The fall armyworm in Africa: a new insect pest in corn crops

A team of researchers from the International Centre of Insect Physiology and Ecology (icipe) in Nairobi and the IRD (French National Research Institute for Sustainable Development) examined one of the key factors in corn yield losses, i.e., the threat of pest insects. Pest insects form complex interactions both with the plant and their parasitoids, natural enemies which develop to the detriment of the pests and ultimately reduce the damage caused by infestations. Specialists call this ‘tritrophic interaction’. 

An American invader

According to Paul-André Calatayud, IRD Research Entomologist at the Genome Evolution and Ecological Behaviour Laboratory in Gif-sur-Yvette, “up until 2016, three moth species, the maize stem borer (Busseola fusca), the African pink stem borer (Sesamia calamistis) and the spotted stem borer (Chilo partellus), were the predominant pests in Sub-Saharan Africa, causing crops losses of between 10% and 80%”  In 2017, a new pest, the fall armyworm, or Spodoptera frugiperda, which is indigenous to the American continent, arrived in Africa. How it arrived exactly is not known. “It was first spotted in Ghana and Benin and then spread rapidly throughout Sub-Saharan Africa, before pursuing its route towards Asia”, the researcher explains. “This rapid development can be explained in part by the fact that the female moth is capable of flying more than 500km on the wind.” The explosive cocktail of “new pest” and “rapid invasion” impacted African corn crops severely; within a few months the armyworm had spread throughout Sub-Saharan Africa. 

One of the questions researchers looked into was to determine whether the armyworm would replace one or more existing corn pests, as has been the case with previous invasions. “We were surprised to observe that this had not occurred here. The new species did not replace existing species in the field, but settled in alongside to become an additional pest”, he continues. One of the reasons behind this co-habitation is that the various pests are not systematically in competition. When corn is in its vegetative stage, before flowering, the armyworm mainly attacks the leaves, while pre-existing stem borers migrate to the stalks of the cereal. The stem borers can even change plants and migrate to sorghum, a crop often grown alongside corn and of little appeal to the fall armyworm.urs changent même de plante et peuvent aller sur du sorgho, fréquemment cultivé avec le maïs – alors que cette céréale attire peu la légionnaire d’automne.