CROPS Russian Wheat Aphid New Russian wheat aphid biotype highlights growing resistance concerns Russian wheat aphid is showing increased cultivar and pesticide resistance, according to entomologist Dr Astrid Jankielsohn of the Agricultural Research Council. She has identified a new biotype, bringing to five the number seen in South Africa. Sabrina Dean reports. he Russian wheat aphid also found that aphid infestation ABOVE: the same active ingredients The Russian wheat (Diuraphis noxia) occurs affects the flour made from aphid produces a over long periods. worldwide where wheat infested wheat plants. variety of effects in “This means a very high T the host plant and is produced at commercial scale. The Food and Agriculture the subsequent likelihood of resistance South Africa currently has five Organization of the United product for which that renders this control biotypes affecting production, Nations regards RWA as a the plant is used. method useless.” An aphid infestation one of which was recorded serious threat to dryland wheat leads to reduced Only a limited selection of for the first time in 2018. It in many developing countries growth due to active ingredients are registered water imbalance was identified by Dr Astrid beset by sporadic drought. In and a reduction for RWA control in South Africa, Jankielsohn, an insect ecologist Europe, Africa, Asia, and North in biomass of the says Jankielsohn. These include at the Agricultural Research and South America, RWA has entire plant. acetamiprid, chlorpyrifos, PHOTOS: DR ASTRID Council’s Small Grains Institute caused crop losses of up to 80%. JANKIELSOHN demeton-S-methyl, dimethoate, in Bethlehem in the Free State. imidacloprid, parathion, The Russian wheat aphid (RWA) CONTROL MEASURES prothiofos and thiamethoxam. is native to south-western Asia According to Jankielsohn, Of these, chlorpyrifos is the and was introduced into many much effort is being invested most widely used in South European countries in the into developing control Africa and elsewhere. 20th century. It was first noted strategies. The two most “It’s systemic and provides as a pest in the Crimea in 1901, widely used of these are fast knock-down of aphids, and hence the common name. chemical control measures is the only active ingredient The aphid, which is up to and breeding for resistance, that can effectively control 2mm long, is light green in but both have drawbacks. RWA after the aphids enter colour and has a tiny spike at “Chemical control results rolled-up wheat leaves in later the rear above its tail, giving in environmental damage, colonisation stages,” she says. the impression of two tails. especially to insect groups Unfortunately, while The insect feeds on leaves, like pollinators, predators and chlorpyrifos is effective on a wide causing them to turn white decomposers. We need those range of insect pest species and is and curl up. The main damage, insects to be present in our used by farmers as a preventative however, is done by the saliva ecosystems,” she explains. application for grain aphids, it of this pest, which is toxic and Problems are also created also kills beneficial insects, such stunts growth. Research has by using chemicals with as pollinators and predators. 36 farmer’s weekly 1 MAY 2020 FAST FACTS • The Russian wheat aphid is a well- established invasive species in South Africa’s wheat production areas. Five biotypes have been recorded here. • Biotype 1 is the only biotype present in the Western Cape. • Dr Astrid Jankielsohn has noted resistance issues for both chemical and genetic control strategies. “The wide and continuous ABOVE LEFT: was first recorded in South “It’s the most virulent ‘The wide and use of one active ingredient continuous use Africa in 1978, and researchers RWA biotype recorded to has the potential of causing of one active have become concerned about date,” adds Jankielsohn. ingredient has resistance in the target insect the potential of increasing resistance among The pest can exist in and also weakening the predator causing resistance different biotypes, as well as the environments in which it has complex,” explains Jankielsohn. being developed in high virulence of new biotypes. established itself, as well as the target insect,’ “When resistance develops in says Dr Astrid adapt to changing environments the target insect, there’s nothing Jankielsohn. by developing new biotypes. to keep this insect under the BAREND WENTZEL THE EASTERN In recent years, Jankielsohn damage threshold in the absence ABOVE RIGHT: FREE STATE WAS has conducted surveys in of the predator complex.” Biological control several of South Africa’s major is key in the fight A DIFFERENT She advocates continual against Russian MATTER, WITH wheat production regions, monitoring of lands and only wheat aphid and has found that RWASA1 infestation. using chemicals when absolutely INFESTATIONS OF is the