Corthylini

 Yellow-banded timber beetle Monarthrum fasciatum, copyright Katja Schulz.

Belongs within: Hylesininae.
Contains: Araptus, Conophthorus, Pityophthorus, Corthylus.

The Corthylini are a cosmopolitan group of bark beetles (not found in Australia) characterised by emarginate eyes, a strongly flattened antennal club, and relatively slender tibiae. A small transverse groove on the rear of the metepisternum allows the elytra to be locked into place when closed. Corthylins may be divided on the grounds of feeding habits between the phloeophagous Pityophthorina and the xylomycetophagous Corthylina, though the two subtribes are less clearly distinct morphologically. Phloeophagous species dominate in temperate regions whereas xylomycetophagous species are predominantly tropical (Wood 1986).

Bark beetles and their hidden harems
Published 24 February 2014
Galleries dug in a grand fir Abies grandis by fir bark beetles Pityophthorus pityographus, photographed by Louis-Michel Nageleisen.

For producers of commercial timber, the above picture would not be a pretty sight. Bark beetles are named after what they feed on: they chew galleries under the bark of trees. In some species that attack otherwise healthy trees, these borings may result in stunted growth or death. The beetles may spread fungal diseases as they move from one tree to another (Dutch elm disease is one example of a well-known disease spread by bark beetles). But on the other hand, many bark beetles play a vital row in nutrient recycling, feeding on already dead and dying trees and breaking down the wood.

The fir bark beetle Pityophthorus pityographus itself, from PaDIL.

The bark beetles belong to a group called the Scolytinae. The scolytines include over 6000 species worldwide (only a relatively small percentage of which, it should be noted, are recognised as significant pests). Oddly enough, they are actually a kind of weevil. The most characteristic feature of most weevils is their elongate snouts, but in scolytines these snouts have been lost (they would probably not be ideal for burrowing through wood). The fir bark beetle belongs to a subgroup of the scolytines called the Corthylini, distinguished from other scolytines by their elytra, which lock down so that a panel on the side of the body called the metepisternum is hidden when the elytra is closed (in other scolytines, it remains at least partially visible), and by the flattened round clubs on their antennae (Wood 1986). The Corthylini are themselves divided into two subgroups, the Corthylina and the Pityophthorina. The two groups are not that easily separated by their morphological appearance, but they are very different in their ecology. The Corthylina don’t live directly under the bark, but deeper in the tree amongst the xylem (the central water-conducting tissue). Corthylinans and ecologically similar beetles, known as ambrosia beetles, live in association with a fungus that grows on the xylem. The beetles, which cannot directly digest the xylem themselves, feed instead on the fungus.

The Pityophthorina, on the other hand, are true bark beetles, with most species feeding directly on the tree’s phloem (the sugar-conducting tissue around the outer part of the tree). Other species in this group burrow in the tree’s seeds, or feed on the pith inside slender stems. The main diversity of Pityophthorina (and of Corthylini in general) is in the Americas, particularly in cooler temperate or tropical highland environments, with over 500 known species in North and South America. Two species, Pityodendron madagascarensis and Sauroptilius sauropterus, are found in Madagascar, while the genus Mimiocurus includes ten species found in Africa and Asia. The largest genus in the Pityophthorina, Pityophthorus, includes about sixty species in Africa and Eurasia in addition to over 300 in the Americas. Wood (1986) suggested that the Eurasian species of Pityopthorus were probably descended from relatively recent migrations from North America, but African and Madagascan species of Pityophthorina may represent more basal lineages.

Female Dendroterus decipiens, photographed by T. H. Atkinson.

As well as their economic and ecological significance, scolytines have attracted attention for the range of breeding behaviours they exhibit (Kirkendall 1983). Bark beetle galleries are not just feeding structures, they are also breeding structures. Females mate and lay their eggs within the galleries, and their larvae hatch and continue to feed there. The Pityophthorina are described as including both monogynous and polygynous species, but these terms refer to the number of females in a gallery, not necessarily the mating habits of the males. In monogynous species, a gallery will be home to only a single female. Construction of this gallery may have been started by the female herself, or it may have been started by a male who was then joined by the female. In most monogynous Pityophthorina, the latter is the case (and the mating system is monogamous as well as monogynous), but the female is the one to start the gallery in the genus Conophthorus (Conophthorus species feed in pine cones, and may be restricted to monogyny by the small spaces available for gallery construction). Conopthorus males that mate with the female may not remain in the gallery, but may leave directly after mating. Males that don’t stay in the gallery can mate with more females, of course, but males who do stay will be able to prevent their mate from mating with another male herself before laying her eggs. Also, by helping to maintain the gallery (or by constructing the gallery himself to begin with), the male may encourage the female to oviposit faster, or improve conditions for the larvae when they hatch.

