Nutrient-mediated germination of Beauveria bassiana conidia on the integument of the bark beetle Dendroctonus ponderosae (Coleoptera:Scolytidae)

Field-collected adult mountain pine beetles, Dendroctonus ponderosae, were inoculated with the white muscardine fungus, Beauveria bassiana, and thoroughly examined externally with a scanning electron microscope. Germinating conidia were found at a very low incidence, and only on antennal clubs. However, mortality of inoculated controls was high, and B. bassiana was confirmed as the causative agent. Germination on the cuticle was greatly increased by sonication of adult beetles. The hypothesis that hemolymph released through sonication-damaged membranes provided a nutrient stimulus that enhanced conidial germination was tested by placing hemolymph or yeast-malt extract broth on the cuticle of otherwise nontreated beetles, which were then inoculated with conidia. Significant germination occurred on the cuticle of these beetles. Therefore, a limiting factor for conidial germination on the sclerotized cuticle of D. ponderosae is probably a lack of sufficient nutrients.