Fusarium Dieback and its Shot Hole Borer Vectors Threaten Native Vegetation in California

Fall 2016 Ecesis, Volume 26, Issue 3

Dead willow trees in Tijuana River Valley.

Dead willow trees in Tijuana River Valley.

The polyphagous shot hole borer (PSHB), Euwallacea sp. nov fornicatus is an invasive beetle that vectors a disease called Fusarium dieback (FD). The disease stops the flow of water and nutrients in over 45 susceptible tree species, which can lead to the death of individual branches or, in severe cases, the entire tree. It is caused by the fungi that the beetle farm in their gallery as a food source: Fusarium euwallaceae, Graphium euwallaceae, and Paracremonium pembeum. PSHB attacks a wide variety of host species. The beetles bore tunnels (galleries) in which to lay their eggs and grow the fungi. A closely related Euwallacea species, the Kuroshio shot hole borer (KSHB), has been detected throughout Orange and San Diego Counties. Morphologically identical to PSHB, it also spreads a fungal disease caused by other species of Fusarium and Graphium.

In 2003, a single PSHB beetle was caught in a CDFA trap in Long Beach. The beetle went unnoticed until 2012 when it was found damaging backyard avocado and urban forest trees in the Los Angeles basin. A rapid monitoring response uncovered the broad host range of the pest-disease complex, but its ability to establish in native vegetation was only gradually recognized.

This wide suitable host range makes native riparian, oak woodland, and mixed evergreen communities highly susceptible to invasion and mortality by FD-SHB (Eskalen et al., 2013). By October 2015, FD-KSHB infested over 280,000 native trees in the Tijuana River Valley in San Diego County, including arroyo willow (Salix lasiolepis), Goodding’s black willow (S. gooddingii), and mule fat (Baccharis salicifolia) (Boland 2016). We continue to confirm SHB attacking native vegetation in many new areas throughout San Diego, Los Angeles, Orange, and Riverside Counties (Eskalen and Lynch pers. obs.).  

These particular plant communities are critical breeding habitat for endangered species such as the least Bell’s vireo (Vireo bellii pusillus), southwestern willow flycatcher (Epidomax traillii extimus), and arroyo toad (Anaxyrus californicus). As FD-SHB kills willows, cottonwood, and mule fat, it can make riparian habitats more susceptible to invasion by giant cane (Arundo donax) and saltcedar (Tamarix spp).

At these early stages of the epidemic, preventative and containment measures can fortunately be effective, providing adequate, rapid assessment of key landscape factors. At present, knowing which management approaches are more effective is limited by the lack of data on how the pest-disease complex spreads across a complex landscape. Monitoring efforts have recently focused on avocado groves (due to immediate investment in research and development by the agricultural industry), but have now expanded to include the broader landscape to allow for regional planning and land management that will reduce or manage this threat within southern California. — by Shannon Lynch and Akif Eskalen, University of California, Riverside (Lynch) and Santa Cruz (Eskalen)

For updates and more information, please visit our lab at www.eskalenlab.ucr.edu.

Literature Cited

Boland, J.M. (2016). The devastating impact of an invasive ambrosia beetle on the riparian habitats of the Tijuana River Valley, California. Peer J Preprints 4:e1964v1 https://doi.org/10.7287/peerj.preprints. 1964v1.

Eskalen, A., R. Stouthamer, S.C. Lynch, P.F. Rugman-Jones, M. Twizeyimana, A. Gonzalez, and T. Thibault. (2013). Host range of Fusarium dieback and its ambrosia beetle (Coleoptera: Scolytinae) vector in southern California. Plant Disease 97(7):938–951.

Lynch, S.C., A. Eskalen, and G.S. Gilbert. (2016). New pest-disease complex threatens California forests. Cal-IPC News. Vol 24, No 2.

Lynch, S.C., et al. (2016). Identification, pathogenicity and abundance of Paracremonium pembeum sp. nov. and Graphium euwallaceae sp. nov. — two newly discovered mycangial associates of the polyphagous shot hole borer (Euwallacea sp.) in California. Mycologia 108(2):313-29; DOI:10.3852/15-063.