Xylosandrus

This page provides a brief introduction to the bark beetle genus Xylosandrus and the work I will be doing for my Ph.D dissertation at Michigan State University. Please check back occasionally, I will be updating this page as I get deeper into my research! In the meantime, have fun exploring the diversity of xyleborines by visiting the Xyleborini PEET Database.

Taxonomic Overview

  • Order Coleoptera = Beetles
  • Family Curculionidae = Weevils
  • Subfamily Scolytinae = Bark Beetles
  • Tribe Xyleborini = Ambrosia Beetles*
  • Genus Xylosandrus
Xylosandrus crassiusculus (Blandford) female (~ 2.8 mm) and male (~1.0 mm).

PEET Who?

My dissertation research at Michigan State is on the systematics of the bark beetle genus Xylosandrus. This work is part of the National Science Foundation (NSF) Partnership for Enhancing Expertise in Taxonomy (PEET) funded project “Monographic research of tropical bark beetles (Scolytinae: Xyleborini): Preservation and modernization of taxonomic expertise.” For my research I will be revising Xylosandrus - establishing species boundaries and distributions, as well as producing evolutionary phylogenies based on morphological and molecular data.

Biology

As is typical of bark beetles (Curculionidae: Scolytinae), Xyleborini beetles construct and inhabit galleries within the woody tissues of host trees. Xylosandrus are “ambrosia beetles”, which means they feed primarily on a symbiotic ambrosia fungus which grows along the walls of their galleries. Colonization begins when a female beetle locates a host tree and initiates gallery construction. As she bores into the tree, the female inoculates the woody tissue lining her tunnels with a suite of microorganisms, including ambrosia fungus (Kirkendall, 1993).

Once the ambrosia fungus has begun growing on the walls of the gallery, the female beetle lays eggs in small clusters at the end of the main tunnel. After hatching, the Xylosandrus larvae feed on the ambrosia lining the tunnel walls. As soon as they have pupated into adults, male beetles mate with their sisters. The females then leave the parent gallery and fly off to establish their own (Kirkendall, 1993)

Xyleborini beetle species frequently inbreed and have skewed (NOT 1:1, female:male) sex ratios. The female to male sex ratio can be as high as 30:1 (Bright, 1968). Xyleborini are also highly sexually dimorphic, with dwarfed, flightless males that bear little resemblance to their female counterparts (Kirkendall, 1993).

Another interesting fact about Xyleborini reproduction is that they are haplodiploid, with males possessing only one set of chromosomes, while the female has the “normal” two (Wood, 1986). This is the same kind of chromosomal system that occurs in many social insects, such as the honey bees and the ants. Being haplodiploid means that famale Xyleborini beetles can produce male offspring without mating.

History of the Genus

  • The genus Xylosandrus was established by Reitter in 1913 with Xyleborus morigerus designated as the type.
  • In 1962, Schedl disagreed and listed Xylosandrus as a synonym of the genus Xyleborus.
  • In 1963 Browne noted differnces that he felt justified the genus and moved 16 species of Xyleborus into Xylosandrus.
  • In 1964 Schedl later agreed with Browne’s assessment and added 8 more species of Xyleborus to Xylosandrus.

Species

  • As of Wood and Bright’s 1992 catalog, there were 52 described species of Xylosandrus worldwide.
  • 5 Xylosandrus species currently occur in North America. Of these, only one is native, X. zimmermanni (Hopkins). The other four, X. compactus (Eichhoff), X. germanus (Blandford), X. crassiusculus (Motschulsky), and X. multilatus (Blandford) have been introduced from the Old World tropics (Rabaglia, 2002 & per. com.).

Diagnostic Characters
(from Bright, 1968)

Females

  • Body: very stout, cylindrical.
  • Frons: convex, punctures shallow, fine; longitudinal carina faint.
  • Antennal club: obliquely truncate, densely pubescent on oblique portion.
  • Pronotum: wider than long; basal and lateral margins with a fine raised line; anterior margin with a row of asperites; anterior slope with erect, closely placed asperites, these arranged in vague concentric rows; posterior portion smooth, sometimes punctate.
  • Scutellum: flat.
  • Elytra: less than 1.3 times longer than pronotum; striae punctured in regular rows, punctures fine; interspaces smooth, very finely punctured, setose; declivity convex, unarmed, ridge of seventh interspace distinct.
  • Anterior coxae: widely separated.

Males

  • Body: much smaller than females, strongly convex.
  • Pronotum: as long as wide, sometimes flattened or concave in anterior portion.
  • Elytra: strongly convex, narrowly rounded behind; strail and interstrail punctures distinct on disc, irregularly placed on sides; ridge of seventh interspace variably developed on declivity.

Literature Cited

Bright, D. E. 1968. Review of the tribe Xyleborini in America North of Mexico (Coleoptera: Scolytidae). The Canadian Entomologist. 100: 1288-1323.

Browne, F. G. 1963. Taxonomic notes on Scolytidae (Coleoptera). Ent. Ber., Amst. 23:53-59.

Kirkendall, L. R. 1993. Ecology and evolution of biased sex ratios in bark and ambrosia beetles. In: Wrensch, D. L. and M. A. Ebbert. Evolution and Diversity of Sex Ratio in Insects and Mites. Chapman & Hall, New York.

Rabaglia, R. J. 2002. Curculionidae: Scolytinae Latreille 1807. In: Arnett, R. H. et al. American Beetles, Volume 2. CRC Press, Boca Raton.

Reitter, E. 1913. Bestimmungs-tabelle der Borkenkafer (Scolytidae) aus Europa und den angrenzenden Landren. Wiener Entomologische Zeitung 32 (Beiheft):1-116.

Schedl, K. E. 1962. Scolytidae und Platypodidae Afrikas. Rev. Ent. Mocamb 5(1):1-593.

Schedl, K. E. 1964. Zur Synonymie der Borkenkafer XIV. 223. Beitrag zur Morphologie und Systematik des Scolytidae. Reichenbachia 2(57):209-223.

Wood, S. L. 1986. A reclassification of the genra of Scolytidae (Coleoptera). Great Basin Naturalist Memoirs 10.

Wood, S. L. and D. E. Bright. 1992. A catalog of Scolytidae and Platypodidae (Coleoptera), Part 2: Taxonomic Index, Volumes A and B. Great Basin Naturalist Memoirs 13.

*Note: There are other groups that are referred to as “ambrosia beetles”, including members of the subfamily Platypodinae.

©2006 S. A. Dole