Imminent danger of RWASA biotype-5 to wheat production in South Africa and neighbouring countries

A virulent Russian wheat aphid in South Africa, biotype-5 (RWASA5 or Diuraphis noxia) is a crop pest that causes yield losses to wheat and barley. It is not only a threat to farmers, but also poses a potential danger to South Africa’s food security and the economy.

The use of resistant wheat cultivars in the eastern Free State increased the evolutionary selection pressures of RWA, producing five RWA with distinct virulence. The discovery of the new highly virulent RWASA5 in the eastern Free State in 2018 suggested it will pose a great risk to the country’s wheat and barley industry as there are no resistant wheat cultivars to manage it. This concerning development prompted experts from the University of the Free State (UFS) to host a multidisciplinary research meeting on the 17 of July 2023, which was attended by the researchers from the University of Johannesburg (UJ), Stellenbosch University (SU), and Agricultural Research Council-Small Grain (ARC-SG) [the whole group is referred to as RWA-group].

Dr Mpho Mafa, lecturer in the Department of Plant Sciences, says the purpose of the meeting was to review and evaluate the potential food security threat associated with RWA, a pest that causes yield losses to wheat and barley.

According to the RWA-group, RWA is present in all countries producing wheat. However, the effects of RWA concerning wheat infestation are well studied and documented in the US and South Africa. The first record of RWA as an invasive pest in South Africa dates back to 1978 when massive yield losses occurred (between 60% and 90%).

Wheat yield loss threatens the economy of the country, farmers’ profits, and farm workers’ job security. But most critically, it threatens food security. As a result, the Russian Wheat Aphid Task Team was formed at the first meeting held at the UFS 40 years ago. This team of scientists studied and developed initial methods to manage the RWA pest and mitigate its effect on wheat, among other tasks.

According to the newly formed RWA-group, an enormous effort by research scientists, government research institutes, and private companies resulted in clear strategies to manage RWA pests. This included the development of integrated pest management (IPM) (such as resistant wheat cultivars, biological control and insecticide application). The resistant wheat cultivars management strategy was effective in suppressing RWA in the wheat-producing regions of the Free State. However, over time the RWA gained its virulence against the resistant wheat cultivars.

Dr Astrid Jankielsohn from ARC-SG, reported that based on her current surveillance data of the RWA pest in the Free State, Northern Cape, and Western Cape wheat production regions, a concerning picture emerged. From her 2022 surveys, the sharp decline in occurrence of less virulent RWA-biotypes (RWASA1 to RWASA4) and a significant increase in the more virulent RWASA5 are eminent.

According to Dr Jankielsohn, since being first recorded in the 2018, RWASA5 has gradually increased and expanded throughout the eastern Free State. There are currently no wheat cultivars on the market in South Africa carrying effective resistant genes to mitigate the effects of RWASA5 on plant health and yield. This poses a severe threat to wheat yield. In simple terms, this means the farmers and research scientists must gear up and work together to prevent a possible RWA infestation epidemic, which will have severe consequences on food security.

The research scientists from the UFS, UJ and SU also pointed out the potential danger of RWASA5 to South African wheat production. Dr Mafa and his colleague, Dr Lintle Mohase, senior lecturer in UFS Department of Plant Sciences, shared the significance of understanding the plant-pest interaction and the use of plant physiology and biochemistry approaches in combating/mitigating the effects of RWA on plant health.

The UFS team, which includes three postgraduate candidates, is currently attempting to elucidate how cell wall reinforces, sugar metabolism and plant defence signalling phytohormones deter RWA feeding and improve plant health and yield. The SU and UJ teams demonstrated the power of molecular biology, while Prof Anna-Maria Botha-Oberholster, a world-leading researcher in this field from SU, demonstrated how the RWA evolution influences their virulence over time. Her colleague, Dr Francois Burger, shared interesting findings showing how we can use bioinformatics to distinguish between the genomes of the South African and US populations or distinguish between genomic chances of the same population over time.

In addition, Dr Vic Nicolis (SU) showed that identifying the aphid effectors that lead to virulence development can assist in neutralising their effects on the wheat plant. Prof Eduard Venter from UJ confirmed that identifying the effector proteins from RWA saliva could lead to a deeper understanding of the plant-pest interaction, as well as formulating RWA-specific pesticides. Ms Nokulunga Mzimela, an employee of ARC and prospective postgraduate student at UFS, will investigate the efficacy of the RWA’s newly developed pesticides in collaboration with UFS and SU.

RWA-group concluded that as the environmental conditions become favourable for RWA infestation in wheat producing regions in South Africa, farmers can expect serious yield losses.

The group recommended an interdisciplinary approach to gear up for potential RWA-outbreaks should we observe them. More alarming is the observation of a potential decrease in insecticide sensitivity in the Western Cape and areas in the western Free State and Northern Cape. Wheat producers should consider alternative measures to aphid control than indiscriminate chemical management.