Ambrosia beetles are beetles of the weevil subfamilies Scolytinae and Platypodinae (Coleoptera, Curculionidae), which live in nutritional symbiosis with ambrosia fungi and probably with bacteria. The beetles excavate tunnels in dead trees in which they cultivate fungal gardens, their sole source of nutrition. After landing on a suitable tree, an ambrosia beetle excavates a tunnel in which it releases spores of its fungal symbiont. The fungus penetrates the plant's xylem tissue, digests it, and concentrates the nutrients on and near the surface of the beetle gallery. The majority of ambrosia beetles colonize xylem (sapwood and/or heartwood) of dying or recently dead trees. Species differ in their preference for different parts of trees, different stages of deterioration, in the shape of their tunnels ("galleries"). However, the majority of ambrosia beetles are not specialized to any taxonomic group of hosts, unlike most phytophagous organisms including the closely related bark beetles.
Classification and Diversity
Until recently ambrosia beetles have been placed in independent families Scolytidae and Platypodidae, however, they are in fact some of the most highly derived weevils. There are about 3,000 known beetle species employing the ambrosia strategy.
Ambrosia beetles are an ecological guild, but not a phylogenetic clade. The ambrosia habit is an example of convergent evolution, as several groups evolved the same symbiotic relationship independently. The highest diversity of ambrosia beetles is in the tropics. In the Paleotropical region, hundreds of species of Xyleborini and Platypodinae are the main agent initiating dead wood decomposition. In the Neotropics, Platypodinae and Xyleborini are joined by the scolytine tribe Cortylini. Compared to the diversity in the tropics, ambrosia beetle fauna in the temperate zone is rather limited. In the Nearctic region it is dominated by a few species from Cortylini, Xyleborini and Xyloterini. In the Palearctic ecozone, significant groups are Xyloterini and Xyleborini, joined by Scolytoplatypodini in the Far East.
The symbiotic relationship Beetles and their larvae graze on mycelium exposed on the gallery walls and on bodies called sporodochia, clusters of the fungus' spores. Most ambrosia beetle species don't ingest the wood tissue; instead, the sawdust resulting from the excavation is pushed out of the gallery. Following the larval and pupal stage, adult ambrosia beetles collect masses of fungal spores into their mycangia and leave the gallery to find their own tree.
A few dozen species of ambrosia fungi have been described, currently in the polyphyletic genera Ambrosiella, Raffaelea and Dryadomyces (all from Ophiostomatales, Ascomycetes). Many more species remain to be discovered. Little is known about the bionomy or specificity of ambrosia fungi. Ambrosia fungi are thought to be dependent on transport and inoculation provided by their beetle symbionts, as they have not been found in any other habitat. All ambrosia fungi are probably asexual and clonal.
During their evolution, most scolytid and platypodid weevils became progressively more or less dependent on fungi regularly co-habiting dead trees.
This evolution had various outcomes in different groups:
Some phloem-eating bark beetles (phloeophages) are probably employing aggressive phytopathogenic fungal associates to kill live trees.
Many of phloem-feeding bark beetles use phloem-infesting fungi as an addition to their diet. Some phloeophages became more or less dependent on such a mixed diet and evolved mycangia to transport their symbionts from maternal trees to newly infested trees. These beetles are called mycophloeophages.
Ambrosia beetles and ambrosia fungi are thus only one end of the spectrum of the weevil-fungus association, where both the beetle and the fungus became completely dependent on each other.