BACULOVIRUS RESISTANCE IN THE NOCTUID SPODOPTERA-EXEMPTA IS PHENOTYPICALLY PLASTIC AND RESPONDS TO POPULATION-DENSITY

Parasite resistance mechanisms can be costly to maintain. We would the refore predict that organisms should invest in resistance only when it is likely to be required. Insects that show density-dependent phase p olyphenism, developing different phenotypes at high and lour populatio n densities, have the opportunity to match their levels of investment in resistance with the likelihood of exposure to pathogens. As high po pulation densities often precipitate disease epidemics, the high-densi ty form should be selected to invest relatively more in resistance. We tested this prediction in larvae of the noctuid Spodoptera exempts. L arvae reared at a high density were found to be considerably more resi stant to a nuclear polyhedrosis virus than those reared in isolation. A conspicuous feature of the high-density phase of S. exempta and othe r phase-polyphenic Lepidoptera is cuticular melanization. As melanizat ion is controlled by the phenoloxidase enzyme system, which is also in volved in the immune response, this suggests a possible mechanism for increased resistance at high population densities. We demonstrated tha t melanized S exempta larvae were more resistant than non-melanized fo rms, independent of rearing density. We also found that haemolymph phe noloxidase activity was correlated with cuticular melanization, provid ing further evidence for a link between melanization and immunity. The se results suggest that pathogen resistance in S. exempta is phenotypi cally plastic, and that the melanized cuticles characteristic of the h igh-density form may be indicative of a more active immune system.