Explanation of formation process of symbiotic relationships between insects and fungi in forests

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Woodwasps are symbiotically associated with Amylostereum fungus spp. Adult females inoculate the fungus during oviposition on sapwood of host trees. Aided by fungal symbiosis, woodwasp larvae can digest sapwood with low nutritional value. Urocerus japonicus, a fungus-carrying woodwasp, oviposits selectively on freshly felled Japanese cedar trees that are presumed to be suitable for propagation of the fungal symbiont. We investigated the potential for growing U. japonicus in old trees by propagating Amylostereum laevigatum in the wood before woodwasp oviposition. Oviposition activity levels were higher on oviposited trees (fungus-inoculated by another woodwasp before oviposition) and on artificially inoculated trees (fungus-inoculated artificially before oviposition) than on control trees. Next-generation adults emerged from artificially inoculated trees but not from previously oviposited and control trees. These results suggest that U japonicus can utilize fungus already propagating in wood. Moreover, we investigated the effect of inoculation season on fungus propagation in the wood. Clear wood discoloration was recognized in summer-inoculated trees but there was almost no discoloration in fall-inoculated trees. In summer-inoculated trees, fungus propagated mainly in discolored areas; in fall-inoculated trees, it propagated widely irrespective of discoloration. These results suggest that fall-inoculated trees would be best for growing these woodwasps.