Life tables, key factor analysis and density relations in natural populations of the legume pod borer Maruca testulalis Geyer (Lepidoptera: Pyralidae) in Western Kenya

Abstract

Data from regular field samples throughout the life cycle for five generations of the legume pod borer, Maruca testulalis on cowpea crops at one site, and three generations at another, were used to construct life tables and through key factor analysis to determine the key mortality factor. The total mortality from egg to adult was 98.4-98.6% at site 1 and 98.2-99.4% at site 2. At both sites the highest mortality occurred between the eggs and the third instar larvae, whilst the fourth instar larvae suffered the lowest mortality. The commonest cause of mortality was classified as disappearance, followed by disease. Parasitism contributed minimally to M. testulalis mortality. At site 1 disappearance at the egg stage was the key mortality factor and was significantly correlated with the total generation mortality. At site 2, disappearance operating at the larva 1 and 2 stage was tentatively identified as the key mortality factor. The data from site 1 showed no significant correlation between population density and mortality at the same stage. This study provides the first detailed analysis of M. testulalis populations and should contribute towards developing control strategies for this important pest of cowpea. Resume-Les donnees receuillies a partie des echantillons reguliers le long du cycle de vie de cinq generations des insectes terebrants de cosses de pois Maruca testulalis, donnees en registrees grace a la recolte faite dans un site, et sur 3 generations dans un autre, etaient utilisees pour etablir des tables de longevite et a travers des analyses des facteurs-cles en vuede determiner le facteur cle de mortalite. Le taux de mortalite entre l'ovulation et I'etat adult etait de 98.4 a 98.6% au site 1 et 98.2 a 99.4% au site 2. Dans tous les 2 sites la mortalite la plus elevee a eu lieu entre l'ovulation et la 3eme phase a I'etat pupal, tandis que la 4eme phase pupate connaissait le taux de mortalite le plus bas. La cause la plus commune de mortalite envisaged etait la disparition, suivie par la maladie. Le parasitisme contribuait tres peu a la mortalite chez le M. testulalis. Au site 1, la disparition a la phase ovulaire etait le facteur cle de mortalite et etait etroitement liee a la mortalite giobale de la generation. Au site 2, la disparition se passant lors des phases 1 et 2 etait identified comme la raison principale de la mortalite. Les donnees obtenues du site n'ont montre aucune correlation entre la densite de la population et la mortalite au meme phase. Cette etude fournit la premiere analyse detaillee des populations M. testulalis et devrait contribuer au developpement des strategies de controle de cette bete qui detruit les cosses de pois.