The effect of larval phase on flight performance of African armyworm moths, Spodoptera exempta (Walker) (Lepidoptera: Noctuidae)

Abstract

AbstractThe characteristics of the high and low density forms of noctuid moths, including Spodoptera exempta (Walker), exhibiting a density-dependent phase polyphenism have frequently been discussed in relation to migration. However, the only previous (unpublished) demonstration of an effect of larval phase on adult flight performance, using a tethered-flight technique, was invalidated by the recent discovery that the principal determinant of flight potential in S. exempta is genetic. When the incidence of prolonged flight was measured in moths derived from genetically-matched (full-sib) samples, there was a clear increase in long flights by females derived from the high-density gregaria phase larvae compared with those from solitaria phase larvae. The reasons for the apparent absence of a similar effect in males is not clear, but it is possible that the tethered-flight technique provides a less reliable index of flight capacity in this sex. The characteristics and significance of phase polyphenism in migratory noctuids are discussed. It is suggested that, in S. exempta and possibly some other comparable species, the high-density phase is adapted to accelerate re-dispersal after populations become concentrated, in order to escape the detrimental consequences of high larval densities.