- "A Study of Landing Behaviour by the Walnut Twig Beetle, Pityophthorus juglandis, Among Host and Nonhost Hardwood Trees in a Northern California Riparian Forest" (https://doi.org/10.1111/afe.12385).
- "Walnut Twig Beetle Landing Rates Differ Between Host and Nonhost Hardwood Trees under the Influence of Aggregation Pheromone in a Northern California Riparian Forest" (https://doi.org/10.1111/afe.12410)
The walnut twig beetle, in association with the fungus, Geosmithia morbida, causes the insect-pathogen complex known as "thousand cankers disease," which wreaks havoc on walnut trees. The insect, measuring about 1.5 millimeters long, is smaller than a grain of rice.
"The first study is one of few bark beetle host selection studies conducted without the use of semiochemical lures," Audley said. "Together, both studies provide strong evidence for directed flight host searching and in-flight, host discrimination behaviors by Pityophthorus juglandis. These papers highlight sources of and provide an ecological context for potential non-host, volatile compounds that may be of use in semiochemical repellents to protect walnut trees from attack by P. juglandis."
The beetle? The walnut twig beetle, Pityophthorus juglandis.
In association with the fungus, Geosmithia morbida,it causes the insect-pathogen complex known as "thousand cankers disease," which wreaks havoc on walnut trees.
Audley will share his research at his exit seminar, "Semiochemical Interruption of Host Selection Behavior of the Invasive Walnut Twig Beetle, Pityophthorus juglandis," set for 4:10 p.m., Wednesday, Dec. 4 in 122 Briggs Hall, UC Davis campus. It's open to all interested persons.
Jackson, who joined the UC Davis doctoral program in September 2015, investigates behavioral chemicals that repel the walnut twig beetle from landing on English walnut trees. He conducts his research in a commercial orchard near Winters.
Audley says in his abstract: "The walnut twig beetle (WTB) is an invasive bark beetle pest of walnut trees in California and throughout much of its recently expanded range across the North American continent. Feeding by the beetle and canker development by the associated fungal pathogen, Geosmithia morbida, constitute the progressive and often fatal, thousand cankers disease. Management efforts to protect walnut trees are currently lacking. Here I present work related to understanding and manipulating WTB chemical ecology. First, we investigate the beetle's host-searching behavior in the context of a dense, native riparian forest habitat. The goal was to establish WTB's inflight sensitivity to host and non-host cues. Next, I present the results of flight-intercept behavioral assays of four potentially repellent volatile compounds: limonene, chalcogran, concophthorin and verbenone, first individually and then in compounds in reducing trap captures in the context of WTB aggregation phermone.
"Finally, we tested the most effective combination on whole walnut trees in a commercial, English walnut orchard. We compared beetle landing rates on treated and untreated trees as a correlate for WTB attacks. I report that we effectively reduced the number of WTB landing on treated trees; however, the repellent effect was spatially limited. Thus, further testing is required prior to recommending a management schedule. This work did provide proof of concept of semiochemical interruption in a hardwood attacking bark beetle system."
Audley, on a path to receive his doctorate in entomology this month, studied with Steve Seybold, who tragically died Nov. 15 of heart disease. Seybold was a lecturer and faculty affiliate with the UC Davis Department of Entomology and Nematology and a forest entomologist and chemical ecologist with the Pacific Southwest Research Station, USDA Forest Service, Davis.
Louie Yang, associate professor, UC Davis Department of Entomology and Nematology and Professor Richard “Rick” Bostock of the UC Davis Department of Plant Pathology serve as mentors. The Bostock lab is heavily involved with the chemistry side of Audley's repellent research.
A native of Washington, D.C., Jackson spent most of his childhood in Atlanta, Ga. He was first introduced to forest entomology while studying at the University of Georgia, Athens, where he received his bachelor of science in wildlife biology and natural resource recreation and tourism in 2009. He went on to receive his master's degree in forestry in 2015 from the University of Tennessee, Knoxville, where he focused his thesis on managing the walnut twig beetle in cut black walnut logs, live edged boards, and nursery stock.
He recently received the 2019 Western Forest Insect Work Conference Memorial Scholarship Award for his research on the chemical ecology of the walnut twig beetle.
His career plans? “I plan to devote my career to conducting chemical ecology-based research of bark and wood boring beetles that threaten trees in forest landscapes in the western U.S.,” Audley said. “In this capacity, I plan to continue adding to the scientific understanding of bark beetle ecology and management.”
Audley aims to engage with the scientific community and public alike in the arena of forest health issues and sound forest management practices. “Our western forests are in dire need of sound forest management to return them to a healthier state, and I plan to conduct and disseminate research to help achieve that goal.”
