Cereal aphids and BYDV

Excellent late summer and early autumn rainfall has promoted the growth of grasses and volunteer cereals in northern Victoria and NSW; these plants are a perfect ‘green bridge’ for both BYDV and the aphid species that vector this virus.

Where have they been reported?

Cereal aphids have been reported from several early sown barley crops northeast of Ouyen in the Victorian Mallee. The populations have been surprisingly high for this time of year, with densities of up to 30 aphids per seedling. In numerous wheat and oat crops east of Mudgee in the Central Tablelands district of NSW, cereal aphids, particularly the oat aphid, have been found in high numbers, up to 15 aphids per tiller. Some of these crops had been treated with imidacloprid seed. The aphids were mostly at the base of the plants. The crops are progressing well. Importantly however, barley yellow dwarf virus (BYDV) symptoms are appearing in some early sown oat crops, planted from late February. (Thanks for these reports*).

Aphids and BYDV in 2013

The previous season can provide some valuable insight into the emerging virus-risk conditions this year. Victorian DEPI Crop Pathologist Frank Henry reports that both aphids and BYDV were prevalent in the Wimmera and Mallee districts of Victoria during spring of 2013. Aphids, primarily the oat aphid (Rhopalosiphum padi), were measured around Horsham in low numbers throughout winter, with populations peaking in September. Symptoms of BYDV were also widespread in the Wimmera and Mallee. Samples originating from Horsham, Mildura, Ouyen and Wodonga were positive for BYDV. BYDV was also observed in cereal crops in the Central Tablelands district of NSW in 2013. These observations point to a heightened risk of BYDV incidence in 2014.

Cereal aphid habits and behaviour

Winged aphids move into crops in autumn from alternative host plants, usually weeds along roadsides and verges where they over-summer. Aphid numbers build-up on the edges of paddocks and tend to form dense colonies on the growing points of a single plant before moving onto surrounding plants. Hence, aphid infestations are usually patchy across the paddock. All aphids are sap-suckers and when in large numbers direct feeding damage that removes the sap causes yellowing and wilting of plants.

More importantly at this time of year, many aphid species transmit plant viral diseases such as BYDV, which can be detrimental and yield limiting, especially in young plants. BYDV is transmitted by aphids from infected grasses, such as perennial ryegrass, and can have a greater impact on cereal crops than direct feeding by aphids. Crops are most susceptible for the first 8 to 10 weeks after emergence; initial infection occurs in young crops (4-8 weeks) when aphids first arrive. Even low densities of aphids can transmit BYDV, with earlier infection resulting in higher levels of damage.

Control options

Using chemical control to keep a crop completely free of aphids is difficult. In aphid and BYDV prone areas, the use of imidacloprid-based seed dressings is advisable; these can provide protection for several weeks after emergence, when the crop is most vulnerable. Research in WA and SA demonstrated that the use of synthetic pyrethroids foliar sprays at 3 and 7 weeks post emergence is effective in minimising BYDV infection and spread within a crop. The pyrethroids act as deter feeding by aphids. Click here for further information about BYDV management.

Our advice

For any remaining late plantings, selecting cereal varieties with greater tolerance to BYDV is an important way to minimise losses in BYDV-prone regions. The Victorian Mallee has only experienced sporadic outbreaks in the past and is generally not a high-risk BYDV region; however in 2013, BYDV was prevalent so there is a chance of carry-over into this season. Growers should remain on the lookout for aphids and symptoms of viruses in their crops, and pay particular attention to areas of paddocks that had a significant green bridge of plant material over summer.

If spraying is necessary, the insecticide to be applied should be chosen carefully. Many chemicals, particularly synthetic pyrethroids, have a detrimental effect on beneficial insects and thus increase the likelihood of subsequent pest outbreaks. Many aphid predators are generalists and feed on other pests, such as native budworm and diamond-back moth. Killing natural enemies early in the season may have implications for the control of these pests later in the season. Once the initial threat of BYDV has passed, the use of “soft” chemicals, such as pirimicarb, will preserve many beneficial insects. Click here for further details on the management of aphids.

Description

The most common aphid vectors of BYDV are the oat aphid (Rhopalosiphum padi) and the corn aphid (Rhopalosiphum maidis). The English grain aphid (Sitobion miscanthi) and the rose-grain aphid (Metopolophium dirhodum) are other known vectors but these are less common species. Oat aphids are similar in appearance to corn aphids, 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. Oat aphids are similar in colour but have a pear shaped body with a rusty red patch at the end of the abdomen. 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.

 

* Sources of field reports of cereal aphids and BYDV

Mohammad Aftab – Pathologist, Vic DEPI (Victorian Wimmera)

Frank Henry – Pathologist, Vic DEPI (Victorian Wimmera)

Andrew McMahen – Agronomist, Landmark (Victorian Mallee)

Phil Stoddart – Consultant (NSW Central Tablelands)

Piotr Trebicki – Entomologist, Vic DEPI (Victorian Wimmera)