Titre de la thèse (en anglais): Mechanisms of resistance in the African maize germplasm to the fall armyworm, Spodoptera frugiperda (J.E. Smith)
Résumé (en anglais): Maize (Zea mays L.) is the third largest crop in the world after rice and wheat. In Africa, it is the most important food crop in terms of area harvested and alone provides more than 30% of the total calories of the human population in sub-Saharan Africa. The fall armyworm, Spodoptera frugiperda (JE Smith) (Lepidoptera: Noctuidae), a pest of maize native to the Americas was first reported in West Africa in 2016, is severely threatening food security in sub-Saharan Africa through the loss of tens of millions of tons of maize production each year according to FAO’s 2018 estimates. In the African context where the majority of maize producers are smallholder farmers with limited access to knowledge and adequate inputs to properly manage this new pest, the use of resistant varieties of the host, obtained either through conventional plant breeding methods or through silica induction (a known inducer of resistance in Grasses against pests), is therefore one of the most effective means of control, compatible with other integrated pest management strategies. The first step is to check whether an increase in the silica content of maize disrupts the development of S. frugiperda larvae. While silica induces a significant increase in stem diameter and height of potted maize plants, it has no influence on the development and mortality of S. frugiperda, ruling out the use of silica in maize resistance to this pest. Some resistant maize varieties have been bred and exist in the Americas against S. frugiperda but none are currently available as they are not adapted to the African continent. The other main objective of this thesis is therefore to develop a strategy to control S. frugiperda in Africa by using resistant varieties derived from African maize germplasm. The first results of the work on breeding and genetic improvement of (sub)tropical maize varieties against S. frugiperda, initiated by the International Maize and Wheat Improvement Center in Kenya (CIMMYT) between 2018 and 2019, indicate that five maize lines out of 1303 genotypes tested in greenhouses under artificial infestation have appreciable levels of resistance to S. frugiperda based on leaf and ear damage. After obtaining hybrids from these lines that are potentially resistant to S. frugiperda, this research is divided into three steps: 1) identify the mechanisms of S. frugiperda resistance in lines and hybrids selected for their resistance, 2) check whether these resistant genotypes are avoided by S. frugiperda females for oviposition, 3) and finally identify the chemical compounds responsible for resistance.