In Kenya, the survey of pest insects in corn crops has revealed that a recently-arrived pest, Spodoptera frugiperda, has not replaced existing pests. The various species of pests are co-inhabiting in fields and calling into question the pest control strategies currently in use to preserve the yields of a crop that is essential for local populations.
Corn is one of the most important cereal crops for Sub-Saharan Africa. In Kenya, for instance, corn is grown primarily by small-scale farmers, and represents a vital means of subsistence.
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.
Co-habitation rather than competition
While from an agricultural perspective the sudden emergence of the armyworm was catastrophic, for the scientific world, it has provided a unique opportunity to study how tritrophic interactions around corn were modified. “Thanks to the data on corn stem borers and their natural predators, which our team has been collecting on the ground since the 2000s, we have been able to monitor the changes in these interactions during and after the introduction of a new invasive species to the system”, says Bonoukpoè Mawuko Sokame, post-doctoral entomologist and specialist in crop protection and dynamic system modelling.
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.
“The issue at present is to understand how these new interactions affect the strategies used control pest species. Will local parasitoids naturally manage to control the armyworm? Will this control negatively impact the organic control currently used against stem borers?” he asks. All these aspects need to be explored, in parallel with studies on the ground. “Agronomy is a science of durability. If we are to understand these complex phenomena, which evolve constantly due to the regular arrival of new pests and the impact of climate change, we must be present on the ground in the long term. The food safety of vast populations relies heavily on the implementation of appropriate agricultural strategies”, Paul-André Calatayud concludes.