Xyleborinus exiguus (Walker)

Kirkendall, Lawrence R. & Ødegaard, Frode, 2007, Ongoing invasions of old-growth tropical forests: establishment of three incestuous beetle species in southern Central America (Curculionidae: Scolytinae), Zootaxa 1588, pp. 53-62 : 57-58

Xyleborinus exiguus (Walker)

( Fig. 1 d)

Records: New to Americas. COSTA RICA, Puntarenas, Osa Conservation Area, Estación Esquinas, 200 m, coll. #2273, July 1993, INBio specimen barcode CRI001839038; same locality, Malaise trap, coll. #3087, June 1994, J. F. Quesada & M. Segura, INBIO barcode INB0003504987; Puntarenas, Golfito, Estaci\n Agujas, Sendero Ajo, 250–350 m elevation, coll. #57259, 18 July 1999, large diameter Brosimum utile (Moraceae) , R. Gonzalez Manual, INBio barcodes INB0003310007, INB0003310009, INB0003310010, INB0003310011, INB0003310012, INB0003310014; Puntarenas, Golfito, Estaci\n Agujas, Sendero Homo, 300 m elevation, coll. #61424, 12 Aug. 2000, in trunk, A. Azofeifa, INB0003127510.

PANAMA, Panama Prov., Gamboa, 99’N 79º44’W, 50 m elevation, 28 Dec. 1995, at lights in second growth forest, F. Ødegaard (1); Darien Prov., Darien National Park, Pirre, Rancho Frio, 30 July 2002, Malaise trap, A. Santos & R. Miranda (1); Colon Prov., San Lorenzo Protected Area, 9º17’N 79º58’W, 130 m elevation, old-growth forest, 22 Sept. 2003 – 30 Oct. 2004, ground level UV light traps, R. Kitching (2), ground level flight intercept traps, A. Tishechkin (19), sticky traps, Y. Basset (14), pit fall traps, E. Medianero (1), canopy fogging, J. Schmidl & J. Bail (1), flight intercept traps at different heights, R. K. Didham, L. L. Fagan & M. Rapp, 0 m (53), 1.3 m (61), 7 m (33), 14 m (91), 21 m (115), 28 m (84), 35 m (26); Panamá Prov, Alto de Espave, 100 m elevation, 19 Dec. 2003, flight intercept trap, L. Guerra (1); Panama Prov., Cerro Azul, 800 m elevation, 4 Mar. 2004, flight intercept trap, L. Guerra (1). In addition, there are many specimens of this species in vials of unidentified Scolytinae collected by Hector Barrios (Univ. Panamá) from flight intercept traps in the dry tropical forest of Parque Natural Metropolitano ( Panamá Prov., close to Panama City) in 1996–1997 (Kirkendall, unpublished identifications).

Diagnosis. This species was identified by direct comparison with Sri Lanka specimens determined by Karl E. Schedl and Malaysian specimens determined by Roger A. Beaver. The overall shape, conical scutellum (diagnostic for Xyleborinus ), and pattern of teeth on the declivital margin ( Fig. 1 d) are characteristic.

Comments. This small, slender Oriental ambrosia beetle occurs naturally from Sri Lanka to Southeast Asia, and Indonesia to Sulawesi ( Beaver, 2005). In what was the first report of the species from West Africa (from Gabon), Xyleborinus exiguus was one of the most numerous of the scolytines collected during intensive insect trapping from Jan.–Mar. 1999 ( Basset et al., 2001; Beaver, 2005).

Multiple specimens have been collected from two localities in southwest Costa Rica (Osa Peninsula and Golfito), and hundreds from scattered localities in central Panama, plus one from Darien National Park in southern Panama. Based on these records, it is probable that there is one continuous population in southern Central America, stretching from southwestern Costa Rica to (at least) the border between Panama and Colombia, but this needs to be corroborated by further collecting. It is only a matter of time before this species becomes established in South America; S. L. Wood did not see specimens of this species during his preparation of a monograph of the bark beetles of South America ( Wood, 2007). It seems likely that this population was introduced to Panama via shipping from Asia, though a West African origin cannot be ruled out.

Xyleborinus exiguus does not seem to be size-selective in host use ( Browne, 1961). Data from Panama indicate that this is a dominant scolytine in the canopy, but unlike X. crassiusculus it shows no height preferences. It is not considered to be an aggressive species ( Browne, 1961); however, all ambrosia beetles are considered potential vectors for introducing pathogenic fungi ( Beaver, 1988; Kühnholz et al., 2001).

Figures

FIGURE 1. Invasive Asian xyleborine ambrosia beetles in Central American forests. (a – c) Xylosandrus crassiusculus: the shape (a), size (2.2 – 2.5 mm), and dull declivity with dense long setae and dense, scattered granules (c) are characteristic. (d) Xyleborinus exiguus: note the plain, convex declivity with a distinctive border of large teeth at the apex. (e – h) Euwallacea fornicatus: arrow (g) indicates suture on the posterior face of the antennal club (upper antenna in figure shows anterior face).