Farmer's Weekly (South Africa)
Russian wheat aphid: sustainable solutions to fight resistance
The Russian wheat aphid ( Diuraphis noxia) occurs worldwide where wheat is produced at commercial scale. South Africa currently has five biotypes affecting production, one of which was recorded for the first time in 2018. It was identified by Dr Astrid Jankielsohn, an insect ecologist at the Agricultural Research Council’s Small Grains Institute in Bethlehem in the Free State.
The Russian wheat aphid (RWA) is native to south-western Asia and was introduced into many European countries in the 20th century. It was first noted as a pest in the Crimea in 1901, hence the common name.
The aphid, which is up to 2mm long, is light green in colour and has a tiny spike at the rear above its tail, giving the impression of two tails.
The insect feeds on leaves, causing them to turn white and curl up. The main damage, however, is done by the saliva of this pest, which is toxic and stunts growth. Research has also found that aphid infestation affects the flour made from infested wheat plants.
The Food and Agriculture Organization of the United Nations regards RWA as a serious threat to dryland wheat in many developing countries beset by sporadic drought. In Europe, Africa, Asia, and North and South America, RWA has caused crop losses of up to 80%.
CONTROL MEASURES
According to Jankielsohn, much effort is being invested into developing control strategies. The two most widely used of these are chemical control measures and breeding for resistance, but both have drawbacks.
“Chemical control results in environmental damage, especially to insect groups like pollinators, predators and decomposers. We need those insects to be present in our ecosystems,” she explains. Problems are also created by using chemicals with
was first recorded in South Africa in 1978, and researchers have become concerned about increasing resistance among different biotypes, as well as the high virulence of new biotypes.
THE EASTERN FREE STATE WAS A DIFFERENT MATTER, WITH INFESTATIONS OF FOUR BIOTYPES
Until 2018, four RWA biotypes were known in South Africa.
“During 2018, there were reports of wheat cultivars, resistant to all four biotypes, which showed RWA damage symptoms in the field at Reitz and Danielsrus,” says Jankielsohn. “Screening of RWA samples collected in these areas confirmed that a new biotype, with additional virulence to the Dnx resistant gene ( see panel) in wheat, was occurring in these areas.” By then, the new biotype, RWASA5, had been recorded at eight different sites in the eastern Free State.
“It’s the most virulent
RWA biotype recorded to date,” adds Jankielsohn.
The pest can exist in environments in which it has established itself, as well as adapt to changing environments by developing new biotypes.
In recent years, Jankielsohn has conducted surveys in several of South Africa’s major wheat production regions, and has found that RWASA1 is the most widely distributed biotype. It occurs mainly in the Western Cape, however, and was the only biotype recorded in that region in 2018.
Although all known biotypes were found in the Free State in 2018, RWA was barely present in the province’s western production regions.
“For example, there was only one land in the Wesselsbron/ Theunissen area that was infested by RWASA1,” notes Jankielsohn.
The eastern Free State was a different matter, with infestations of all four biotypes, as well as the first recording of the new biotype, mostly in the Reitz area, but also in Clocolan.
Interestingly, the virulent new RWASA5 topped the
biotype complex in the region, with a share of 15,4%, followed by RWASA3 (7,7%), RWASA4 (5,8%), RWASA2 (3,85%) and RWASA1 (1,92%).
By 2019, Jankielsohn still noted low incidences of infestations in the western Free State areas, but saw that RWASA5 populations had increased and spread throughout the entire eastern Free State region.
According to her, different management practices are being employed in different areas, particularly those that differ in terms of seasonal rainfall. Farmers in the Western
Cape, for example, use solely chemical control strategies.
“In the Western Cape, they never rely on resistant wheat cultivars. Farmers grow susceptible plants and rely on chemicals to overcome RWA,” she explains. This is why only RWASA1 infestations have been recorded in that region. However, there are indications of growing insecticide resistance there.
In contrast, farmers in the Free State tend to favour cultivar resistance management. This, however, has resulted in the creation of the four additional biotypes, all deemed more