Epizootiological observations on a nuclear polyhedrosis of the African armyworm Spodoptera exempta (Walk.)

Abstract

Field observations were carried out on a natural outbreak of a nuclear polyhedrosis virus of the armyworm Spodoptera exempta. Records were kept in four study sites with varying larval density, vegetation cover, and climatic conditions. Biotic factors influencing disease outbreaks were also noted. Initial mortality was recorded 11 days after larval outbreak, and the highest mortality occurred 10 days later. The epizootic was highest in sites of high larval density, wide fluctuations of daily temperatures (range 16.2 °C), and high relative humidity. Rainfall was recorded in all disease outbreak sites. An acridid Acrotylus patruelis fed on larval cadavers in the field. Polyhedral inclusion bodies recovered from the gut of A. patruelis caused disease when fed to third instar S. exempta larvae in the laboratory. Predation on larvae by the pied crow Corvus albus in some observation sites reduced larval numbers, and may have prevented NPV from rising to epizootic levels in such areas. In sites of high polyhedrosis incidence, the disease led to larval population collapse in 3 weeks following disease outbreak.