Polyphagous shot hole borer

Euwallacea sp.

Last updated by: 
Faith Campbell

Polyphagous shot hole borer (Euwallacea sp.) and Fusarium fungus

NOTE: the Polyphagous shot hole borer and Fusarium fungus complex attacking avocadoes and other trees in California are completely unrelated organisms to Laurel wilt, despite both being beetle-fungi complexes that affect avocado. Click here to read about Laurel wilt, an ambrosia beetle Xyleborus glabratus and associated fungus Raffaelea lauricola.

 

In late winter 2012, attention was drawn to a new insect/pathogen complex in Southern California as a result of damage to several backyard avocado trees in residential neighborhoods and a commercial avocado grove in Los Angeles County.  The insect/pathogen complex is now known to be found over a wide area including most of Los Angeles County and parts of Orange County.  Smaller outbreaks are found in San Bernardino and Riverside counties.  An outbreak discovered in late 2013 in San Diego County – was later determined to be a second species of insect, the Kuroshio shot hole borer.[i]  [see map]. This outbreak is 60 miles from the nearest outbreak, and indications are that the presence in San Diego is the result of a separate introduction, not spread from Los Angeles.

In 2014, the disease was detected for the first time in a commercial avocado nursery.  A “grower alert” suggested steps growers should take to avoid spreading the disease.  The focus was on disposal of wood  from infested trees – including as firewood, pruning debris, or chips; and on ensuring that twigs and leaves are not present in bins used to ship the fruit.  Growers are also advised to inspect not just their avocado trees but other hosts (see below) growing near their orchards. [iii]

The original disease is caused by a new, previously undescribed fungus -- Fusarium euwallacea -- that forms a symbiotic relationship with the beetle.  The beetle transports the fungus; in turn, the beetle larvae live in galleries within the tree and feed on the fungus.  The beetle has been named the polyphagous shot hole borer (PSHB).  This beetle is morphologically indistinguishable from the tea shot hole borer, Euwallacea fornicatus, an ambrosia beetle from Asia. However, scientists suspect that the California beetle is a new species, based on the large differences in DNA sequence between the beetle invading California and beetles from tea plantations in Asia. The beetle discovered in California is smaller than a sesame seed (about 0.1 inch in length). The identical new beetle species has been causing damage to commercial avocado orchards in Israel since 2009. [iv]  The polyphagous shot hole borer is native to Vietnam.[i] It has been in California since at least 2003, when it was found on some black locust (Robinia pseudoacacia). [v]

The second beetle, named the Kuroshio shot hole borer, is from Taiwan.  It transports a different group of fungi. 

Studies at the Los Angeles County Arboretum and Huntington Library, Arts Collections, and Botanical Gardens have identified more than 200 species of tree, shrub, or vine that are attacked by the PSHB; these plants are in 58 plant families from every continent except Antarctica. [vi]
 (More recent studies have found the beetle in more than 300 species.  T. Coleman, presentation to the North American Forest Insect Working Group, June 2016.) The Fusarium fungus was detected in 54% of these trees – 113 species.  Among the trees attacked by the PSHB are 11 species native to the southern California; 13 agriculturally important trees; and 53 species widely used in urban plantings.  Analysis of a typical metropolitan area’s tree survey indicates that these latter constitute more than half of all trees planted in urban areas of southern California.  [vi]
The costs to cities and homeowners of dealing with the disease could be substantial; among hosts are such large trees as coast live oak and several Eucalyptus.

 Thirty eight of the tree species are known reproductive hosts for PSHB.  To date, 14 trees have been confirmed as reproductive hosts for the Kuroshio shot hole borer; more might be detected by continuing research.[i]  

When beetles attack a tree, there are several potential outcomes: [xii]

1. The beetle is repelled and causes no infection. This has been observed in 20 species of trees. Investigators are trying to figure out what features of the tree might repel the beetle.

2. The beetle drills into the tree and transmits the fungus, but doesn’t produce offspring. This has been observed in over 50% of the tree species attacked. Scientists don’t know the final outcome of this interaction. If the beetle has penetrated to the xylem, this could cause dieback of branches. Damage could also make the tree more prone to attack from other pest species.

3. The beetle drills into the tree, fungus infects the tree, and the beetle produces offspring in the tree. This has been seen in about 8% of the tree species attacked, and these species are considered reproductive hosts of PSHB. Some trees seem to suffer mild symptoms like branch die-back, while others are killed outright. 

The greatest injury is caused to box elder, sycamore, cottonwood, and willows.  The willows might re-sprout from the roots … but the PSHB has been sown to attack all size classes (Coleman, 2016).

 

We present here a partial list of reproductive hosts for PSHB; those that are also reproductive hosts of KSHB are marked by an asterisk.  Full list available here http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=19197

 Hosts native in southern California:

  • Box elder (Acer negundo)                             
  • Big leaf maple (Acer macrophyllum) *         
  • California Sycamore (Platanus racemosa)*
  • Red Willow (Salix laevigata)
  • Goodding's black willow (Salix gooddingii)
  • Coast live oak (Quercus agrifolia)
  • Engelmann Oak (Quercus engelmannii)       
  • Valley oak (Quercus lobata)                         
  • Cottonwood  (Populus fremontii)                 
  • Black cottonwood (Populus trichocarpa)     
  • Mesquite (Prosopis articulata)                     

 

Hosts that are exotics but widespread in southern California:

  • Avocado (Persea americana)*
  • Castor bean (Ricinus communis)
  • English Oak (Quercus robur)
  • London plane (Platanus x acerifolia)
  • Coral tree (Erythrina corallodendon)*
  • Blue palo verde (Cercidium floridum)
  • Palo verde (Parkinsonia aculeata)
  • Brea (Cercidium sonorae)
  • Weeping willow (Salix babylonica)
  • Red  Flowering Gum  (Eucalyptus ficifolia

