Audley, who conducts his research in a commercial orchard near Winters, investigates behavioral chemicals that repel the walnut twig beetle from landing on English walnut trees. The walnut twig beetle (Pityophthorus juglandis), in association with a canker-producing fungus, Geosmithia morbida, causes the insect-pathogen complex known as thousand cankers disease.
“The disease threatens both native black walnuts across North America and agriculturally important English walnut, particularly in California,” said Audley, who received the $1000 award at the recent conference in Anchorage. “My research focuses on improving our understanding of the walnut twig beetle's chemical ecology and developing a semiochemical repellent tool to manage the threat.”
The scholarship memorializes Mark Duane McGregor, a bark beetle management specialist who died in April 1990 while conducting forest entomology research in Idaho. The scholarship has since expanded to honor other deceased WFIWC members.
Audley, who expects to receive his doctorate in entomology by December 2019, is co-advised by Steve Seybold, 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; Associate Professor Louie Yang of the UC Davis Department of Entomology and Nematology, and Professor Richard “Rick” Bostock of the UC Davis Department of Plant Pathology. The Bostock lab is heavily involved with the chemistry side of Audley's repellent research. Seybold and Bostock have shared a California Department of Food and Agriculture grant with the doctoral student.
Audley is invited to deliver a plenary lecture on his dissertation at the WFIWC meeting next year in Calgary, Alberta.
Audley, who was born in Washington D.C., but spent most of his childhood in Atlanta, Ga., was first introduced to forest entomology while he was a student 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 then received 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 (WTB) in cut black walnut logs, live edged boards, and nursery stock. He joined the UC Davis doctoral program in September 2015.
“My academic and work history have shaped my interest in researching invasive bark and wood boring beetles,” Audley said in his award application. “I am fascinated with the biology and ecology of these invaders and their interactions with native, often naïve host trees. My plan is to continue to pursue research pertaining to the ecology and management of invasive bark and wood boring beetles, focusing on chemical ecology and semiochemical disruption.”
His research at UC Davis includes conducting field-based research relating to the chemical ecology of WTB in northern California walnut orchards and native riparian forests; assisting on several WTB-related research projects, including a trial on the efficacy of emamectin benzoate in protecting walnuts from WTB and thousand cankers disease (TCD), and a project assessing the impact of WTB and TCD on the productivity of English walnut orchards in California. In addition, Audley has monitored and sorted the trap catches from numerous walnut twig beetle flight intercept traps, maintained year round in northern California and performs sorting and identification for similar traps maintained by the Tennessee Department of Agriculture.
Active in WFIWC, Audley delivered a presentation on the walnut twig beetle in 2018 as part of an invasive species symposium. He also serves as the student representative on the Founders' Award Committee. At UC Davis, he teaches and mentors students on forest entomology.
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.”
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.
Between 2005 and 2016, the disease killed nearly 60 percent of the 210 specimens of southern California black walnut mature trees in the USDA Agricultural Research Service's National Clonal Germplasm Repository Juglans Collection near Winters, Seybold said. “This is only an estimate and the true proportion of the mortality is likely much higher, as only six of the 210 trees were rated as having healthy crowns in August 2016."
Seybold estimated that the E Street tree is about 150 years old, "maybe older" and around 50 feet in height. It measures almost 65 inches--or just over five feet--in diameter.
“The walnut twig beetle is also significant because it is the consummate invasive species; it is small enough to travel under the bark of modest-sized pieces of barked wood and it can withstand relatively dry conditions that it might encounter during transit,” Seybold said. “We believe that it has moved from isolated Arizona black walnut trees along creeks and rivers in the desert Southwest to nearly the entire western USA wherever walnut trees of any species have been planted or grew naturally. It has also been transported to Europe and established significant populations in Italy.”
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.”
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.
Hishinuma, who is completing her doctorate, studies the walnut twig beetle, Pityophthorus juglandis, which in association with a newly described fungus, Geosmithia morbida, causes thousand cankers disease (TCD) of walnut and butternut trees.
Hishinuma works with major professor Mary Louise Flint and is co-advised by chemical ecologist and forest entomologist Steve Seybold of the Pacific Southwest Research Station, USDA Forest Service, Davis, an affiliate of the department. Flint is an Extension specialist emeritus with the UC Davis Department of Entomology and Nematology and a former associate director with the UC Integrated Pest Management Program.
“USDA will provide some financial support during the end stages of her Ph.D. thesis, in exchange for 320-640 hours of work and training over the next 14 months,” said Seybold. Then, in June 2016 her position will be converted to that of a permanent entomologist with the USDA Forest Service office in San Bernardino after she has completed her thesis. She will be responsible for forest insect survey, detection, and management on four national forests in southern California ranging from San Diego to Monterey counties.
