Wheat producers in South Africa are being warned of a Russian wheat aphid outbreak in crops in two major wheat producing regions in the country.
Although experts stress that the outbreak is not yet a crisis, farmers in the Free State and Western Cape are urged to be vigilant and conduct necessary inspections.
The potentially damaging return of the Russian wheat aphid (Diuraphis noxia) was flagged by aphid and insect ecology experts from the Agricultural Research Council (ARC). They warn that the eastern Free State and Western Cape are most vulnerable to the outbreak at this stage.
The aphid is a small, green insect of about 2 mm long that feeds on wheat, barley and grasses. The resurgence of the pest is believed to be as a result of wheat production volumes rising in these parts.
Outbreak caused by production increase and climate conditions
In the recent years, farmers, specifically in the Free State, have been planting less wheat, switching to other crops yielding higher profits. This was bad news for the Russian wheat aphids.
However, farmers are now returning to planting wheat. Furthermore, research confirms that the current spring drought season, in which Russian wheat aphid outbreaks usually occur, has the potential to aggravate the outbreak even further.
Dr Vicki Tolmay from the ARC says that while the pest holds no major threat at this point, warnings should not be underestimated.
The pest’s low-threat status is largely due to the wheat crop nearing the end of the season and the aphids not surviving in irrigated, wet areas.
Dr Tolmay explains that the eastern Free State has been flagged due to the likelihood of a resistance breaking biotype occurring on cultivars in the region. A biotype is a group of organisms that is either the same or nearly the same.
She says, “Despite (wheat) cultivars having genetic resistance to the aphid, a biotype the cultivar is not resistant to or a new biotype could occur and damage the crop.”
Irregular numbers detected in Western Cape
Major concern has been raised for the Western Cape, where high numbers of Russian wheat aphids are being detected. Outbreaks of the aphid have never been a cause of concern for the province.
Although not in vast numbers, Dr Tolmay warns that the distribution of the aphid is certainly widening.
“We are seeing a wider distribution of Russian wheat aphid in this area and although populations are small, we do not want them to pop to pest status. We do not know why the aphid is more widespread this season, it may be due to climate change or resistance to insecticides,” Dr Tolmay explains.
While there is no proof to back her theory, they hope to register a project to test whether the aphids are resistant to insecticides.
Aphid outbreaks are controllable
Experts say farmers can keep the number of aphids in their fields down. They are calling on farmers to inspect their fields and make sure that they do not have damaging levels of RWA building up.
Besides planting resistant wheat cultivars, farmers have the option of chemical control.
However, Dr Astrid Jankielsohn, a specialist in the species at the ARC, warns that this is risky. Russian wheat aphids are able to build up resistance against the active ingredients in the chemicals used against them.
“The best way to avoid this is to only spray chemicals when it is absolutely necessary and not preventatively,” she says.
She advises farmers to monitor their fields for the aphids regularly to determine if there are populations present and if these populations are increasing and reaching damaging levels that can influence the yield.
Dr Jankielsohn adds that in order to determine the specific biotype of the aphids in a field, the insects must be screened at the ARC-Small Grain Institute. Producers who wish to know which biotype is present on their farm can contact Dr Jankielsohn at jankielsohna@arc.agric.za.
