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

摘要

Parasite resistance mechanisms can be costly to maintain. We would thereforepredict that organisms should invest in resistance only when it is likely to berequired. Insects that show density–dependent phase polyphenism, developingdifferent phenotypes at high and low population densities, have the opportunityto match their levels of investment in resistance with the likelihood ofexposure to pathogens. As high population densities often precipitate diseaseepidemics, the high–density form should be selected to invest relatively more inresistance. We tested this prediction in larvae of the noctuid Spodopteraexempta. Larvae reared at a high density were found to be considerably moreresistant to a nuclear polyhedrosis virus than those reared in isolation. Aconspicuous feature of the high–density phase of S. exempta and otherphase–polyphenic Lepidoptera is cuticular melanization. As melanization iscontrolled by the phenoloxidase enzyme system, which is also involved in theimmune response, this suggests a possible mechanism for increased resistance athigh population densities. We demonstrated that melanized S. exempta larvae weremore resistant than non–melanized forms, independent of rearing density. We alsofound that haemolymph phenoloxidase activity was correlated with cuticularmelanization, providing further evidence for a link between melanization andimmunity. These results suggest that pathogen resistance in S. exempta isphenotypically plastic, and that the melanized cuticles characteristic of thehigh–density form may be indicative of a more active im