In polygynous species, such as most Pityophthorus species, a single gallery will be home to multiple females. In most polygynous Pityophthorina, a single male will co-habit with a harem of females. A few pityophthorinans of the genus Araptus are inbreeding polygynes: males do not leave their parent gallery, but instead mate with their sisters before the latter leave the gallery. Inbreeding species seem to show remarkable control over sex ratios in the population, with many more female larvae produced than males. Interestingly, monogynous and polygynous galleries tend to differ in physical structure: monogynous galleries tend to be simple and direct, with only one or two arms extending along or across the host plant from the central nuptial chamber. Polygynous galleries, on the other hand, may have several arms radiating from the nuptial chamber, with each arm probably being built by a separate female.

Systematics of Corthylini

Characters (from Wood 1986): Frons usually dimorphic, either or both sexes feebly to strongly modified in sculpture and ornamentation; eye emarginate; scape usually elongate, rarely strongly flattened, funicle 1- to 5-segmented, club strongly flattened, sutures present or obsolete, frequently 1 and/or 2 septate; when present, sutures on posterior face little if any displaced toward apex; anterior slope of pronotum usually asperate, anterior margin frequently armed, lateral and basal margins frequently marked by a fine, raised line; procoxae contiguous, tibiae rather slender, their lateral margins rarely armed by more than four socketed teeth; metepisternal spine obsolete, replaced by small, transverse groove, thus making it possible for elytra in locked position to cover at least posterior two-thirds of metepisternum; vestiture usually hairlike, scales rarely present.

<==Corthylini [Amphicranidae, Araptidae, Corthyli, Pityophthoridae]
    |--PityophthorinaW86
    |    |  i. s.: Dacnophthorus Wood 1975W86
    |    |           `--*D. clematis [=Gnathophthorus clematis]W86
    |    |         Mimiocurus Schedl 1957 [incl. Micracidendron Schedl 1957, Mimiophthorus Schedl 1957]W86
    |    |           |--*M. acuminatusW86
    |    |           |--M. montanus [=Brachydendrulus montanus, *Mimiophthorus montanus]W86
    |    |           `--*Micracidendron’ montanum non Brachydendrulus montanusW86
    |    |         Styphlosoma Blandoford 1904W86
    |    |           `--*S. granulatumW86
    |    |         Phloeoterus Wood 1984W86
    |    |           `--*P. burseraeW86
    |    |         AraptusW86
    |    |         Spermophthorus Costa Lima 1929W86
    |    |           `--*S. apuleiaeW86
    |    |         Pityoborus Blackman 1922W86
    |    |           `--*P. comatus [=Crypturgus comatus]W86
    |    |         Pityotrichus Wood 1962 [=Pityophilus Blackman 1928 non Bedel 1889]W86
    |    |           `--*P. barbatus [=*Pityophilus barbatus]W86
    |    |         Gnatholeptus Blackman 1943W86
    |    |           `--G. shannoni [=Pityophthorus shannoni; incl. *Gnatholeptus mandibularis]W86
    |    |         Pityodendron Schedl 1953W86
    |    |           `--*P. madagascarensisW86
    |    |         Sauroptilius Browne 1970W86
    |    |           `--*S. sauropterus [=Xyleborus sauropterus]W86
    |    |         Phelloterus Wood 1971W86
    |    |           `--*P. tersusW86
    |    |--+--ConophthorusCG05
    |    |  `--PityophthorusCG05
    |    |--Pseudopityophthorus Swaine 1918CG05, W86 [incl. Xenophthorus Wood & Yin 1986A02]
    |    |    |--*P. minutissimus [=Crypturgus minutissimus]W86
    |    |    |--P. fagi Blackman 1931A02
    |    |    `--P. pubipennisCG05
    |    `--Dendroterus Blandford 1904CG05, W86 [incl. Plesiophthorus Schedl 1940W86, Xylochilus Schedl 1956W86]
    |         |--*D. mexicanusW86
    |         |--D. perspectus [=*Plesiophthorus perspectus]W86
    |         |--D. sallaei [incl. *Xylochilus insularis]W86
    |         `--D. striatusCG05
    `--CorthylinaW86
         |--CorthylusW86
         |--Gnatharus Wood & Yin 1986W86
         |    `--*G. tibetensisW86
         |--Glochinocerus Blandford 1904W86
         |    `--*G. retusipennisW86
         |--Brachyspartus Ferrari 1867 [=Thylurcos Schedl 1939]W86
         |    `--*B. moritzi [=*Thylurcos moritzi]W86
         |--Microcorthylus Ferrari 1867W86
         |    `--*M. parvulusW86
         |--Corthycyclon Schedl 1951W86
         |    `--*C. ustumW86
         |--Corthylocurus Wood 1966W86
         |    `--*C. barbatus [=Brachyspartus barbatus]W86
         |--Metacorthylus Blandford 1904W86
         |    |--*M. nigripennisW86
         |    `--M. velutinus [=*Paracorthylus velutinus]W86
         |--Amphicranus Erichson 1836 [incl. Piezorhopalus Guerin-Meneville 1838, Steganocranus Eichhoff 1878]W86
         |    |--*A. thoracicus [incl. *Piezorhopalus nitidulus]W86
         |    `--A. dohrni [=*Steganocranus dohrni]W86
         |--Tricolus Blandford 1905 [incl. Pterocyclonoides Schedl 1970]W86
         |    |--*T. ovicollisW86
         |    |--T. octodentatus [=*Pterocyclonoides octodentatus]W86
         |    `--T. peltatusW86
         |--Gnathotrichus Eichhoff 1869 (see below for synonymy)W86
         |    |--G. materiarius (Fitch 1858)A02 (see below for synonymy)
         |    |--G. pilosus (Leconte 1868)W86, B38 (see below for synonymy)
         |    |--G. primus [=*Prognathotrichus primus]W86
         |    |--‘Ancyloderes’ saltoni Blackman 1938B38
         |    `--‘Cryphalus’ sulcatus [=*Gnathotrichoides sulcatus]W86
         |--Gnathotrupes Schedl 1951 (see below for synonymy)W86
         |    |--*G. bolivianusW86
         |    |--G. caliculus [=*Gnathocortus caliculus]W86
         |    |--G. impressus [=*Gnathoglochinus impressus]W86
         |    |--G. nothofagi [=*Gnathomimus nothofagi]W86
         |    `--G. terebratus [=*Gnathotrypanus terebratus]W86
         `--Monarthrum Kirsch 1866 (see below for synonymy)W86
              |--*M. chapuisiW86
              |--M. cincinnatum [=*Trypocranus cincinnatus]W86
              |--M. comatum [=*Eupteroxylon comatum]W86
              |--M. cristatum [=*Cosmocorynus cristatus]W86
              |--M. fasciatumW86 [=Bostrichus fasciatusW86, *Corthylomimus fasciatusW86, Pterocyclon fasciatumG38]
              |--M. ingens [=*Phthorius ingens]W86
              |--M. laterale [=*Pterocyclon laterale]W86
              `--M. rufipes [=*Anchonocerus rufipes]W86

