Competitive interactions between the larvae of Heliothis armigera (Hübner) and Heliothis punctigera Wallengren (Lepidoptera: Noctuidae)

Abstract

Experiments were conducted in the laboratory and glasshouse to investigate the role of competitive interactions between the larvae of Heliothis armigera and H. punctigera (Lepidoptera:Noctuidae) in regulating the relative abundance of the two species on cotton plants. Under uniform laboratory conditions H. armigera and H. punctigera larvae differed in their cannibalistic behaviour; the frequency of cannibalism in H. armigera was primarily encounter-dependent while in H. punctigera it appeared to be independent of density. When larvae of both species were reared together on artificial diet the survivorship of H. punctigera was depressed compared to that observed when reared by itself at the same overall density. By contrast, the survivorship of H. armigera was not affected. Both species required a similar absolute time for development on cotton plants and thus developmental characteristics appear unlikely to affect the outcome of interspecific interference competition in one generation. Nevertheless, the interference advantage possessed by H. armigera may be enhanced over a number of generations because this species passes through the pupal stage more rapidly than H. punctigera. Although the resource utilization patterns of H. armigera and H. punctigera larvae on cotton plants overlapped broadly there were quanlitatitive difference in feeding preference (as Leaves were acceptable to H. punctigera than H. armigera and the development of a preference for large bolls occurred later during the larval period for H. punctigera than H.armigera. Mixed species trials on cotton plants showed that the greater complexity of the cotton plant environment provided an opportunity for the two species to partition food resources according to their feeding preferences. Under these conditions aggressive interference behaviour is directed primarily at conspecifics. Thus, although H. amigera is potentially able to limit the population density of H. punctigera in a uniform environment, intraspecifc interference encounters may be more important in regulating the population density on plants. The relevance of these findings to the regulation of field populations is discussed.