 

Hosts that are native or widespread exotics in the Southeastern states:

  • Box elder (Acer negundo) (repeated from above)
  • Liquidambar (Liquidambar styraciflua)
  • Japanese wisteria (Wisteria floribunda)       
  • Tree of heaven (Alianthus altissima)

 

Hosts that are sold interstate in the nursery trade (note that PSHB, at least, has attacked branches as small as 2.5 cm – Coleman, 2016):

  • Japanese maple (Acer palmatum)
  • Camelia (Camellia semiserrata)
  • Chinese holly (Ilex cornuta

 

Avocado, maples, coast live oak, and sweetgum are in very different botanical families (Laureaceae, Aceracae, Fagaceae, Hamamelidaceae) -- so the host range could be quite large.  Thirty three families contain both species that are hosts of the beetle and/or the fungus; and species that apparently do not host them. [xiv]

 

Given the large number of tree types that can be infested by one or both of the beetles, and the growing list of species that support beetle reproduction, it appears highly likely that the disease complex could be transported in firewood.  Firewood suppliers and customers, as well as regulatory agencies should take action to ensure that this does not happen.

 

Symptoms of PSHB attack and fungus infection differ among tree species. For illustrations of the symptoms on various species, visit www.eskalenlab.ucr.edu

 

Some California ecosystems are at particular risk because they are dominated by susceptible tree or shrub species.  These vulnerable ecosystems are mixed evergreen forests, oak woodlands, foothill woodlands, & riparian habitats. [vii]  Already, the Kuroshio shot hole borer (KSHB) is causing considerable damage in the Tijuana River Valley Regional park.  More than 144,000 plants are estimated to be infested or dead.  In San Diego County alone, more than 58,000 acres of riparian woodlands are at risk.[i]  The trees being killed are arroyo willow, black willow and castor bean [http://www.californiaavocadogrowers.com/articles/polyphagous-and-kuroshio-shot-hole-borer-recent-finds].

Drought does not appear to be a contributing factor as both beetles thrive in riparian and well-watered trees.[i]

 

For more information on this pest, please visit:

 

 

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[i] California Forest Pest Council. 2015.  2015 California Forest Pest Conditions. http://bofdata.fire.ca.gov/hot_topics_resources/2015_california_forest_pest_conditions_report.pdf

[ii] Richard Stouthamer, at the 25th USDA Interagency Research Forum on Invasive Species, January 7 – 10, 2014.

[iii]GROWER ALERT: Polyphagous Shot Hole Borer / Fusarium Fungus Found in Commercial Avocado Grove in Escondido, CA. California Avocado Commission. http://californiaavocadogrowers.com/articles/polyphagous-shot-hole-borer-fusarium-fungus-found-commercial-avocado-grove-escondido-ca; visited September 24, 2014.

[iv] A. Eskalen, pers. comm. September 2012. Arakelian, G. email to several recipients 13 March 2012.

[v] Eskalen, A., Stouthamer, R., Lynch, S. C., Twizeyimana, M., Gonzalez, A., and Thibault, T. 2013. Host range of Fusarium dieback and its ambrosia beetle (Coleoptera: Scolytinae) vector in southern California. Plant Dis. 97:938-951.

[vi]Eskalen, A., Stouthamer, R., Lynch, S. C., Twizeyimana, M., Gonzalez, A., and Thibault, T. 2013. Host range of Fusarium dieback and its ambrosia beetle (Coleoptera: Scolytinae) vector in southern California. Plant Dis. 97:938-951.

[vii] Eskalen, A., Stouthamer, R., Lynch, S. C., Twizeyimana, M., Gonzalez, A., and Thibault, T. 2013. Host range of Fusarium dieback and its ambrosia beetle (Coleoptera: Scolytinae) vector in southern California. Plant Dis. 97:938-951.

[viii] Eskalen, A., Stouthamer, R., Lynch, S. C., Twizeyimana, M., Gonzalez, A., and Thibault, T. 2013. Host range of Fusarium dieback and its ambrosia beetle (Coleoptera: Scolytinae) vector in southern California. Plant Dis. 97:938-951.

[ix] Eskalen, A., Stouthamer, R., Lynch, S. C., Twizeyimana, M., Gonzalez, A., and Thibault, T. 2013. Host range of Fusarium dieback and its ambrosia beetle (Coleoptera: Scolytinae) vector in southern California. Plant Dis. 97:938-951.

[x] Gevork Arakelian, Ph.D. Senior Biologist, Los Angeles County Department of Agricultural Commissioner, pers. comm. January, 2013.

[xi] Eskalen, A., Stouthamer, R., Lynch, S. C., Twizeyimana, M., Gonzalez, A., and Thibault, T. 2013. Host range of Fusarium dieback and its ambrosia beetle (Coleoptera: Scolytinae) vector in southern California. Plant Dis. 97:938-951.

[xii] University of California.  Pests and Diseases of southern California oak trees. http://ucanr.edu/sites/socaloakpests/Polyphagous_Shot_Hole_Borer/#  accessed 24 June, 2013;

[xiii] A. Eskalen, email to colleagues, December 2013. 

[xiv] Eskalen, A., Stouthamer, R., Lynch, S. C., Twizeyimana, M., Gonzalez, A., and Thibault, T. 2013. Host range of Fusarium dieback and its ambrosia beetle (Coleoptera: Scolytinae) vector in southern California. Plant Dis. 97:938-951.

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