Variation in wing length of the African armyworm, Spodoptera exempta in East Africa during 1973-74.
Wing lengths of nearly 6000 Spodoptera exempta (Wlk.) adults were measured in samples caught at light-traps at various sites in East Africa between December 1973 and July 1974. There were systematic variations in mean wing length between different populations at different times and places. A sampling unit called the station-month is defined as all the moths of one sex caught at one particular station in one particular month. The variance between station-months was greater than that between days within station-months, but less than that between observations within days. In general, wing length was at a minimum in February and rose to reach a maximum at the end of the season in July. In any one month, wing lengths were usually larger at more northerly stations. A sample from one particular night at Ilonga (Tanzania) consisted of moths whose wing length and sex-ratio differed significantly from those caught there at other times; this suggests that there was an influx of moths from a different geographical source on this night. Measurements were also made on samples from emergence sites in Kenya, and the results compared with the main analysis: they provide further evidence for geographical variation. Possible causes and consequences of the variation are discussed.