A study of the recently arrived fall armyworm in Africa suggests that the pest has formed one single interconnected population on the continent. An examination of highly variable parts of their genomes suggested the fall armyworms were using long-distance flight and prevailing winds to frequently move throughout Africa, resulting in population mixing (Sci. Rep., doi: 10.1038/s41598-021-00298-3).
‘The crop most impacted [in Africa] is maize,’ notes Roger Day, an entomologist with the inter-governmental organisation CABI in Nairobi, Kenya. Maize makes up almost half of the calories and protein intake in eastern and southern Africa and one-fifth in West Africa.
If insect resistance emerges, it is likely to spread rapidly across the African continent. ‘This would mean that farmers would have to use different insecticides to treat the pest,’ says Amy Withers, of Lancaster University and Rothamsted Research in the UK. ‘Since we know an adult can fly up to 300 miles and across borders, it will be important to try to detect the pest early and act as soon as possible.’
The fall armyworm (Spodoptera frugiperda) was first detected in Nigeria in 2016. It spread rapidly through Africa and on to Asia in 2018, arriving in Australia in 2020, and inflicted billions of dollars of damage through lost harvests. It is an indigenous moth throughout the Americas and the caterpillar feeds on crops including maize, rice, sugarcane, potato, cotton and millet.
‘It can burrow into crops and go undetected until the crop has already been destroyed,’ says Withers, who looked at populations from Ghana, Kenya, Malawi, Rwanda, Sudan and Zambia. Numerous synthetic pesticides can kill the feeding caterpillar, but resistance to organo-phosphate and pyrethroid insecticides has been detected in fall armyworms in China.
A 2017 CABI report that Day co-wrote recommended a combination of control methods, rather than insecticides only. ‘The first part of management is to avoid populations building up in the first place,’ advises Day. ‘Avoiding the use of pesticides unless absolutely necessary – as indicated by field scouting – allows the natural enemies to exercise some control.’
The species likely can survive in warm, tropical parts of Africa throughout the year, complicating control. In the Americas, it is naturally migratory and killed by frost. There does not seem to have been repeated incursions from the Americas to Africa, says Withers: ‘We didn’t find any evidence in our data that there had been further introductions from North or South America since the first introduction in 2016.’
Day says management practices such as companion cropping, inter-cropping and maintaining plant diversity can help, as well as keeping crops healthy. ‘Maize varieties that are partially resistant to the pest have been found, but are not widely available yet,’ he notes. ‘Genetically modified maize containing Bt genes is effective, but only permitted in a few countries in Africa.’
Meanwhile, a separate paper in review (Sci. Rep., doi: 10.21203/rs.3.rs-1025368/v1) concludes that there is limited movement of the pest around the continent. ‘So it looks as though the picture is complicated,’ comments Day.