What makes a scolytine?

Scolytine ambrosia beetles are actually snoutless weevils (Curculionidae)! Members of this subfamily are characterized by having the rostrum (snout) absent or reduced, eyes that are never round, elbowed antennae with a club, and a protibia which is armed by more structures (ex. denticles, tubercles, rugae, etc.) than a single apical spine. They are small in size, 0.7–12.0 mm in length, with most species around 3.0 mm long, body narrowly elongate to broadly ovate.

The group Scolytinae contains over 6,000 species. But they are diverse not only in terms of species, but also in their ecology, morphology, and genetics. Scolytines are found in all regions of the world but are most abundant and diverse in the tropics. They reproduce within plant tissues, and different species specialize on different plant parts: from bark to xylem and from roots to fruit; a few species even reproduce in forbs and grasses. Most species, however, feed either on phloem in dead trees (those are called bark beetles) or on symbiotic fungi (ambrosia beetles). Bark beetles feed almost exclusively on phloem and cambium of declining or dead trees. They create tunnels termed ‘galleries’ in the phloem along which they lay eggs. Larvae hatch and typically create their own galleries radiating away from that constructed by the parents. These galleries are visible when bark is removed and may be diagnostic for species (Hulcr et al. 2015).

The ambrosia beetles tunnel directly into the sapwood of their host where they cultivate a symbiotic fungus. The fungus is transported in specialized structures termed ‘mycangia’. The beetles and their larvae either consume only the fungus or a combination of wood and fungus. Ambrosia beetle galleries are harder to see without dissecting the wood. The largest group of ambrosia beetles is the tribe Xyleborini. Xyleborini contain over 1,000 species, including many important pests and globally widespread and abundant invasive beetles, and as such are some of the most encountered scolytine groups. Xyleborini is the focal group of this identification tool.

How to identify scolytines from other beetles that may be confused with them?

Several other beetle groups can be easily confused with scolytines, including cossonine weevils (another weevil subfamily), bostrichids (Bostrichidae, powder post beetles), ciids (Ciidae, minute tree-fungus beetles), and anobiids (Ptinidae: Anobiinae, death watch beetles). These groups of beetles have similar feeding habits or are often caught in scolytine traps. Superficially, they have similar body shapes as scolytines, but round eyes distinguish them. Bostrichids and anobiids can be further distinguished by the antennae which are neither elbowed nor clubbed, and cossonines by the presence of a large apical spine on the protibia. One scolytine tribe, Scolytini, also possesses a protibia with only an apical spine, but the eyes are never round. Phloeotribini bark beetles possess a lamellate antennal club similar to Bostrichidae, but again, the eye is emarginate.

Is it a xyleborine ambrosia beetle?

Xyleborine ambrosia beetles can be distinguished from all other scolytines by the combination of the following characters: eyes emarginate, tibiae flat and broad with convex outer edge and possessing at least 6 denticles (very few exceptions with as few as 4 denticles), pronotum strongly convex and armed with asperities on the anterior slope, antennal club dorsoventrally flattened, and depressed submentum. Equally important are biological characters: all Xyleborini are haplodiploid inbreeders (each family has usually only one male, dwarfed, flightless, and haploid) and obligate fungus farmers. Their dependence on fungi means that almost all species have a mycangium, but that may or may not be externally visible.

What tools are necessary to identify xyleborine specimens?

Due to the small size of many xyleborines, a high-powered dissecting scope and a good light source are necessary for identification. We strongly recommend that specimens are examined dry, as many of the diagnostic characteristics are obscured or not visible while the specimen is in ethanol; this is exceptionally true for Cyclorhipidion, Microperus, and Xyleborinus species. The specimen can then be returned to alcohol once the identification has been made. In some cases, access to a DNA sequencing facility will be necessary, especially to identify males and larvae.

What if the specimen doesn’t match any of the included species?

This tool allows for the identification of all known species from mainland Southeast Asia and Taiwan. If a specimen does not match any of the included species, it is quite possible that it is either represents new record for the region, or it is a species new to science. Should this scenario arise, please contact one of the content authors.