An insect-resistant future for cowpeas in Nigeria and beyond
Cowpeas (Vigna unguiculata) are a vital protein-rich crop in West Africa and play a critical role in safeguarding food security there. However, the largest producer, Nigeria, still needs to import around 500,000 tonnes per year to meet internal demand. This is because insects can reduce its cowpea production by up to 80 per cent.
The lepidopteran Maruca vitrata called the pod-borer is the major pre-harvest pest. The major post-harvest or storage pest is the coleopteran Callosobruchus maculatus known as the cowpea weevil.
Nigeria and neighbouring countries could become self-sufficient in cowpeas, and possibly even develop export markets if these two problem pests could be controlled. With the help of the African Agricultural Technology Foundation (AATF), Donald Danforth Plant Science Center, and the Institute of Agriculture Research our team at CSIRO has developed an in-built genetic solution for the pod-borer. It has now been commercially released, and we are on the way to solving the weevil problem.
Fixing problem one: the pod-borer
In December 2019 Nigeria approved the world’s first genetically modified (GM) cowpea for release for commercial use. The transgenic variety, named SAMPEA 20-T, carries a bacterial insecticidal gene, cry1Ab, from Bacillus thuringiensis (Bt). This was developed by an international team, led by the African Agriculture Technology Council (AATF) in collaboration with CSIRO, the Institute for Agricultural Research at Ahmadu Bello University, and the Institute for International Crop Improvement at the Donald Danforth Plant Science Center.
Developing pod-borer resistant (PBR) cowpeas using classical plant breeding was not possible due to the lack of natural resistance. Instead, farmers were using chemical insecticides to control the pest. This practice has serious health risks because many farmers do not understand the product formulation and instructions. There is also a lack of protective clothing for the spraying of insecticides.
Additionally, widespread usage of insecticides brings other unintended consequences. Beneficial insects that predate on pests are common off-target victims, while the odds of chemical resistance increase the more they are used.
Because of this, the team, supported by the US Agency for International Development, has developed a Bt-based PBR cowpea that provides in-built and full protection against Maruca that allows a cheap, safe and accessible solution to the problem. Apart from the health benefits, the sowing of PBR cowpea is expected to bring in over USD$336 million over the next 25 years. That would be a return of USD$26 for each dollar invested in the project.
New improved versions of the PBR cowpea carrying more than one gene are needed in order to avoid the development of resistance in the insect populations. By stacking two or more Bt genes we will be able to produce a cowpea with long-lasting resistance.
Our team is now working on the development of a second generation of PBR cowpea carrying the cry1Ab and the cry2Ab genes. These genes were provided by Bayer Crop Science on a humanitarian basis. Smallholder farmers can access the new PBR varieties royalty free.
The development of the new double Bt cowpea is well underway. Field trials have been successfully completed in Nigeria, Ghana and Burkina Faso.
A promising weapon against problem two: the cowpea weevil
While the PBR cowpea has been produced and is approved for deployment, there is no biotech solution yet for the stored grain pest – a bruchid called the cowpea weevil.
Bruchid infestation very often starts in the field when grain moisture content is high, and it continues in storage. The larvae of this weevil burrow into the seeds to feed inside. When the adult female emerges, about four weeks later, she can lay up to 100 eggs (seen as white specks in the image above) spreading the infestation rapidly.
Damage to seeds can affect the farmer, the trader, the retailer, the processor and the consumer. The economic impact of this seed predation is very large because it reduces the selling price, reduces the germination rate. It also forces farmers to sell the grain immediately after harvest, when prices are low, to avoid losses in storage.
The storage pest issue can be a major constraint to cowpea production, trading and processing. A common method to control this pest is the use of another set of chemical insecticides. However, as explained before, insecticides can be a serious health risk. When used to kill bruchids it is the main cause of high pesticide residues in cowpea grains on the market.
Given the scale of the problem, African breeders are now advocating for a biological solution that is cheap, safe and easy to deploy allowing in-built protection against the cowpea weevil in the same way that SAMPEA 20-T can help control pod-borers.
To meet this demand, the team has produced cowpeas carrying a gene encoding a bean α-amylase inhibitor (αAI). Important preliminary studies have shown these cowpeas to be resistant to the cowpea weevil.
This product is still at the proof-of-concept stage and needs to be repeated at scale. It must undergo a full molecular characterisation, biosafety studies, and efficacy tests in the granary. While promising, it still has a long journey ahead of it before it will be commercially available. PBR cowpea took approximately 20 years from concept to farm release.
However, the successful release of PBR cowpea will, we hope, expedite regulatory approvals, and also illustrate that science can overcome real-world challenges.
Eventual deployment of weevil resistant cowpea would bring large economic benefits to farmers, traders, processors, and consumers. It will also have a health benefit because for the first time since the introduction of DDT in sub Saharan Africa almost 70 years ago, people would have the prospect of consuming cowpea with low insecticide residues.
A future for agricultural biotechnology abroad and in Australia
The small list of biotechnologically improved food crops now includes PBR cowpea. Examples of other available crops include virus resistant papaya and beans, insect resistant eggplant, Golden Rice, and drought-resistant wheat. PBR cowpea’s deployment constitutes a good example of Australian science contributing to the United Nations Sustainable Development Goals, which the international community has committed to address by 2030.
SAMPEA 20-T provides full protection against pod-borers and does not have any yield penalty. It is expected to be adopted also in Ghana, Burkina Faso and possibly Niger. Certified seed companies have been engaged to guarantee the purity of the seeds. They expect to reach a minimum of 25 per cent uptake by farmers in Nigeria.
The release of a GM crop in Africa is a significant achievement as many countries in the continent are still reluctant to adopt biotechnological approaches. While controversy about GM crops still exists, it is important to recognise that a success such as this is an advancement for the adoption of the technology.
In Australia, crops that could potentially benefit from this technology include rust-resistant wheat, the drought-tolerant HB4 wheat, high fibre wheat, iron and zinc-fortified wheat, and fusarium-resistant bananas. Internationally, some examples include blight resistant potato and antioxidant enriched tomatoes.
PBR cowpea’s release shows the world how GM solutions can play a crucial role in our future.