Community ecologist Rachel Vannette, assistant professor, is coordinating the weekly seminars. (See list of seminars)
Steve passed away Friday, Nov. 15 at a Sacramento hospital of a heart condition. Born in 1959 in Madison, Wisconsin, he was 60 years old. He was one of the pioneering scientists researching the newly discovered thousand cankers disease (TCD), caused by the walnut twig beetle, Pityophthorus juglandis, in association with the canker-producing fungus, Geosmithia morbida. He was a worldwide authority on the insect, fungus and disease. (See obituary in Davis Enterprise)
In his memory, he was honored with a moment of silence this week at the Entomological Society of America meeting in St. Louis, Mo.
The tributes are pouring in:
Nematologist Steve Nadler, professor and chair of the UC Davis Department of Entomology and Nematology: "Steve was an excellent chemical ecologist whose research on insect pests of trees proved to be of great importance to landscapes throughout North America.”
Chemical ecologist Walter Leal, UC Davis Department of Molecular and Cellular Biology and past chair of the UC Davis Department of Entomology (now the Department of Entomology and Nematology). "Chemical ecology lost a champion, forestry entomology lost an ally, and we will miss a friend and colleague. Steve served the International Society of Chemical Ecology as Councilor, the Journal of Chemical Ecology as Associate Editor, and mentored many undergraduate and graduate students and postdoctoral scholars. He elucidated biochemical mechanisms related to bark beetle pheromones, leaving behind a legacy of papers and review articles, some of which have already been cited almost one thousand times. I copied from him the style of praising colleagues when citing their good work by adding the advert 'elegantly' like in the following sentence. Steve Seybold elegantly demonstrated how bark beetles make their pheromones."
Doctoral student Jackson Audley of the Seybold lab: "Steve Seybold was a brilliant scientist and an integral component of the forest entomology community, especially here in the western USA. I admired the amount of time he spent out in the field. I have often heard it said that once a field biologist obtains a Ph.D., that they often become something of an 'armchair biologist', not Steve. I have never seen him happier than when he was out on long field excursions, hunting down various trees and tree pests. I hope to emulate that characteristic in my own career. Although my time working with Steve now feels cut abruptly and unfairly short, he imparted a great deal of wisdom upon me in that time. I am incredibly grateful to have had Steve as a mentor and a friend. He contributed a great deal to making me into the scientist I am today. He will be sorely missed."
Seybold lab alumnus Andrew Graves (now a zone leader entomologist with the Forest Service, Forest Health Protection, New Mexico Zone): "We are all shocked and saddened. Steve and I worked together for nearly 20 years. A faithful mentor, constant teacher, an incessant researcher to whom the time of day or length of time were meaningless, a good friend who contributed much to our world. A too early passing and a great loss for us all. He was a good man and will be missed by many. If I had to guess, he would've been disappointed he didn't finish that last manuscript."
Seybold lab alumnus Stacy Hishinuma, who went on to accept a position in the Pacific Southwest Region, San Bernardino: "Steve was a pivotal person in my life. I worked with him for more than 10 years and during that time he pushed me to have the highest scientific standards and taught me all I know about research. He was an involved mentor and cared deeply about the success of his students. His love was expressed through the red marks on our manuscripts, the time he spent helping us with presentations, and the pop quizzes on insect identification. When I started graduate school, people advised me to be the first person in and the last person to leave. This was impossible in the Seybold lab since Steve regularly started working before dawn. It was somehow always startling when I would email Steve at 4:00a, before heading to sleep, and he would immediately respond. Steve's work ethic was unprecedented. It was inspiring to see his passion for research. His love for science was only rivaled by his love for his daughters. I'll always miss the conversations we had driving to field sites, eating at his go-to restaurants, and geeking out about bark beetles. Thank you for everything Steve. I hope I can help carry on your legacy through my own work as a forest entomologist."
We knew him as an incredible scientist with an immense curiosity, an outstanding teacher, and a kind and caring friend. A few of the last news stories we wrote about him:
- A Sign of the Times: Why This Black Walnut Tree Is Dying
- Meet the Extreme Insects at the Bohart Museum of Entomology Open House
- Walnut Twig Beetle and Fungus Has Caused 'Profound Damage to Black Walnut Trees
Steve is survived by his widow, Julie Tillman, and daughters Natalie, 11, and Emily, 17. "Steve loved his work and his daughters with his whole self and heart," Julie said. "He will be sorely missed by his family and friends, his lab family and his huge network of professional colleagues."
A visitation will be held from 5 to 7 p.m. Friday, Nov. 22, at Smith Funeral Home in Davis. A service will be held on Saturday, Nov. 23, at Saint James Church in Davis at 10 a.m. with interment at a later date near his father at Forest Hill Cemetery in Madison. Memorials in his name may be made to the American Heart Association, National Parks Conservation Association or the Entomology and Nematology Student Support fund at UC Davis.
