Baculovirus resistance in the noctuid Spodoptera exempta is phenotypically plastic and responds to population density.

摘要

Insects that show density-dependent phase polyphenism, developing different phenotypes at high and low population densities, have the opportunity to match their levels of investment in resistance with the likelihood of exposure to pathogens. As highpopulation densities often precipitate disease epidemics, the high-density form should be selected to invest relatively more in resistance. This prediction was tested in larvae of the noctuid Spodoptera exempta in the laboratory. Larvae reared at a highdensity (6 larvae/12-ml pot) were found to be considerably more resistant to a nuclear polyhedrosis virus than those reared in isolation (1 larva/12-ml pot). A conspicuous feature of the high-density phase of S. exempta is cuticular melanization. As melanization is controlled by the phenoloxidase enzyme system, which is also involved in the immune response, this suggests a possible mechanism for increased resistance at high population densities. It was demonstrated that melanized S. exempta larvae weremore resistant than non-melanized forms, independent of rearing density. It was also found that haemolymph phenoloxidase activity was correlated with cuticular melanization, providing further evidence for a link between melanization and immunity. It issuggested that pathogen resistance in S. exempta is phenotypically plastic, and that the melanized cuticles characteristic of the high-density form may be indicative of a more active immune system.