Field development and quality changes in successive generations of Spodoptera exempta Wlk., the African armyworm.
Abstract
Three successive generations of Spodoptera exempta (Wlk.) were observed on cereal crops and grasses at Salisbury Research Station, Rhodesia, during January-May 1954. Generation developmental periods were estimated and the numbers of adults emerging from under cereals and from under indigenous grasses were compared. The rates of development and phase differences of larvae, which fed either on oats or grasses in the field, were measured for one generation; these indicated that although development of passive forms [cf. RAE/A 45, p 81] was slower, the ultimate sizes of the larvae and adults differed little from those of active forms. Large size differences were found between generations; a population of small adults was predominantly female, laid fertile eggs on the night of emergence and appeared not to migrate; populations of larger adults were of even sex ratio, longer preoviposition period and mostly migrants. The sequence suggested that although most adults of the first generation migrated after emerging from pupae in the ground, the smallest laid eggs locally. Their larvae were small, passive, heavily parasitised and most of the adults which developed from the survivors laid eggs locally. Vigour was regained in the third generation; this was expressed by the presence of large larvae of both passive and active forms, few parasites and large migrant adults.