Polyphagous shot hole borer and Kuroshio shot hole borer (Euwallacea spp.) and Fusarium fungus
NOTE: the Polyphagous and Kuroshio shot hole borers and their associated fungi – primarily Fusarium – 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 (California Forest Pest Council 2015). [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 (California Avocado Commission 2014).
The original disease is caused by a new, previously undescribed fungus – Fusarium euwallacea – that forms a symbiotic relationship with the beetle. The beetles transport the fungus; in turn, the beetle larvae live in galleries within the tree and feed on the fungus. The two beetles – the polyphagous shot hole borer (PSHB) and Kuroshio shot hole borers (KSHB) – are morphologically indistinguishable from the tea shot hole borer, Euwallacea fornicatus, an ambrosia beetle from Asia. However, scientists suspect that the California beetles are new species, based on the large differences in DNA sequences between the beetles invading California and beetles from tea plantations in Asia. The beetles discovered in California is smaller than a sesame seed (about 0.1 inch in length). The PSHB has been causing damage to commercial avocado orchards in Israel since 2009 (Eskalen pers. comm. 2012). The PSHB is native to Vietnam (California Forest Pest Council 2015). It has been in California since at least 2003, when it was found on some black locust (Robinia pseudoacacia) (Eskalen et al. 2013).
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 (Eskalen et al. 2013). (More recent studies have found the beetle in more than 300 species (Coleman, 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 (Eskalen et al. 2013).
Scientists have completed an initial analysis of urban forests in cities in three regions of southern California – the Inland Empire, Coastal Southern California, and Southwest Desert (McPherson, G. to Kabashima, 2016). Together, these comprise 4,244 sq. miles and have 20.5 million residents. The scientists found that:
Approximately 26.8 million trees, 37.8% of the region’s 70.8 million trees, are at risk. They include :
- 4.5 million coast live oaks,
- 1.4 million ash,
- 1.3 million sycamores and plane trees,
- 2.9 million stone fruit or flowering Prunus species,
- 2.5 million avocadoes, and
- 1.8 million citrus trees.
The cost for removing and replacing the 26.8 million trees would be approximately $36.2 billion. This amounts to $1,768 per capita.
The value of ecosystem services forgone each year due to the loss of these trees is $1.4 billion.
These estimates are conservative because they: do not include costs associated with damage to people and property from tree failures, as well as increased risk of fire and other hazards; may undervalue benefits of trees to human health and well-being; and do not include newly detected host species or the shot borers’ spread.
Thirty eight of the more than 300 tree species that are attacked by the PSHB are known to be 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 (California Forest Pest Council).
When beetles attack a tree, there are several potential outcomes (University of Southern California):
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 shown to attack all size classes (Coleman, 2016).
In September 2017, scientists announced that the Fusarium euwallaceae fungus – the primary fungus transported by the PSHB – can infect almond trees (Prunus dulcis). California growers – primarily in the San Joaquin Valley – produced $5.33 billion worth of almonds in 2015 (California Agricultural Production Statistics – https://www.cdfa.ca.gov/statistics/ ).
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)
- Arroyo willow (Salix lasiolepsis)
- Goodding’s black willow (Salix gooddingii)
- Coast live oak (Quercus agrifolia)
- Engelmann Oak (Quercus engelmannii)
- Valley oak (Quercus lobata)
- Canyon live oak (Quercus chrysolepis)
- Fremont Cottonwood (Populus fremontii)
- Black cottonwood (Populus trichocarpa) *
- White alder (Alnus rhombifolia)
- Blue palo verde (Cercidium floridum)
- Palo verde (Parkinsonia aculeata)
- Mesquite (Prosopis articulata)
- Mule Fat (Baccharis salicifolia)
- California buckeye (Aesculus californica)
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)*
- Brea (Cercidium sonorae)
- Weeping willow (Salix babylonica)
- Red Flowering Gum (Eucalyptus ficifolia)
- Tree of heaven (Ailanthus altissima)
- Kurrajong (Brachychiton populneus)
- Black mission fig (Ficus carica)
- Japanese beech (Fagus crenata)
- Dense logwood/Shiny xylosma (Xylosma congestum)
- Black Poplar (Populus nigra)
- Carrotwood (Cupaniopsis anacardioides)
- Kentia Palm (Howea forsteriana)
- King Palm (Archontophoenix cunninghamiana)
- Tamarix (Tamarix ramosissima)
- Almond (Prunus dulcis)
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 (Eskalen et al. 2016).
