POTENTIAL RATES OF INCREASE OF SOLITARIOUS AND GREGARIOUS PHASES OF THE AFRICAN ARMYWORM SPODOPTERA-EXEMPTA (LEPIDOPTERA, NOCTUIDAE)

1. The African armyworm Spodoptera exempta exhibits a phase polyphenis m with insects capable of transforming from one 'phase' to another, wh ich involves both physiological and behavioural changes. Amongst these changes, evidence from the laboratory and the field suggest that, giv en equivalent conditions of temperature and food, solitary phase insec ts take longer to complete their life cycles than gregarious phase ins ects, and the latter's generations are more synchronized. There is als o some evidence that females of the solitary form sometimes lay more e ggs. 2. Using data from field and laboratory studies, estimates of the rates of increase of each form were calculated from Leslie matrix mod els. Assuming equivalent mortality rates for both phases, asynchronous development for the solitaries and synchronous development for the gr egarious forms, with the latter having lower fecundities than the soli taries, the models suggest that the gregarious forms have higher value s of their intrinsic rates of increase (r) than solitarious population s. These results are explored using a range of parameter values includ ing variation in the percentage of solitaries with delayed development . 3. The possible ecological and evolutionary significance of variatio ns in rates of increase of gregarious and solitary populations are bri efly discussed.