Resin exudation and resinicolous communities on Araucaria humboldtensis in New Caledonia

Abstract

Conifers of the endemic species Araucaria humboldtensis on Mont Humboldt in New Caledonia exhibit extensive resin exudation. The resin flows of these threatened trees are here shown to be induced by two beetle species, which bore into branches and branchlets, leading to abundant outpouring of resin, which gradually solidifies into often drop-shaped resin bodies. The exudate is colonized by a resinicolous and likely insect-vectored ascomycete, Resinogalea humboldtensis, which is only known from Mont Humboldt. The fungus grows into fresh resin and eventually develops ascomata on the surface of solidifying resin. The solidified resin is also colonized by another fungus, a dematiaceous hyphomycete. Based on protein coding (CO1, CAD, ArgK) and ribosomal (LSU) genes, the larger branch-boring beetle is a weevil of the tribe Araucariini, which represents the sister group of all other cossonine weevils. The smaller beetle species belongs to the longhorn beetles (Cerambycidae). The strong host specificity of the Araucariini, along with the occurrence of two unique fungi, suggests that the resin-associated community is native and has evolved on the endemic conifer host. The formation of large amber deposits indicates massive resin production in the past, but the environmental triggers of exudation in Mesozoic and Cenozoic ecosystems remain unclear. Our observations from Mont Humboldt support the notion that the occurrences of small drop-shaped amber pieces in Triassic to Miocene amber deposits were linked to ancient insect infestations.