Resistance of maize to fall armyworm (Spodoptera frugiperda) in Africa

Resistance of maize to fall armyworm (Spodoptera frugiperda) in Africa

The fall armyworm (FAW), Spodoptera frugiperda (Lepidoptera: Noctuidae), a maize pest native to the Americas, was first reported in West and Central Africa in 2016, before invading nearly all sub-Saharan Africa (SSA) in 2017. This new pest causes an estimated $9.4 billion in annual maize yield losses of maize and severely threatens food security in SSA. Faced with this threat, several means of control have been launched by several national and international agricultural research institutions. In the context of the search for rapid solutions, other than the use of synthetic pesticides, the first work undertaken consisted in using silica, a known inducer of plant resistance to biotic and abiotic stresses. A greenhouse study showed that an increase in the silica content of the plant disturbed the development of the larvae of one of the two African populations of FAW. In parallel, a strategy to control FAW in Africa by using host plant resistance from tropical maize germplasm was developed. In this context, an intensive screening against FAW was conducted by artificial infestation in the greenhouse in Kenya between 2017 and 2018 on about 3000 maize lines available in the germplasm collection of the International Maize and Wheat Improvement Center (CIMMYT). Among these lines, four (CKSBL10008, CML71, CML125, and CML370) were found to be resistant to FAW. However, information on the heredity, mechanisms and chemical basis of resistance of these lines was not known. This thesis showed that: 1) for heredity of resistance, heterosis values indicated an increase in resistance of hybrids over the average of inbred line parents. General (GCA) and specific (SCA) combining ability as well as reciprocal were a highly significant source of variation for the inheritance of resistance. CML71 and CKSBL10008 proved to be the best general combiners based on GCA effects for leaf feeding damage. Non-additive genetic effects were highly source of variation for the inheritance of resistance for FAW larval survival, suggesting that heterosis breeding would be the best strategy for improving that resistance trait in the two FAW-resistant inbred lines CML71 and CKSBL10008. 2) Regarding the mechanisms of resistance, CML71 and CKSBL10008 showed the highest level of resistance by antibiosis on leaves due to the low relative growth rates (RGR) of larvae. These two lines were also found to be less preferred by CLA caterpillars due to a lower proportion of larvae found on leaf portions of these genotypes. The non-preference for feeding and low RGR of larvae on these lines suggest biochemical resistance to FAW. 3) Metabolomic analysis revealed that CML71 and CKSBL10008 possess specific metabolites that are absent or less abundant in susceptible lines. From this study, at least 11 metabolites, whose effects against crop pests and diseases have been reported in the literature, were found to be good candidates to explain the resistance to FAW, such as non-protein amino acids like GABA. This thesis reveals that CML71 and CKSBL10008 are very promising lines for breeding native genetic resistance to FAW in tropical maize in SSA.