The tea shot-hole borer beetle (TSHB, Euwallacea fornicatus) causes serious damage in plantations of tea, Camellia sinensis var. assamica, in Sri Lanka and South India. TSHB is found in symbiotic association with the ambrosia fungus, Monacrosporium ambrosium (syn. Fusarium ambrosium), in galleries located within stems of tea bushes. M. ambrosium is known to be the sole food source of TSHB. Six naphthoquinones produced during spore germination in a laboratory culture broth of M. ambrosium were isolated and identified as dihydroanhydrojavanicin, anhydrojavanicin, javanicin, 5,8-dihydroxy-2-methyl-3-(2-oxopropyl)naphthalene-1,4-dione, anhydrofusarubin and solaniol. Chloroform extracts of tea stems with red-colored galleries occupied by TSHB contained UV active compounds similar to the above naphthoquinones. Laboratory assays demonstrated that the combined ethyl acetate extracts of the fungal culture broth and mycelium inhibited the growth of endophytic fungi Pestalotiopsis camelliae and Phoma multirostrata, which were also isolated from tea stems. Thus, pigmented naphthoquinones secreted by M. ambrosium during spore germination may prevent other fungi from invading TSHB galleries in tea stems. The antifungal nature of the naphthoquinone extract suggests that it protects the habitat of TSHB. We propose that the TSHB fungal ectosymbiont M. ambrosium provides not only the food and sterol skeleton necessary for the development of the beetle during its larval stages, but also serves as a producer of fungal inhibitors that help to preserve the purity of the fungal garden of TSHB.
Keywords: Ambrosia beetle; Ambrosia fungus; Ectosymbiont; Fungal metabolites; Inhibitory activity; Insect pest; Naphthoquinones.