Density dependent phase polyphenism in the African Armyworm Spodoptera exempta (Lepidoptera: Noctuidae)

Reeson, Andrew Francis (1999) Density dependent phase polyphenism in the African Armyworm Spodoptera exempta (Lepidoptera: Noctuidae). University of Southampton, Doctoral Thesis.

Abstract

Larvae of the African armyworm Spodoptera exempta (Lepidoptera: Noctuidae) show density dependent phase polyphenism, with individuals developing one of two alternative phenotypes; the phenotype into which an individual develops is determined by the population density experienced during the first few days of larval development. In common with many other phase polyphenic Lepidoptera, the high density form of S. exempta has a melanised cuticle (at low densities they are usually green or brown) and develops more rapidly than the low density form. In a number of polyphenic species, larvae and adults from crowded populations have been found to be more active than those reared at low density. This was also found to be the case for S. exempta, with larvae reared in crowds shown to move significantly further than those reared in isolation. This behaviour may enable them to compete for food more effectively, or to disperse away from crowded areas. The cuticular melanisation seen in crowded S. exempta is controlled by the prophenoloxidase enzyme system. The same enzyme system also plays a key role in the insect immune response, suggesting a link between phase polyphenism and pathogen resistance. Viral bioassays showed that the black, crowded form of S. exempta has an LD₅₀ more than ten times greater than that of the solitary form. Moreover, among larvae reared in isolation, which show considerable colour variation, black individuals suffered significantly lower viral morality than green or brown larvae. Assays of haemolymph phenoloxidase activity showed that there is a correlation between cuticular melanisation and phenoloxidase levels in the haemolymph, supporting the hypothesis that cuticular melanisation is indicative of higher phenoloxidase levels and a more active immune response. However, there were no differences in phenoloxidase activity between black solitary and crowded larvae, while there were large differences in viral mortality.

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