Impacts of the invasive shot hole borer (Euwallacea kuroshio) are linked to sewage pollution in southern California: the Enriched Tree Hypothesis
By: John M. Bolandand Deborah L. Woodward
The Kuroshio Shot Hole Borer (KSHB, Euwallacea kuroshio) and the Polyphagous Shot Hole Borer (E. whitfordiodendrus; Coleoptera: Curculionidae: Scolytinae) have recently invaded southern California and are attacking live trees in commercial agriculture groves, urban parks and native riparian forests. Among native forests the worst impacts observed to date have been in the Tijuana River Valley in south San Diego County, where approximately 30% of the native willows (Salix spp.), or120,000 trees, have died as a result of a KSHB infestation. This paper examines wood densities, wood moisture contents, KSHB infestation rates, and KSHB-induced mortality rates in two willow species (Salix lasiolepis and S. gooddingii) at sites near and far from sewage input. Comparisons were made on two spatial scales: broadly among sites within San Diego County; and locally among sites within the Tijuana River Valley. The results showed that, on average, willow trees growing closest to sewage pollution had significantly lower wood density, higher wood moisture content, higher KSHB infestation rates, and higher KSHB-induced willow mortality rates than those growing farther away. We present the Enriched Tree Hypothesis to explain the link between sewage pollution and KSHB impacts; it is as follows: (A) Riparian trees subject to nutrient enrichment from frequent sewage pollution grow quickly, and their fast growth results in wood of low density and high moisture content. If attacked by the KSHB, the trunks and branches of these nutrient-enriched trees provide an environment conducive to the fast growth of the symbiotic fungi upon which the KSHB feeds. With an abundant food supply, the KSHB population increases rapidly and the trees are heavily damaged by thousands of KSHB galleries in their trunks and branches. (B) Riparian trees not subject to frequent sewage pollution grow more slowly and have denser, drier wood. Conditions in their trunks and branches are not conducive to the fast growth of the KSHB's symbiotic fungi. The KSHB generally ignores, or has low abundances in, these slow-growing trees. This new hypothesis explains current patterns of KSHB impact in San Diego County and focuses attention on the important roles of the environment and preexisting conditions of trees in determining the extent of KSHB impact .It highlights the Tijuana River Valley as an unusual site due to high sewage inputs and predicts that the high KSHB-induced willow mortality seen there should not occur in other natural riparian habitats in southern California. Most importantly, by identifying sewage pollution (or nutrient enrichment) as a major risk factor for KSHB impacts, the hypothesis ratchets down the KSHB-threat level for most riparian sites in southern California and directs attention to other nutrient-enriched sites as those most at risk.
The complete article can be found at:
Two newly published scientific journal articles that address SHB species delineation are:
- Gomez et al. Species Delineation Within the Euwallacea fornicatus. in Insect Systematics and Diversity, (2018) 2(6): 2; 1–11. (Attached) Richard Stouthamer and Paul Rugman-Jones, both with UC Riverside, are two of the co-authors
- Hulcr, J & Landers, J. January 7, 2019. So Many Shot Hole Borers: New research charts four nearly identical species. in Entomology Today. https://entomologytoday.org/2019/01/07/so-many-shot-hole-borers-new-research-charts-four-nearly-identical-species/
Examples of ambrosia beetle impact on willows. (A top) KSHB excavate galleries within a trunk and push the sawdust tailings out of their entrance holes. (B lower) Trees can be undermined by many galleries and snap in high winds. [These pictures show the extremes – infested trunks do not always look like A, and infested trunks do not always break, like B. Images: John Boland]
So Many Shot Hole Borers: New Research Charts Four Nearly Identical Species
by Jiri Hulcr, Ph.D., and Jackson Landers
When an insect spends most of its life in total darkness, it doesn't much matter what color it is. So, it comes as no surprise that so many species of bark and ambrosia beetles maintain the same brown hue as they slowly tunnel through wood and feed on a fungus that they carry with them into their trees. This similarity of appearance has been taken to an extreme in what has turned out to be a cryptic species complex. What was once referred to as the “tea shot hole borer” is actually four distinct species who all look almost exactly the same.
The tea shot hole borer first attracted notice from North American entomologists when it appeared in Florida in 2012, appearing harmless. But the beetles were first described as an economically significant pest in Sri Lanka in 1968. In 2009, the beetles were found eating through avocado and street trees in Israel. Then, in 2012 in California, with a bang, avocado trees were being attacked and killed. Given the value of global avocado crops and tea plants, entomologists had to start taking a closer look at these beetles.
Many taxonomists eventually came around to the idea that they were looking at three identical species rather than one: the tea shot hole borer (from southern Southeast Asia), the Kuroshio shot hole borer (originating in the Pacific Islands), and the polyphagous shot hole borer (presumed native to northern Southeast Asia). A 2017 paper authored by Richard Stouthamer and his team from the University of California, Riverside, first designated those three clades and established common names for them.
The four newly delineated species of the Euwallacea fornicatus species complex are, from left to right, E. fornicatior, E. fornicatus, E. whitfordiodendrus, and E. kuroshio. The wood-boring beetles known as various kinds of “shot hole borers” are so similar in morphological characteristics while also variable in body dimensions that their appearance can't be reliably used for differentiating specimens. (The four shown here vary in size but some of their cousins within each species can all range from 1.8 to 2.9 millimeters long.) New research has used molecular genetic techniques to identify the different species within the complex. (Photo credit: Demian Gomez)
And read more from the folks at UC:
Come Learn About Field Identification of Invasive Shot Hole Borers
We're holding two early December trainings on invasive polyphagous and Kuroshio shot hole borer biology, identification, surveillance, and management of infested trees and downed wood. We'll cover these topics in the classroom, then head outside to see infested trees and learn how to identify signs of shot hole borer damage, set up a monitoring program, and sample trees.