It's about 150 years old, 50 feet in height, and measures about five feet in diameter. And it's dying.
What's killing it is thousand cankers disease (TCD), an emerging insect-fungus complex.
It's killing a lot of black walnut trees.
In fact, TCD has caused profound damage to black walnut trees not only in urban areas of California and other western states, but in Pennsylvania, Tennessee and Virginia, according to a newly published review by UC Davis-affiliated scientists and their colleagues.
The article, “Status and Impact of Walnut Twig Beetle in Urban Forest, Orchard and Native Forest Ecosystems,” published in the Journal of Forestry, updates the spread of the disease, and chronicles the role of the bark beetle, Pityophthorus juglandis, and the canker-producing fungus, Geosmithia morbida, in killing walnut trees, especially black walnuts.
Native to southwestern United States and northern Mexico, the bark beetle, about half the size of a grain of rice, “has invaded urban, orchard and native forest habitats throughout the United States, as well as Italy,” said lead author and forest entomologist Steven Seybold of the Pacific Southwest Research Station, USDA Forest Service, Davis, and a lecturer and researcher with the UC Davis Department of Entomology and Nematology.
Walnut twig beetles (WTB) tunnel into branches and trunks of walnut (Juglans) where they create galleries for mating and reproduction. They carry spores of the fungus into their galleries, and the resulting fungal infection causes formation of cankers, which coalesce and girdle branches and stems.
“The walnut twig beetle is a significant pest of very large trees because it sequentially attacks the small branches--though ironically not the twigs--all the way down the trunk to the soil line,” said Seybold, a pioneering scientist of TCD, who first found TCD evidence in Davis in 2008. “Most bark beetle species are not this thorough in using all of the phloem tissue in their plant hosts. In Davis right now, in the courtyard next to Sophia's Thai Restaurant, 129 E St, the tiny beetle is gradually killing the largest northern California black walnut tree in the city. It has taken nearly a decade, but the crown of the massive tree is nearly completed killed.”
Seybold estimated that the E Street tree is about 150 years old, "maybe older." It measures almost 65 inches or just over five feet.
Seybold noted that the disease is “unique because of its multifaceted negative impact on walnut trees involved in landscaping, food production, and forestry. Walnut trees are valuable ecologically and for food and timber, so the walnut twig beetle is a good model in which to study the impact of a bark beetle on forest and agro-ecosystem services.”
The five co-authors of the synthesis article include Stacy Hishinuma and Andrew Graves, two USDA forest entomologists with UC Davis connections. Hishinuma, who works in the Pacific Southwest Region, San Bernardino, and holds a doctorate in entomology from UC Davis, studied in the Seybold and Mary Lou Flint labs, UC Davis Department of Entomology and Nematology. Graves, who works in the Southwestern Region, Albuquerque, N.M., is a former postdoctoral fellow in the UC Davis Department of Plant Pathology.
Other co-authors are Professor William Klingeman III of the Department of Plant Sciences, University of Tennessee, and forest entomologist Tom Coleman with the USDA Forest Service's Southern Region, Asheville, N.C..
UC Davis doctoral student Jackson Audley of the Seybold lab and Richard Bostock lab (Plant Pathology), contributed photos of dead and dying walnut trees in the Davis area, along with UC Davis doctoral student Corwin Parker and Hishinuma. Audley, who is researching an ensemble of behavioral chemicals that repel the walnut twig beetle from landing on English walnut trees, conducts his research in a commercial orchard near Winters.
“WTB is one of a few invasive bark beetles in North America where expanding distribution and impact have been pronounced enough to affect other species, communities, and ecosystems to the extent that services provided by urban forests, agroecosystems, and wildland areas have been altered,” the co-authors concluded in their paper. “We envision that ecological impacts of WTB will continue to unfold across a wider geographic area to affect various types of key services, i.e., provisioning (e.g., timber and nontimber products); regulating (e.g., air and water quality/quantity, climate regulation); and cultural (e.g., recreation, aesthetics, shade) services.”
Scientists first collected the beetle in North America in 1896 in New Mexico, 1907 in Arizona, 1959 in California, and 1960 in Mexico, but never considered it a major pest of walnut trees until black walnuts began deteriorating and dying in New Mexico in the early 2000s. Walnut tree mortality that occurred in the early 1990s in the Wasatch Mountains of Utah and in the Willamette Valley of Oregon is now attributed to TCD.
“Currently, good cultural practices and sanitation of infested materials are the primary strategies for disease management within orchards and also for prevention of spread of the disease and vector to regions with low rates of infection,” according to the UC Statewide Integrated Pest Management Program (UC IPM)>
UC IPM recommends that trees with less than 50 percent live crown be removed to reduce the buildup of walnut twig beetles and inoculum in the trunk and larger scaffold branches. "Chemical control with either fungicides or insecticides is not recommended for management of thousand cankers disease," UC IPM says.
