What are ambrosia beetles?

The term ‘ambrosia beetles’ refers to groups of scolytines that share the habit of culturing symbiotic fungi. However, ambrosia beetles are not a taxonomic group. This ambrosia habit has evolved at least 16 times in different scolytines, and each of the independent lineages has its own unique biological and morphological characteristics (Johnson et al. 2018). All ambrosia beetles tunnel directly into the sapwood of their tree host, cultivate a fungus garden, and transport the fungi in specialized structures termed mycangia. But because each of the groups have evolved the habit separately, their fungi are different, details of their ecology and genetic systems are different, even the mycangia are on different parts of their bodies.

Xyleborini ambrosia beetles

The scolytine tribe Xyleborini represents the largest radiation of ambrosia beetles. This tribe of over 1100 species evolved very recently, only within 20 mya (Jordal and Cognato 2012). This rapid radiation means that some genera are not readily diagnosed, and many species within the genera are typically very morphologically similar. Many closely related species of Xyleborini only differ by to subtle morphological differences, or the variable morphological features do not correspond with species boundaries.

Like bees, ants, and wasps, xyleborines possess a haplodiploid reproductive system in which diploid females produce diploid daughters and haploid sons. In Xyleborini, the haplo-diploidy is also combined with extreme inbreeding in which most males mate only with their sisters. Xyleborine broods exhibit strong sex ratio bias with many more females produced than males. Males are dwarfed, flightless, and rarely leave their natal gallery. Males typically look quite different from females and can be distinguished by their smaller size and impressed area on the anterior half of the pronotum. This inbreeding mating system is what has allowed xyleborines to rapidly diversify and makes them ideally suited to colonize new geographic areas because only a single female is needed to establish a population (Smith and Hulcr 2015).

Nearly all xyleborines possess mycangia, cuticular pouches specialized for storing spores of their symbiotic ambrosia fungus. There are three main types of mycangia: oral, mesonotal, and elytral. Some mesonotal and elytral mycangia are externally marked by dense patches of setae at the pronotal base and scutellum respectively. Several xyleborine genera, including Ambrosiophilus and Diuncus, contain species that steal ambrosia fungus from other xyleborines by excavating their galleries close to the host beetle and parasitizing the fungus growing around it for their own galleries. This habit termed ‘mycocleptism’ has also independently evolved several times in scolytines (Hulcr and Cognato 2010b).

In general, ambrosia fungus farming freed xyleborines from strict specificity to particular host trees, as the beetles consume fungus rather than host tissue and are thus not limited by the host’s secondary chemistry. They tend to be known from a wide diversity of hosts. If a species is host specific, it is at the family level rather than the generic or specific level. Due to their association with fungi, it is unsurprising that xyleborines can also serve as tree fungal disease vectors, leading to death of the infected host (Smith and Hulcr 2015).