Phenoloxidase Activity, Lack of Bactericidal Immunity, and Oral Susceptibility of Tobacco Budworm (Lepidoptera: Noctuidae) Larvae to Serratia marcescens

A 20% difference in phenoloxidase activity was found between hemolymph obtained from uninoculated Heliothis virescens (F.) larvae and from larvae inoculated with heat-killed Serratia marcescens , indicating a lack of immunity and perhaps a lack of larval recognition of S. marcescens by the phenolxidase system. No bactericidal (0%) humoral immunity to pigmented and capsulated S. marcescens could be detected in cell-free hemolymph from fifth instars. This was true for hemolymph obtained from uninoculated larvae and from inoculated larvae 24 h after inoculation of heat-killed S. marcescens . After oral infection of larvae with S. marcescens , larval mortality was 17.4%, pupation decreased by 17.4%, and adult emergence decreased by 21.5%, when uninfected and infected insects were compared.