Density-dependent prophylaxis in the mealworm beetle Tenebrio molitor L-(Coleoptera : Tenebrionidae): cuticular melanization is an indicator of investment in immunity

If there are costs involved with the maintenance of pathogen resistance, th en higher investment in this trait is expected when the risk of pathogenesi s is high. One situation in which the risk of pathogenesis is elevated is a t increased conspecific density. This paper reports the results of a study of density-dependent polyphenism in pathogen resistance and immune function in the mealworm beetle Tenebrio molitor. Beetles reared at high larval den sities showed lower mortality when exposed to a generalist entomo-pathogeni c fungus and a higher degree of cuticular melanization than those reared so litarily The degree of cuticular melanization was a strong indicator of res istance, with darker beetles being more resistant than lighter ones regardl ess of rearing density. No differences were found between rearing densities in the levels of phenoloxidase, an enzyme key to the insect immune respons e. The results show that pathogen resistance is phenotypically plastic in T . molitor suggesting that the maintenance of this trait is costly.