Gnathotrichus Eichhoff 1869 [incl. Ancyloderes Blackman 1938, Gnathotrichoides Blackman 1931, Paraxyleborus Hoffmann 1942, Prognathotrichus Bright 1972]W86

Gnathotrichus materiarius (Fitch 1858)A02 [=Tomicus materiariusW86; incl. *G. corthyloidesW86, Xyleborus dupreziW86, *Paraxyleborus dupreziW86]

Gnathotrichus pilosus (Leconte 1868)W86, B38 [=Cryphalus pilosusW86, *Ancyloderes pilosusW86, Cryphalus pilosulus Leconte 1868B38, Pityophthorus pilosulusB38, Pseudopityophthorus pilosulusB38]

Gnathotrupes Schedl 1951 [incl. Gnathocortus Schedl 1975, Gnathoglochinus Schedl 1975, Gnathomimus Schedl 1975, Gnathotrypanus Wood 1968]W86

Monarthrum Kirsch 1866 [incl. Anchonocerus Eichhoff 1878, Corthylomimus Ferrari 1867, Cosmocorynus Ferrari 1867, Eupteroxylon Eggers 1936, Phthorius Eichhoff 1878, Pterocyclon Eichhoff 1869, Trypocranus Eichhoff 1878]W86

*Type species of generic name indicated

References

[A02] Anderson, R. S. 2002. Curculionidae Latreille 1802. In: Arnett, R. H., Jr, M. C. Thomas, P. E. Skelley & J. H. Frank (eds) American Beetles vol. 2. Polyphaga: Scarabaeoidea through Curculionoidea pp. 722–815. CRC Press.

[B38] Blackman, M. W. 1938. Ancyloderes, a new genus of Scolytidae. Proceedings of the Entomological Society of Washington 40 (7): 204–206.

[CG05] Cognato, A. I., N. E. Gillette, R. C. Bolaños & F. A. H. Sperling. 2005. Mitochondrial phylogeny of pine cone beetles (Scolytinae, Conophthorus) and their affiliation with geographic area and host. Molecular Phylogenetics and Evolution 36: 494–508.

[G38] Gurney, A. B. 1938. Minutes of the 489th regular meeting of the Entomological Society of Washington. Proceedings of the Entomological Society of Washington 40 (4): 111–114.

Kirkendall, L. R. 1983. The evolution of mating systems in bark and ambrosia beetles (Coleoptera: Scolytidae and Platypodidae). Zoological Journal of the Linnean Society 77: 293–352.

[W86] Wood, S. L. 1986. A reclassification of the genera of Scolytidae (Coleoptera). Great Basin Naturalist Memoirs 10: 1–126.

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