$30.00 Registration fee includes lunch, a ISHB Field Guide, and ISHB Demonstration Kit
Continuing Education Units from DPR have been requested, check back for updates.
Speakers include Sabrina Drill, UCCE Natural Resources Advisor; Bea Nobua-Behrmann, UCCE Research Scientist; Kim Corella, Forest Pest Specialist, CalFire; and Paul Rugman-Jones, Research Entomologist, UC Riverside.
Ventura County - Ojai - Dec.6
Meiners Oaks (Ojai) Class & Field Training at Saint Thomas Aquinas Church
Thursday December 6, 2018, 10am – 2:30pm
Los Angeles County - Gardena - Dec. 7th
Gardena Class & Field Training at Gardena Moneta Mason Lodge & Gardena Willows Wetland Preserve
Friday December 7, 2018, 10am – 3:00pm
- Author: Tunyalee Martin
UC ANR Integrated Pest Management Program
A sugar volcano is one symptom that shows your avocado tree might be infected with Fusarium dieback, a fungi spread by a beetle called the shothole borer. But what you might see if your tree is being attacked by shothole borer, varies among the different kinds of tree hosts. The symptoms—staining, sugary exudate, gumming and beetle frass—are often noticed before the tiny beetles (1.5–2.5 mm) are found.
As its name suggests, these beetles bore into trees. Near or beneath the symptoms, you might notice the beetle's entry and exit holes into the tree. The female tunnels into trees forming galleries, where she lays her eggs. Once grown, the sibling beetles mate with each other so that females leaving the tree to start their own galleries are already pregnant. Males do not fly and stay in the host tree.
Shothole borers have a special structure in their mouth where they carry two or three kinds of their own novel symbiotic fungi. Shothole borers grow these fungi in their tree galleries. It's these fungi that cause Fusarium dieback disease, which interrupts the transportation of water and nutrients in the host tree. Advanced fungal infections will eventually lead to branch dieback.
Early detection of infestations and removal of the infested branches will help reduce beetle numbers and therefore, also reduce the spread of the fungus.
- Chip infested wood onsite to a size of one inch or smaller. If the branch is too large to chip, solarize them under a clear tarp for several months
- Avoid movement of infested firewood and chipping material out of infested area
Avocado is one tree host. Shothole borers successfully lay eggs and grow fungi in many tree hosts, with some of these trees susceptible to the Fusarium dieback disease. For more information about tree host species, where the shothole borer is in California, and what symptoms look like in other tree hosts, visit the UC Riverside Eskalen Lab website.
Californians can help in the fight against invasive species by learning and participating during California Invasive Species Action Week, June 2–10.
During the week, spend your lunch with us learning the latest about invasive tree killing pests, aquatic nasties like quagga mussels and nutria, and how the invasive weed/wildfire cycle is altering our ecosystems! http://ucanr.edu/sites/invasivelunch/
Content in this post taken from the UC IPM Avocado Pest Management Guidelines. Faber BA, Willen CA, Eskalen A, Morse JG, Hanson B, Hoddle MS. Revised continuously. UC IPM Pest Management Guidelines Avocado. UC ANR Publication 3436. Oakland, CA.
And more about Shot Hole Borers
A recent publication from CA Fish and Wildlife has some wonderful photos of trees that have been known to be attacked by either or both of the Shot Hole Borers, Polyphagous or Kuroshio. The publication can be found at:
The most current listing of hosts and the distribution of the beetles can be found at Akif Eskalen's website:
Shot hole borers (SHB) and Fusarium dieback (FD) represent an increasing threat to many plant species in southern California. Two similar species of invasive shot hole borer exist in southern California, the Polyphagous shot hole borer (PSHB) and the Kuroshio shot hole borer (KSHB) (Euwallacea spp). Shot hole borers are ambrosia beetles forming a symbiotic relationship with the fungi. PSHB symbionts are Fusarium euwallaceae, Graphium euwallaceae, and Paracremonium pembeum, while KSHB forms a symbiotic relationship with two novel species of Fusarium and Graphium (Cooperband et al. 2016) (Lynch et al. 2016) (Eskalen, Stouthamer 2015). These fungi clog the host's vascular tissue leading to branch dieback and eventually tree death. The beetles are not pathogenic in their native habitats in Southeast Asia, preferring instead to infect stressed trees (Hulcr and Dunn 2011). The two shot hole borer species found in southern California differ primarily in the fungal species they associate with but can be treated the same for the sake of management.
SHB infestations begin when females burrow natal galleries into the host, introduce symbiotic fungi, and lay eggs. Females are capable of producing haploid male offspring in the absence of mating while mated females produce diploid female offspring. Eggs mature into adults in six weeks. Beetle emergence occurs over a period of weeks during the spring and summer. Dispersal of females to other hosts is primarily wind-driven, but is also likely facilitated by firewood and green waste movement. Males are incapable of flight; they remain on the host tree but may enter other natal galleries (Umeda, Eskalen, and Paine 2016). Beetle dispersal can occur over large distances if winds allow.
The loss of wildland trees, particularly in riparian areas, poses a threat to numerous threatened and endangered species such as the least Bell's vireo (Vireo bellii pusillus). Destroyed forests give way to invasive species such as giant reed (Arundo donax) and castor bean (Ricinus communis) (Boland 2016). Specifics of this emerging threat are poorly understood and while research is ongoing this guide is intended as an identification aid for land managers in areas where SHB may not yet be discovered.
(references are found at the end of the publication)