Geosmithia associated with bark beetles and woodborers in the western USA: taxonomic diversity and vector specificity

ABSTRACT

Fungi in the genus Geosmithia (Ascomycota: Hypocreales) are frequent associates of bark beetles and woodborers that colonize hardwood and coniferous trees. One species, Geosmithia morbida, is an economically damaging invasive species. The authors surveyed the Geosmithia species of California and Colorado, USA, to (i) provide baseline data on taxonomy of Geosmithia and beetle vector specificity across the western USA; (ii) investigate the subcortical beetle fauna for alternative vectors of the invasive G. morbida; and (iii) interpret the community composition of this region within the emerging global biogeography of Geosmithia. Geosmithia was detected in 87% of 126 beetle samples obtained from 39 plant species. Twenty-nine species of Geosmithia were distinguished, of which 13 may be new species. Bark beetles from hardwoods, Cupressus, and Sequoia appear to be regular vectors, with Geosmithia present in all beetle gallery systems examined. Other subcortical insects appear to vector Geosmithia at lower frequencies. Overall, most Geosmithia have a distinct level of vector specificity (mostly high, sometimes low) enabling their separation to generalists and specialists. Plant pathogenic Geosmithia morbida was not found in association with any other beetle besides Pityophthorus juglandis. However, four additional Geosmithia species were found in P. juglandis galleries. When integrated with recent data from other continents, a global pattern of Geosmithia distribution across continents, latitudes, and vectors is emerging: of the 29 Geosmithia species found in the western USA, 12 have not been reported outside of the USA. The most frequently encountered species with the widest global distribution also had the broadest range of beetle vectors. Several Geosmithia spp. with very narrow vector ranges in Europe exhibited the similar degree of specialization in the USA. Such strong canalization in association could reflect an ancient origin of each individual association, or a recent origin and a subsequent diversification in North America.

KEYWORDS:

• Entomochoric fungi
• Geosmithia morbida
• Pityophthorus juglandis
• subcortical beetles
• symbiosis

Acknowledgments

We are grateful to Andrew D. Graves for assistance with sampling; Celie Korittová for molecular analyses; and Anthony I. Cognato for assistance with insect identification. This work was supported by the Czech Grant Agency (grant no. 16-15293Y), the Long-Term Research Development Project (grant number RVO 67985939), and Ministry of Education, Youth and Sports of the Czech Republic (grant number LO1509) and by funds provided by the USDA NIFA Western Region IPM Center and Critical Issues—Emerging and New Plant and Animal Pests and Diseases grant programs; the USDA Forest Service Forest Health Monitoring Program (Detection Monitoring Grant No. INT-DM-09-01 and Evaluation Monitoring Grant No. INT-EM-B-11-03); and USDA Forest Service Special Technology Development Grant R4-2011-01 (administered by A. S. Munson). In-kind support was provided by the USDA Forest Service Pacific Southwest Research Station. J.H. was supported by USDA Forest Service cooperative agreements 11-CA-11330129-092 and 12-CA-11420004-042, and by NSF DEB 1256968.

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