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 (Eskalen et al. 2016). In San Diego County alone, more than 58,000 acres of riparian woodlands are at risk (California Forest Pest Council). The U.S. Fish and Wildlife Service is concerned that loss of riparian willows will undermine efforts to protect the least Bell’s vireo, an endangered bird species.
The disease’ impact might vary depending upon ecological factors. In the Tijuana River valley along the California-Mexico border, the Kuroshio shot hole borer (KSHB) caused considerable damage initially. After two years, 88% of the willows in the riparian area – 355,510 trees – were infested. An estimated 24% (95,791 trees) had been killed. Nearly all of the infested and killed trees grew in the wettest parts of the riparian forests (Boland 2018).
By 2017, however, the willows were resprouting vigorously. By late 2017, these resprouts had created a new forest canopy that was about 5 meters tall. (Previously, the canopy had been about 20 meters tall). The median rate of infestation of these resprouting willows was only 6% in 2017, down from 97% in 2015-2016. Some insect boring holes have healed. The new willow canopy appears to support continued breeding by the least Bell’s vireo (Boland 2018).
Willows growing in drier parts of the Tijuana River valley had rarely been attacked initially, but were increasingly infested over time. In 2017, the median infestation rate was 78%, up from 9% in 2015-16. However, few trees have been killed (Boland 2018).
Dr. Boland suggests that the severity of the initial attack reflected the vulnerability of “soft trees”. Trees growing in the wetter parts of the Tijuana River Valley are inundated by sewage from the Mexican city. As a result of this artificial fertilization, they grow quickly and their wood is less dense (Boland 2018).
As expected, the damage to native willows has led to proliferation of invasive plants, especially castor bean, salt cedar, and giant reed (Boland 2018).
For more information on these pests, please visit:
- UC Polyphagous Shot Hole Borer Pest Information
- UC Riverside, Center for Invasive Species, PSHB Species Profile
- UC Riverside, Eskalen Lab research page
Boland, J.M. 2016. The impact of an invasive ambrosia beetle on the riparian habitats of the Tijuana River Valley, California. Southwest Wetlands Interpretive Association, Imperial Beach, CA, US https://peerj.com/articles/2141.pdf
Boland, J.M. 2018. The Kuroshio Shot Hole Borer in the Tijuana River Valley in 2017-18 (Year Three): Infestation Rates, Forest Recovery, and a New Model. A Final Report for the US Navy & US Fish and Wildlife Service and Southwest Wetlands Interpretive Association
California Avocado Commission. 2014. GROWER ALERT: Polyphagous Shot Hole Borer / Fusarium Fungus Found in Commercial Avocado Grove in Escondido, CA. http://californiaavocadogrowers.com/articles/polyphagous-shot-hole-borer-fusarium-fungus-found-commercial-avocado-grove-escondido-ca; visited September 24, 2014.
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
Coleman, T. 2016. Presentation to the North American Forest Insect Working Group, June 2016.
Eskalen, A. Pers. comm. September 2012. Arakelian, G. email to several recipients 13 March 2012.
Eskalen, A. Email to colleagues, December 2013.
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.
McPherson, G. 2016. USDA Forest Service. Memorandum to John Kabashima Re: Potential Impact of PSHB and FD on Urban Trees in Southern California, April 26, 2016
Moreno, K., J.D. Carrillo, F. Trouillas, A. Eskalen. 9/24/2017 Almond (Prunus dulcis) is susceptible to Fusarium euwallaceae, a fungal pathogen vectored by the Polyphagous Shot Hole Borer in Calif | Plant Disease. http://apsjournals.apsnet.org/doi/abs/10.1094/PDIS-07-17-1110-PDN
Stouthamer, R. 2014. Presentation at the 25th USDA Interagency Research Forum on Invasive Species, January 7 – 10, 2014.
University of California. Pests and Diseases of southern California oak trees. http://ucanr.edu/sites/socaloakpests/Polyphagous_Shot_Hole_Borer/# accessed 24 June, 2013;