The Kuroshio Shot Hole Borer in the Tijuana River Valley
The Kuroshio Shot Hole Borer (KSHB) is a tiny beetle that invaded San Diego County in 2013. It was first seen in some avocado groves but then in 2015, it became very abundant in the native riparian forests in the Tijuana River Valley. It immediately caused extensive damage to those forests and as it is now also being found in many other sites in southern California, the authorities are extremely concerned that other sites are going to be impacted as badly as the Tijuana River Valley.
Dr. John Boland
Luckily Dr. John Boland was studying the willow trees in the valley in 2015 and he immediately switched his focus to the KSHB’s impact in the valley. Here we present a summary of the four main storylines coming out of his intensive five-year study of the beetles in the valley:
- The KSHB in the valley went through a 5-year boom-and-bust cycle;
- The KSHB severely damaged the native willow forests in the valley but the forests are now rapidly recovering;
- The research has uncovered two mysteries about the KSHB – one we think is solved and the other still needs to be solved; and
- The research has led to a prediction about KSHB’s likely impact at other sites in southern California and several management recommendations.
KSHB biology
When KSHB (Euwallacea kuroshio) attack a tree, the females drill into the trunk and create galleries of tunnels in the wood by pushing sawdust ‘tailings’ out of the entrance hole. They inoculate the tunnel walls with a fungus (e.g., Fusarium sp.), and live in the tunnels eating the fungus and reproducing. Within a few weeks new females emerge, and start another gallery in either the natal tree or a new tree.
The beetles are tiny (~2 mm in length) and seldom seen, however if there are enough of them they can damage and even kill trees via their tunneling activities, which undermine the structure of the tree trunks.
The Tijuana River Valley and the riparian forest
The Tijuana River Valley in San Diego County, California, is a coastal floodplain of approximately 3,700 acres at the end of a 1,730 square mile watershed. The Tijuana River is an intermittent stream that typically flows strongly in winter and spring and is mostly dry in summer. For decades, the Tijuana River has been polluted with sewage and industrial waste as it flowed through the city of Tijuana, Mexico, and when it flows through the Tijuana River Valley it is one of the most polluted rivers in California.
The riparian forest that grows around this river is one of the largest in coastal southern California. The forest is dominated by just two tree species: the black willow (Salix gooddingii) and the arroyo willow (Salix lasiolepis). Both willow species are high on the list of KSHB’s preferred hosts.
STORYLINE # 1. The KSHB in the valley went through a 5-year boom-and-bust cycle
Annual surveys of infestation rates showed that the KSHB population went through a rapid outbreak and a rapid decline over a five-year period, with the infestation rates peaking in Fall 2016. The early increase in population occurred while the KSHB was attacking the willows in the Wet Forests and the later decrease in population occurred while the KSHB was attacking the willows in the Dry Forests.
The KSHB boom-and-bust is now complete. In fall 2019 it was difficult to find any trees infested with KSHB. While it lasted the KSHB’s population explosion was very destructive: it has been estimated that the KSHB infested more than 375,000 willows and killed more than 122,000 willows in the valley.
This boom-and-bust cycle occurred naturally, with no management interventions to control the spread or severity of the outbreak.
The graph on the right shows annual canopy loss caused by the KSHB, i.e., area of annual NDVI loss of at least 0.1 within riparian forests. NDVI is a metric of vegetation health.
Click on each polygon to check the graphs displaying infestation, mortality and canopy loss percent rates.
Two examples of rapid destruction by KSHB - at Hollister St. Bridge and Dairy Mart Rd. Bridge
Satellite image analyses conducted by Dr. Kellie Uyeda determined spatial and temporal patterns of canopy loss. The normalized difference vegetation index (NDVI), a metric of vegetation health, was calculated for each year from 2015 - 2019.
The remote sensing analyses support the results seen in the ground surveys. First, in the early years of the KSHB infestation, the most dramatic vegetation losses were observed in the Wet Forests and in later years the vegetation losses were observed in the Dry Forests, with lower levels of vegetation loss. Second, the KSHB’s impact was greatest in 2016-17 and since then it has been tapering off.
The NDVI losses (red areas) show the progression of the KSHB-induced damage in the valley. Negative NDVI losses (blue areas) show the areas where the forests have been regrowing.
STORYLINE #2. The extensively-damaged willow forests are rapidly regrowing
Two examples of the rapid recovery of the forests
These results show that a KSHB-damaged forest has the ability to rapidly restore itself. The speed of willow recovery is remarkable.
The willow forests have recovered in three ways: by the seeding of new willows (‘seedlings’), by the resprouting of the surviving heavily-KSHB-damaged willows (‘resprouts’), and by the survival of a few, scattered mature infested willows (‘Big Trees’).
Seedlings
After the KSHB had damaged the adult willows many willow seedlings recruited onto the sunny and moist river beds. Most of the willow seedlings were scattered within the forest, but three large stands of seedlings became established in three units.
Resprouts
A KSHB-damaged willow tree produces new upright branches (green arrows) that grow from adventitious buds on the surviving trunk (red arrow).
Big Trees
A few old willows survived the KSHB invasion and remain as scattered Big Trees. It is likely that they will play an important role in the recovery of willows in the river valley.
This overview of a Wet Forest in 2019 shows the two-layered canopy: the sparse, emergent layer created by the few remaining Big Trees, and the denser, lower layer composed of resprouting willow trees and willow seedlings.