“The highly competitive internship and guaranteed position are a credit to her and her achievements,” Seybold said.
Seybold and Flint assisted her in developing the internship, as did Richard “Rick” Bostock, UC Davis professor of plant pathology.
Hishinuma won the 2013 Western Forest Insect Work Conference Memorial Scholarship for her research on TCD and presented her work at group's 65th annual conference, held March 31-April 3, 2014 in Sacramento. She also received two scholarships from the California Garden Clubs, Inc. (CGCI) and a McBeth Memorial Scholarship to support her research on TCD.
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 east of the Mississippi to states in the heart of the valuable black walnut timberlands. Most recently it was reported from Indiana. Latest collection records show that the beetle and pathogen are now known from nine states in the western United States and seven states in the eastern USA. In 2013 the disease was also reported in Italy marking the first time that it occurred in Europe.
Seybold's research group has led the effort to characterize the disease in California and to develop a nationwide detection program for the beetle. They recently published two papers in the journal PLOS ONE that characterize the genetic diversity and invasion patterns of both the pathogenic fungus and the beetle in the United States. Scientists believe that TCD occurs only on walnut, butternut, and wingnut, but it is most damaging to native black walnuts, Juglans californica, J. hindsii, and J. nigra although the disease has been recorded on at least 10 species of walnuts or their hybrids in California. Often the first symptoms of TCD are flagging and yellowing leaves and branch dieback, Seybold said. Affected branches show sap staining and pinhole-sized beetle holes. Beneath the surface are dark stains caused by the fungus.
Yigen Chen and Steve Seybold continually trapped the reddish-brown insect, about a third of the size of a grain of rice, for three years along Putah Creek in Davis, Calif., and recorded its daily and seasonal flight behavior. They lured the insect into the traps with a synthetic version of its aggregation pheromone.
“We discovered the collective and interactive effects of four environmental factors on the crepuscular (twilight) flight behavior of the insect,” said lead author Chen, a research entomologist and project scientist with the UC Davis Department of Entomology and Nematology. “We found that the optimal trapping conditions are a combination of moderate to warm temperatures, around 79 to 81 Fahrenheit; low light intensity; low wind speed, 0.6 to 2.5 miles per hour; and moderate barometric pressure, 755 to 757.”
“Understanding the walnut twig beetle's seasonal flight cycle and factors that govern its flight are critical first steps in the early detection of invasive species prior to implementing pest eradication or integrated pest management (IPM) programs,” they wrote in their research article, “Crepuscular Flight Activity of an Invasive Insect Governed by Interacting Abiotic Factors,”published in the Aug. 26 edition of the Public Library of Science, PLOS ONE.
However, when coupled with a hitchhiking fungus, Geosmithia morbida, it causes what is known as thousand cankers disease. The beetles create numerous galleries beneath the bark, resulting in fungal infection and canker formation. The large numbers of cankers led to the name, thousand cankers disease.
As the disease advances, the health of the tree declines and eventually it dies, sometimes within a three-year period, said Seybold, who has been studying the beetle and the newly discovered fungus with its barrel-shaped spores since 2008.
When male beetles initiate new galleries, they produce an aggregation pheromone. As the population increases, the flight response of males and females similarly increases.
Mating disruption or interruption of insect aggregation is crucial to controlling such insect pests as the walnut twig beetle in IPM programs, the entomologists pointed out. “Understanding the interactions among abiotic environmental factors on flight activity, should increase the efficacy of these methods in a specific IPM program to control the beetle,” Chen said.
“The primarily crepuscular flight activity had a Gaussian relationship with ambient temperature and barometric pressure but a negative exponential relationship with increasing light intensity and wind speed,” they wrote in their abstract. “A model selection procedure indicated that the four abiotic factors collectively and interactively governed P. juglandis diurnal flight.”
New knowledge of the primary periods of seasonal flight (May‒July and September‒October) “provides some guidance for when semiochemical-based interruption of aggregation may be applied most efficaciously,” they said..
Seybold, one of the first scientists to study the beetle and fungus in California, served on a scientific team that developed guidelines and trapping methods for the beetle. In 2010 and 2011, the team discovered and later patented the aggregation pheromone for the beetle and conducted scientific trials in northern California.
Late in the summer of 2011, the team demonstrated the efficacy of the pheromone as a flight trap bait. The bait lures both male and female beetles into a small plastic funnel trap.
