African ArmywormSpodoptera Exempta (Walker) (Lepidoptera: Noctuidae) and Wind Convergence in the Kenya Rift Valley, May 1970

Abstract

1970 the most important end of season outbreaks of African armyworm in Kenya were near Nakuru in the Rift Valley; much of the oviposition was inferred to have taken place in the first half of May. The seasons highest catch of Spodoptera exempta in the local light trap ,occurred on 9/ 10th May, coincident with a synoptic-scale weather disturbance which lay almost stationary over the area during 8-lOth May; detailed observations on the wind fields were made on two ,of those days by a specially instrumented aircraft. It was estimated that large numbers of moths were being produced at the time only at places distant from Nakuru, but all upwind from there while this convergent wind system was present. Later in the month further laying occurred in Nakuru area, again coincident with increased light trap catches and the persistence of convergent winds which would have held moths emerging locally trapped in the area. The wind fields are described and their effect on the contemporary S. exempta moths which produced the Rift Valley infestations discussed. It is considered that the persistence of wind convergence in the area led to the laying occurring there.