Which brings us back to the massive black walnut tree that is dying on E Street in Davis. If it dates back to 1868, that's the year the city of Davis was founded. Its namesake, Jerome C. Davis, owned a stock farm on the site.
Who was in the White House and who was in the California governor's mansion that year? Andrew Johnson and Henry Haight, respectively. That was also the year that trustees founded the University of California in Oakland; Clark Kerr became the first president. And 1868 was the year Memorial Day was first observed in the United States...and when author Louisa May Alcott published the first volume of her coming-of-age novel, Little Women.
The little tree in Davis became of age, too, growing into a giant tree offering shade, shelter and sustenance. Who would have thought that a tiny insect, half the size of a grain of rice, would play a major role in its demise?
It's about the size of a grain of rice but it's a killer.
That's the walnut twig beetle, Pityophthorus juglandis, which in association with a newly described fungus, Geosmithia morbida, causes thousand cankers disease, wreaking havoc on native black walnut trees.
Enter doctoral student Jackson Audley of the Steve Seybold lab, UC Department of Entomology and Nematology. He will present a seminar on "Protecting Trees from Bark Beetle Attacks Utilizing Semiochemical Repellents: A Case Study with the Walnut Twig Beetle” at a seminar from 4 to 5 p.m., today (Wednesday, Oct. 25) in Room 3001 of the Plant and Environmental Science Building.
The seminar is part of the UC Davis Forest Biology Research Center Seminar Series and his presentation is a preview of his thesis project.
"In this talk I will discuss the theoretical construct of semiochemical repellents for protecting trees from attack by bark beetles and demonstrate the process of developing a repellent tool with my work on the walnut twig beetle, Pityophthorus juglandis, the vector of thousand cankers disease in walnuts," Audley says in his abstract. "Bark beetles can constitute a significant threat to tree health and are a significant disturbance agent in forest ecosystems. Semiochemicals have been successfully utilized to protect individual to stands of susceptible trees from various bark beetle pests. Most of this work has been conducted in coniferophagous bark beetle systems. My work here at UC Davis has been focused on bringing those same techniques into studying an invasive, hardwood-attacking bark beetle system."
Audley received his bachelor's degree in wildlife biology and natural resource recreation and tourism in 2009 from the University of Georgia, and his master's degree in forestry in 2015 from the University of Tennessee. As a master's student, he did research on the walnut twig beetle, including treatment tests.
The walnut twig beetle is believed to be native to Arizona, California, New Mexico and Mexico. In 2006, plant pathologist Ned Tisserat and entomologist Whitney Cranshaw of Colorado State University identified the pathogen in declining black walnut trees in central Colorado. The disease has now spread throughout much of the United States.
"Often the first symptoms of thousand cankers disease (TCD) are flagging and yellowing leaves and branch dieback," says Seybold, one of the pioneering TCD researchers. He is a chemical ecologist with the USDA Forest Service, Pacific Southwest Research Station, Davis, and a lecturer/researcher with the UC Davis Department of Entomology and Nematology. "Affected branches show sap staining and pinhole-sized beetle holes. Beneath the surface are dark stains caused by the fungus."
"The fungus enters the tree through the feeding or reproductive activities of the beetle, and colonizes and kills the phloem and cambium of the branches and main stem," according to the UC Integrated Statewide Integrated Pest Management Program website, content that's the work of Steve Seybold and entomologist Mary Lou Flint, then associate director of of Urban and Community IPM, and now UC Extension entomologist emerita. The disease gets its name from the large number of dark cankers that rapidly develop on affected branches.
The UC Davis Forest Biology Research Seminars, which began Oct. 11, are held Wednesdays at 4 p.m. in Room 3001 of the Plant and Environmental Science Building.
The remaining schedule includes:
Nov. 1 – Alison Scott, postdoctoral student, UC Davis Department of Plant Sciences, “The Polyploid Origin and Evolutionary History of California Redwoods”
Nov. 15 – Brian Smithers, Ph.D student, Ecology Graduate Group, "Mechanisms of Range Shifts in Great Basin Sub‐Alpine Bristlecone Pine Forests."
Nov. 29 – Sarah Bisbing, assistant professor, Forest Ecosystem Science, Department of Natural Resources and Environmental Science, University of Nevada, Reno, "Looking Backward to See Forward: Causes and Consequences of Altered Disturbance and Climate on Western Conifer Forests.”
Dec. 6 ‐ Ryan Tompkins, Forest Silviculturist, U.S. Forest Service/Plumas National Forest, “Sierra Nevada Silviculture in the New World Order: Challenges, Uncertainties, and Opportunities”
For more information, contact David Neale at email@example.com.