Aphids and BYDV

District agronomist, Phil Bowden (NSW DPI), reports finding oat aphids (Rhopalosiphum padi) and corn aphids (Rhopalosiphum maidis) on several grass weeds around Cootamundra, in the South West slopes district of New South Wales. Although they have not yet been found on any crops, this should serve as a reminder to be on the lookout for aphids as they may move into establishing crops from surrounding weeds and grasses. Aphids that arrive in crops in autumn and persist in low numbers over winter can lead to large, damaging populations that peak in late winter and early spring. Importantly, aphids can also transmit several important plant viruses.

Both oat aphids and corn aphids are vectors of barley yellow dwarf virus (BYDV), one of the most important diseases of cereals worldwide. BYDV can cause significant losses in cereal crops, particularly when transmission occurs to young plants. The risk of BYDV transmission increases when summer rainfall results in the growth of weeds and grasses that act as alternative host plants for aphids. Effectively controlling these weeds is a good way to prevent the build up of aphid numbers as it removes the ‘green-bridge’ between cropping seasons. If chemical control of aphids is required in establishing crops, selective insecticides (such as pirimicarb) are available, which are aphid specific and less harmful to other invertebrates. In some cases a border spray can be effective in preventing aphid numbers building-up in crops.

Insecticide seed dressings are another option that provides protection to young seedlings from attack by aphids, as well as several other important establishment pests (eg. earth mites). Key Account Manager, Ken Blowers (Bayer CropScience), says it is important to ensure seed has been professionally treated so the maximum benefits are gained. Seed treatment equipment needs to be calibrated correctly to achieve proper grain coverage across each and every seed. Studies in 2008 found that some seed treatments ranged between 50% and 120% of the label rates across a single batch of seed. This inconsistency will result in variable performance in the field. Ken says this is important with seed treatment insecticides (and fungicides) where specific amounts of product are required on each and every seed.

Corn aphids and oat aphids (sometimes called ‘cereal aphids’) are similar in appearance but can be distinguished in the field relatively easily. Corn aphids have an oblong shaped, light green to olive coloured body with two dark areas on the abdomen near the base of the cornicles. They have antennae, which extend about one-third the body length. Oat aphids are similar in colour but have a pear shaped body with a rusty red patch at the end of the abdomen, and have antennae about half their body length. Corn aphids tend to occur mostly on barley, whereas oat aphids are generally found on oats and wheat; but both species may attack all cereals. Both corn aphids and oat aphids can attack crops at any stage and heavy infestations can result in yield losses. Click here for more information on these aphids and other species which also occur in cereal crops.

Click here for images of corn aphids and here for images of oat aphids.