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More on oat aphids

Oat aphids (Rhopalosiphum padi) continue to be major pests of cereals across most regions in Victoria and New South Wales. Aphids are known to affect cereals directly through feeding damage and indirectly when they introduce and spread barley yellow dwarf virus (BYDV).  The spread of BYDV by aphids is only damaging if aphids transmit the virus early in the first 8-10 weeks following emergence.

Direct feeding damage from aphids occurs when colonies of aphids develop on stems, leaves and heads, usually in the tillering and later stages of crop growth through to head filling. In most years, cereal aphids do not require chemical control in spring; however it is important to accurately monitor aphid numbers within crops. Low aphid numbers will not cause yield loss, although feeding damage to cereals caused by very large numbers of aphids can occur from late tillering onwards.

There have been several reports of “higher than usual numbers” of aphids across most regions, which is likely – at least in part – to be the result of mild autumn conditions which favoured the build-up of aphid numbers at crop establishment. Spraying for cereal aphid feeding damage is considered worthwhile if 50% of tillers have >15 aphids. Trials in Western Australia have shown that at this density (when crops are expected to yield 3 t/ha or more), feeding damage may cause reductions in yields by up to 10% and also reduce seed size, leading to a possible downgrading of the grain quality.

Western Australian trial results showed that the yield benefits from controlling aphids occurred where sprays were applied early (e.g. at stem elongation to flag leaf emergence) and just as numbers reached threshold levels. Unfortunately, late application of insecticides in situations where aphids have been at damaging levels for some time, or when applied to crops with advanced growth stages, will have less impact. Late aphid infestations on cereals will most likely have a larger affect on grain quality rather than yield.

In previous years, where aphid numbers have reached the threshold or been slightly under, populations have typically declined naturally within a short period of time. In these situations, it's hard to demonstrate a return on the value of the spray applied. However, if crops are under stress, usually from lack of moisture, aphids are likely to have a larger effect. Keep in mind that rainfall will typically not cause appreciable reductions in cereal aphid numbers in spring.

Waiting until threshold levels are reached before spraying is the recommended practice. In many years, aphid numbers will never build up to damaging feeding levels. Routine and unnecessary application of insecticides will also encourage the build up of resistance in aphids and other non-target insects.

Many insecticides (e.g. organophosphates and synthetic pyrethroids) are disruptive to natural enemies; which should be encouraged as a natural way of suppressing aphid numbers. Natural enemies are a reliable form of control during the warmer days of spring and have been reported to be building-up in many crops at present. If chemical control is necessary, consider ‘softer’ chemicals (such as pirimicarb) which are aphid-specific and less harmful to other insects. For images of natural enemies (including hoverflies, lacewings and ladybirds) click here.

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