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 polyphenism with insects capable of transforming from one 'phase' to another, which involves both physiological and behavioural changes. Amongst these changes, evidence from the laboratory and the field suggest that, given equivalent conditions of temperature and food, solitary phase insects take longer to complete their life cycles than gregarious phase insects, and the latter's generations are more synchronized. There is also some evidence that females of the solitary form sometimes lay more eggs. 2. Using data from field and laboratory studies, estimates of the rates of increase of each form were calculated from Leslie matrix models. Assuming equivalent mortality rates for both phases, asynchronous development for the solitaries and synchronous development for the gregarious forms, with the latter having lower fecundities than the solitaries, the models suggest that the gregarious forms have higher values of their intrinsic rates of increase (r) than solitarious populations. These results are explored using a range of parameter values including variation in the percentage of solitaries with delayed development. 3. The possible ecological and evolutionary significance of variations in rates of increase of gregarious and solitary populations are briefly discussed.