most widely distributed necessary to protect yield. FOUR BIOTYPES biotype. It occurs mainly in In addition, practices such the Western Cape, however, as crop rotation and inter- and and was the only biotype cover-cropping will limit the Until 2018, four RWA biotypes recorded in that region in 2018. availability of RWA host plants were known in South Africa. Although all known biotypes and prevent large populations “During 2018, there were were found in the Free State from surviving from one reports of wheat cultivars, in 2018, RWA was barely season to the next, she says. resistant to all four biotypes, present in the province’s which showed RWA damage western production regions. BREEDING FOR RESISTANCE symptoms in the field at Reitz and “For example, there was only Turning to the second control Danielsrus,” says Jankielsohn. one land in the Wesselsbron/ measure, breeding for resistance, “Screening of RWA samples Theunissen area that was infested Jankielsohn says that its collected in these areas confirmed by RWASA1,” notes Jankielsohn. drawback “is that insect pest that a new biotype, with The eastern Free State was a populations eventually adapt additional virulence to the Dnx different matter, with infestations to overcome resistance and resistant gene (see panel) in wheat, of all four biotypes, as well as render the strategy useless”. was occurring in these areas.” the first recording of the new She has been tracking RWA By then, the new biotype, biotype, mostly in the Reitz distribution, as well as RWASA5, had been recorded area, but also in Clocolan. investigating resistance problems, at eight different sites in Interestingly, the virulent for several years. The pest the eastern Free State. new RWASA5 topped the 1 MAY 2020 farmer’s weekly 37 CROPS Russian Wheat Aphid virulent than RWASA1. “The conclusion is that we are creating resistance,” says Jankielsohn. TACKLING THE PROBLEM Jankielsohn is convinced that the solution is to focus on sustainable practices that will address current challenges, as well as facilitate future control. She suggests, for example, that farmers change cropping practices by moving away from monoculture and following intercropping programmes. She also advocates decreasing host plants for RWA, as well as creating diversity within crops. “Don’t plant the same cultivar with the same genetic background in the same area every year, for example,” she says. She also calls on approaches that limit insecticide resistance. “Stop preventative spraying. Rather scout and only spray when necessary.” biotype complex in the Cape, for example, use solely ABOVE: The Russian wheat ‘AS THE NUMBER region, with a share of 15,4%, chemical control strategies. aphid is pale followed by RWASA3 (7,7%), “In the Western Cape, they green and up to OF BIOTYPES RWASA4 (5,8%), RWASA2 never rely on resistant wheat 2mm in length. HAVE INCREASED, (3,85%) and RWASA1 (1,92%). cultivars. Farmers grow SO RESISTANCE By 2019, Jankielsohn still noted susceptible plants and rely on low incidences of infestations chemicals to overcome RWA,” TO THE DISEASE in the western Free State she explains. This is why only HAS BEEN LOST’ areas, but saw that RWASA5 RWASA1 infestations have been populations had increased and recorded in that region. However, spread throughout the entire there are indications of growing Jankielsohn also advises eastern Free State region. insecticide resistance there. farmers to alternate pesticide According to her, different In contrast, farmers in the Free products with different active management practices are State tend to favour cultivar ingredients so as to prevent being employed in different resistance management. This, creating populations resistant areas, particularly those that however, has resulted in the to specific active ingredients. differ in terms of seasonal creation of the four additional Another solution Jankielsohn rainfall. Farmers in the Western biotypes, all deemed more champions is biological control. “One could increase suitable habitats for other insects Resistant genes such as predators. Sufficient numbers of predator insects According to Dr Astrid Jankielsohn, there are 11 genes could eventually maintain RWA associated with resistance to Russian wheat aphid (RWA), threshold populations,” she says. namely Dn1 to Dn9, Dnx and Dny. For a time after RWASA1 • Based on a presentation given was identified in South Africa in 1978, many of the Dn genes by Dr Astrid Jankielsohn at the could be used in breeding cultivars resistant to RWA. annual Combined Congress of “Initially, there were plenty of cultivars with resistance to RWASA1. crop production, soil, horticulture As the biotypes increased, resistance was lost, so that nowadays and weed sciences in January. there are only limited Dn genes able to overcome RWA,” she says. • Email Dr Astrid Jankielsohn at [email protected]. ▪FW 38 farmer’s weekly 1 MAY 2020.