Virus disease of the African armyworm,Spodoptera exempta(Wlk.)

Abstract

Field and laboratory studies of virus disease in the armyworm Spodoptera exempta (Wlk.) were carried out in East Africa in 1962–64. A nuclear polyhedrosis virus was detected microscopically in pupae, prepupae and all larval instars except the first, and was found to be widely distributed in most areas subject to outbreaks of armyworm in East Africa. The signs and course of the disease in larvae of S. exempta are described. Attempts made in the laboratory to infect S. triturata and other species of Spodoptera with virus from S. exempta failed. In unsterile conditions of maintenance, almost all laboratory cultures from wild-caught female moths sustained heavy infections. The appearance of the symptoms of polyhedrosis was accelerated at higher temperatures, and the incidence was higher in more crowded cultures. Transmission took place from contaminated leaves to larvae and probably from one larva to another. There was evidence also of generation-to-generation transmission through the egg.The effect on polyhedra of immersion in alkali solutions was investigated; it. was found that immersion for 15 minutes in 0·2 per cent. caustic potash effectively dissolved the matrix and inactivated the virus. When all equipment was sterilised in this way (or, later, with 0·4% caustic potash) apparently virus-free cultures were obtained by rearing the larvae from wild-caught females in isolation and breeding from those that survived to become adults.The possible use of the virus for control of outbreaks of armyworm in the field is discussed; it is concluded that there is at present insufficient evidence to show whether such a measure would be effective.