Melanism and disease resistance in insects

Abstract

There is growing evidence that insects in high‐density populations invest relatively more in pathogen resistance than those in low‐density populations (i.e. density‐dependent prophylaxis). Such increases in resistance are often accompanied by cuticular melanism, which is characteristic of the high‐density form of many phase polyphenic insects. Both melanism and pathogen resistance involve the prophenoloxidase enzyme system. In this paper the link between resistance, melanism and phenoloxidase activity is examined inSpodopteralarvae. InS. exempta, cuticular melanism was positively correlated with phenoloxidase activity in the cuticle, haemolymph and midgut. MelanicS. exemptalarvae were found to melanize a greater proportion of eggs of the ectoparasitoidEuplectrus laphygmaethan non‐melanic larvae, and melanicS. littoraliswere more resistant to the entomopathogenic fungusBeauveria bassiana(inS. exemptathe association between melanism and fungal resistance was non‐signficant). These results strengthen the link between melanism and disease resistance and implicate the involvement of phenoloxidase.