Why Sesamia nonagrioides (Lefèbvre) (Lepidoptera : Noctuidae) is a pest only in portions of its geographic range: the influence of climate

Abstract

The stem borer Sesamia nonagrioides (Lefèbvre) (Lepidoptera : Noctuidae) is a major pest of maize in Mediterranean Europe and in several countries of sub-Saharan West Africa but not in East Africa where it lives on wild host plants along wet places. Sampling of Sesamia species in maize fields in Côte d'Ivoire during five years revealed high densities of S. nonagrioides in the southern forest regions, and showed that it was absent in crops in the northern half of the country, a savannah region where Sesamia calamistis was the major pest. A regression analysis showed that rainfall and temperature explained 81% of the variation of the density of S. nonagrioides in maize crops. The model predicts no infestation in the cool and dry conditions of East Africa. This conclusion is in agreement with the results of the study of migration from mitochondrial genes that showed that the species moved highly only along wet places in East Africa. In southern France, where S. nonagrioides is a pest of maize, the model predicts no infestation. This contradiction may be explained by (i) the selection of individuals adapted to cold and dry climate, (ii) The diapause, that occurs only in the palearctic region and (iii) the possible diffusion by humans. Heavy damage in maize crops by this pest in France are mentioned only from 1836, and were not mentioned in a comprehensive book on maize by Parmentier in the late eighteenth century. The colder temperatures of the little ice age may explain that no infestation occurred in maize fields at that time. In West Africa, S. nonagrioides is absent in the countries East of Togo in spite of favourable climate, which is likely due to the Dahomey gap, a dry region that splits the forest.