Fortunately, the heavily-damaged Wet Forest units recovered considerably, and in some places the forests are almost back to their pre-KSHB condition. However, in other places they have failed to return because of the expansion of the invasive plant arundo, Arundo donax.
STORYLINE # 3. Two mysteries
A. Why are the recovering willows not being re-attacked by the KSHB?
The recovering willows in the Wet Forests are forming forests similar to what was present before the KSHB invasion, but the trees are not being substantially re-attacked by the KSHB. Why?
We don’t know the answer but suggest these three possibilities:
1. An ‘induced response’ of the trees. It is possible that the infested willows have changed their chemistry as a result of the borer attack, and this has increased the resistance of the surviving trees to further borer attacks.
2. Overall forest structure. It is possible that the willows, though individually suitable, no longer present a suitable group target for the KSHB.
3. A disease or predator. It is possible that the KSHB population in the valley is now being kept low by a pathogen, parasite, parasitoid or predator.
It will take further research to solve this mystery. But understanding it will provide essential information about the KSHB invasion in southern California and about shot hole borers in general.
B. Why was the KSHB’s impact different in different parts of the valley?
The distribution of KSHB within the Tijuana River Valley was not random; they infested and killed willows growing in or near the main channel significantly more than willows growing far from the water.
On the right you can see a wet site where the mortality rate was high, and a dry site where the mortality rate was low. Black willow was the most abundant tree species in both sites. Why the difference?
We think we know the answer to this mystery:
We think it has to do with the sewage pollution in the river. Sewage contains the most important plant nutrients (nitrogen, phosphorus and potassium) in abundance and so the willows growing in or near the polluted channel water were growing vigorously. The KSHB targeted these fast-growing willows because the sap in these trees was being nutrient loaded in two ways – phloem sap was loaded with sugars from the fast-growing leaves, and xylem sap was loaded with nutrients from the enriched soil. These extremely high nutrient conditions in the wood allowed for the fast growth of the KSHB’s symbiotic fungi and ideal conditions for the rapid population growth of the KSHB (the Enriched Tree Hypothesis).
Some kind of link between the environment and shot hole borer impact had been suspected but not previously identified. Hulcr and Stelinski (2017) noted that “in ambrosia beetle research, the role of the environment and preexisting conditions of the trees has not yet been well appreciated, even though it appears to determine the impact of these beetles.” The Enriched Tree Hypothesis directly links the environment (enriched water) and the preexisting condition of the trees (vigorous, fast growing willows) with the impact of the KSHB (tens of thousands of KSHB per host tree, which cause the trunk to snap and the canopy to collapse).
STORYLINE # 4 – A prediction and management recommendations
4A. A prediction
The Enriched Tree Hypothesis suggests that the Tijuana River Valley is an unusual case, and that the extreme sewage pollution in the Tijuana River makes the trees growing near the main channel unique in the region. The rapid invasion by the KSHB, the subsequent collapse of the willow canopy, and the many thousands of willow deaths in the valley should not be expected to occur in more normal, unpolluted riparian habitats elsewhere in southern California.
4B. Management recommendations
A. Do not cut down and remove infested trees thinking that they are going to die. Willows can survive very heavy infestation rates.
B. Remove Arundo in order to improve the riparian forests in the valley. Arundo is degrading the forests and needs to be removed for the willows to fully recover.
C. Continue to plant willows in restoration sites. Use ‘natural restoration’ methods wherever you can.
D. When searching for KSHB in other parts of San Diego County search in nutrient-enriched areas, e.g., near storm drain outfalls.
E. In urban forests do not over-fertilize or over-water trees. Nutrient-enriched and fast-growing trees are more vulnerable to KSHB infestation.
RESEARCH DRAWN UPON
Boland, J.M. 2016. The impact of an invasive ambrosia beetle on the riparian habitats of the Tijuana River Valley, California. PeerJ 4:e2141; DOI 10.7717/peerj.2141. Available online at: https://peerj.com/articles/2141.pdf .
Boland, J.M. 2017. The Ecology and Management of the Kuroshio Shot Hole Borer in the Tijuana River Valley. Final Report for US Navy, US Fish and Wildlife Service and Southwest Wetlands Interpretive Association. 43 pages. Available online at: https://trnerr.org/wp-content/uploads/2019/04/Boland-KSHB-Apr-2017_FINAL.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. Final Report for US Navy, US Fish and Wildlife Service and Southwest Wetlands Interpretive Association. 74 pages. Available online at: https://trnerr.org/wp-content/uploads/2018/07/Boland-KSHB-April2018-FINAL.pdf
Boland, J.M. 2019. Ecology and Management of the Kuroshio Shot Hole Borer in the Tijuana River Valley in 2018-19 (Year Four). Final Report for US Navy and Southwest Wetlands Interpretive Association. 60 pages. Available online at: https://trnerr.org/wp-content/uploads/2020/05/Boland-KSHB-TRValley2019.pdf
Boland, J.M. and D.L. Woodward. 2019. Impacts of the invasive shot hole borer (Euwallacea kuroshio) are linked to sewage pollution in southern California: the Enriched Tree Hypothesis. PeerJ. Available online at: https://peerj.com/articles/6812.pdf .
Boland, J.M. and K. Uyeda. 2020. Ecology and Management of the Kuroshio Shot Hole Borer in the Tijuana River Valley in 2019-20 (Year Five). Final Report for US Navy and Southwest Wetlands Interpretive Association. 70 pages. Available online at: https://trnerr.org/wp-content/uploads/2020/05/KSHB-TRValley2020.pdf
