Oat aphids & beneficial invertebrates

Agronomist, Greg Toomey (Landmark), reports noticing a decline in numbers of oat aphids (Rhopalosiphum padi) in several wheat crops around Elmore, in the Northern Country district of Victoria. Over the past 2-3 weeks aphid numbers have decreased from approximately 100 per plant, to less than 20 per plant. In addition to this, Greg also reports noticing a significant increase in numbers of aphid parasitic wasps and aphid ‘mummies’ being seen in crops. It is likely that the decrease in aphid numbers is due to a combination of parasitism, and the suboptimal conditions following the significant rainfall received over the past fortnight.

Aphid parasitoid activity is often first noticed by the presence of aphid ‘mummies’ (buff to bronze coloured, bloated/enlarged aphids). These wasps are small (approximately 1-3 mm long), usually dark in colour and difficult to detect. Females lay their eggs into the bodies of live aphids and the developing larva feeds inside, eventually killing the aphid. The adult wasp then emerges through an exit hole in the body. Click here for images of aphid parasitoids.

Beneficial hoverfly larvae (Family: Syrphidae) have been identified for research agronomist, Tony Single (AgriTech), from a barley crop east of Narrandera in the Riverina district of New South Wales. Tony estimates finding approximately 30-40 larvae per m² in trial plots and surrounding crop. Coinciding with this, Tony says the particular crop had significant numbers of oat aphids present prior to the hoverfly larvae appearing, and these have now all but disappeared. Hoverfly larvae were also found in lower numbers in nearby wheat and pulse crops.

Hoverfly larvae prey on aphids by spearing them with their mouth hooks and sucking out the body contents. They are grub or maggot-like, up to 10 mm long, blind, legless and transparent cream to green or brown in colour. It is important not to confuse hoverfly larvae with larvae of the diamondback moth (Plutella xylostella), or any other lepidopteran pests such as armyworms. Adult hoverflies feed on pollen and honeydew, grow up to 10 mm long and have a flattened body with black and yellow markings. They can hover and fly swiftly, and may be confused with bees or wasps. Click here for images of hoverflies.

Oat aphids have also been reported in numbers ranging from quite low to well above threshold levels by agronomists in Donald, in the Victorian Wimmera, and near Lake Bolac, in the Western district of Victoria. For oat aphids and corn aphids (Rhopalosiphum maidis) (often referred to as ‘cereal aphids’) a commonly publicised threshold is to consider control when there are 10-20 aphids on 50% of tillers. However, it is important to remember that any economic threshold should be considered to be a flexible guideline only, and is likely to vary with factors such as the crop growth stage, weather conditions and crop vigour (e.g. stressed crops are more susceptible to aphid feeding damage).

Oat aphids are significant pests of cereal crops and grasses in southern Australia. They have a light green to olive coloured body which is pear shaped, with a rusty red patch at the end of the abdomen. Oat aphids are generally found on oats and wheat, however they may attack all cereals. They can attack crops at any stage and heavy infestations can result in yield losses.

Click here for images of oat aphids.