The fungal symbionts in the gallery system and the mycangia of the four ambrosia beetles, Scolytoplatypus mikado, Xylosandrus mutilatus, X. brevis, and X. crassiusculus, were studied using both isolation experiments and SEM microscopy. In the galleries, one of Ambrosiella spp. was predominant during the larval growing periods, but its relative dominance among all the associated fungi of each beetle species gradually decreased during development of the larvae. In contrast, yeasts (mainly Candida sp.) and Paecilomyces sp. continuously dominated in the galleries after eclosion. Ambrosiella sp. was consistently stored in the mycangia in all adult stages, except the teneral or overwintering adults during which other fungi were dominant. No fungal spores occurred in the mycangia of the adult beetles reared under aseptic conditions from the pupal stage, while only Ambrosiella sp. was stored in those from the teneral-adult stage. These results suggest that : 1) each scolytid beetle is associated with at least three fungal species, among which Ambrosiella sp. is the most essential food, -resource for development of the broods and 2) immediately after eclosion, new female adults may take a few associated fungal species, with no or incomplete selection, into their mycangia from the walls of the cradles.
Positive correlations between the diametral size of infested logs and fecundity per gallery and between the fecundity and survival rate of brood in each gallery, as observed in S. mikado, also suggested that ambrosia beetles would have the potential to increase abruptly over their endemic levels in response to increased availability of host trees.