cereals from 400kg - 1800 kg/ha. The yield losses result from two factors; direct feeding, and infection by barley yellow dwarf virus (BYDV). Both aphid species can transmit this virus disease. The proportion of yield lost to each factor is uncertain, and probably varies greatly between seasons and locations.
Management and Control:Several parasites and predators attack the cereal aphids, e.g. lady birds, lacewings, wasps, flies. These biological control agents are only effective when aphid numbers are low and may be encouraged by the use of insecticides that are solely aphicides and 'soft' on beneficial insects. Under optimum conditions where aphids have the ability to rapidly multiply the use of insecticides may be necessary.
Climatic factors such as temperature, rainfall intensity and wind are critical in affecting the rate of development of aphid colonies. However, short-term weather changes are generally not effective in reducing their numbers. Adverse weather with the passage of cold fronts will delay aphid development, but not eliminate them unless it is extreme and prolonged. Dense crop canopies provide protection from such elements.
Aphids can appear suddenly and breed up quickly, so frequent crop inspections are required. They may disappear just as quickly after prolonged heavy rain or as the crop matures.
Aphids are often difficult to find on young plants because they hide in the rolled youngest leaf. An easy way to detect them is to take several tillers and place them in a plastic bag. If aphids are present they will emerge in a few hours and can be seen on the inside of the bag.
The application of insecticides is advisable when aphid infestations reach a spray threshold of 15 - 20 aphids per tiller on 50% of tillers. This is recommended for crops past the tillering stage (Z25). If aphids are found before the Z25 stage, the spray threshold is reduced to 10 aphids per tiller on 20% of the tillers. There are a number of insecticides available for the control of cereal aphids. Usually anti feeding insecticide such as cypermethrin or alpha-cypermethrin are used for early sprays to prevent virus infection and organophosphate insecticides or pirimicarb are used to kill late infestations at the flag leaf stage of the crop.
In areas regularly affected by BYDV, spraying at the early tillering stage of the cereal is advisable for crops with a yield potential greater than 2 t/ha. If aphids are detected before tillering then immediate spraying with synthetic pyrethroids is usually warranted to prevent early viral infection of the crop. Ideally the crop should be sprayed before the aphids arrive.
In areas where aphids and virus are a common problem an initial spray of 125 mL/ha of Fastac or Dominex100 at the 3-5 leaf stage of the cereal followed by a second spray at he same rate 3 weeks later is often used. Areas that have had rains in March/April and volunteer cereals or grasses growing well before planting are more likely to have BYDV problems and early spraying is more likely to be profitable.
The symptoms of BYDV are
Wheat - yellow green stripes near the base of the leaves turning into an overall reddening and leaf tip death with time.
Barley - leaf yellowing and whole plant stunting occasionally with increased tillering and sterile tillers.
Oats - leaf yellowing turning to mauve red with time.
BYDV is carried in the salivary glands of the aphid so it is present wherever the host aphid occurs. (in contrast CMV of lupins is "dirty stylus" type of virus and is transmitted from one infected plant to the next)
Related Species:Similar Species:
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