The walnut twig beetle, native to the southwestern United States and Mexico, and widely distributed in Colorado, Arizona, California, and New Mexico, has now been detected throughout much of the United States: in nine western and five eastern states. In 2013, it was reported in northern Italy.
The earliest symptom of thousand cankers disease is yellowing foliage that progresses rapidly to brown wilted foliage, then finally branch mortality. Branch mortality and decline of the tree crown are one of three major symptoms of thousand cankers disease. The others are numerous small cankers on branches and the trunk, and holes and other evidence of tiny bark beetles.
- Thousand Cankers Disease and the Walnut Twig Beetle in California (UC IPM)
- Walnut Twig Beetle (USDA Forest Service)
- Pest Alert, Walnut Twig Beetle and Thousand Cankers Disease (Colorado State University)
- Thousandcankers.com (This site is a collaborative effort between the Northeastern Area State and Private Forestry, the USDA Forest Service Northern Research Station, the Purdue University Department of Forestry and Natural Resources, the Hardwood Tree Improvement and Regeneration Center, the American Walnut Manufacturers Association, and the Walnut Council.)
In announcing the awards, Provost Ralph Hexter noted (1) that Tatiossian’s research on the walnut twig beetle makes a significant contribution to establishing an integrated pest management plan; (2) that her manuscript, “Flight Response of the Walnut Twig Beetle, Pityophthorus juglandis, to Aggregation Pheromones Produced by Low Densities of Males”; is in preparation for submission to the Journal of Chemical Ecology; and (3) that her poster received attention at a national entomology conference for the ceramic bark beetle she sculpted.
Hexter presented awards of excellence to Brenda Marin-Rodriguez and L. Carolina Tavarez. In addition to Tatiossian, honorable mentions went to Amanda Steele, biomedical engineering; Rachel Borthwell, biological sciences and art history; and Lindsey Black, history.
A photo of Tatiossian and Black appears as the cover photo on the UC Davis Undergraduate Facebook site at https://www.facebook.com/UCDavisUndergraduateEducation. Her poster, including the ceramic bark beetle she crafted, is mounted on the third floor of Briggs Hall, next to the administration office of the UC Davis Department of Entomology and Nematology.
Tatiossian, who joined the Research Scholars Program in September 2011, graduated from UC Davis in three years (she achieved the top grade point average in entomology) and is currently working in the laboratory of Diane Ullman, professor of entomology and associate dean for undergraduate academic programs in the College of Agricultural and Environmental Sciences.
Among those nominating her or supporting her nomination were her mentor, chemical ecologist and forest entomologist Steve Seybold of the Davis-based Pacific Southwest Research Station, USDA Forest Service, and an affiliate of the UC Davis Department of Entomology; professor Jay Rosenheim who co-founded and co-directs the Research Scholars Program in Insect Biology; and Diane Ullman.
Rosenheim noted that Tatiossian was a member of the first cohort of undergraduates recruited to the program. “I witnessed her tremendous determination to develop independent research skills” and she “succeeded in all phases of the project, from design, data collection, data analysis and manuscript preparation.”
Among the 30 students who have entered the program since 2011, “Kristina is absolutely the standout in terms of motivation and enthusiasm for research,” Rosenheim said. “She leaped at the opportunity to learn how to become an independent researcher. Kristina will generate the first-lead authored publication for any student in our program—hopefully, the first of many. In this sense, she has already been a trailblazer for our program.”
Seybold noted that Tatiossian “worked on the host-finding behavior of a major pest of walnut trees, the walnut twig beetle. This is a nationally significant pest that spreads a disease of live trees called thousand cankers disease (TCD). The condition threatens not only the English walnuts that form the basis of the California nut industry, but also the black walnuts that represent over $500 billion in growing stock value of fine wood products in the eastern U.S.
“Kristina formulated her research project in fall 2011 and spring 2012 and then carried it out in spring and summer 2012. As she developed the project, she also applied to the Department of Entomology for a McBeth Scholarship, which she was awarded in summer 2012. The award helped her offset the costs of her research supplies and funded her travel to several scientific meetings.”
“Kristina collected a live population of the walnut twig beetle from a traditional orchard habitat in the southern Central Valley, reared the insects to the adult stage, and re-introduced the adults into freshly cut black walnut branch sections. Once the male beetles had begun producing their aggregation pheromones (attractants) in the branch sections, Kristina used the branch sections as lures to attract new males and females into flight traps. Using this basic technique she was able to establish that as few as 1 to 5 male beetles would provide a threshold of flight behavioral attraction in the field. This finding has ramifications for establishing an integrated pest management program for the walnut twig beetle nationwide.”
Tatiossian developed and displayed her poster at the 60th Annual Meeting of the Entomological Society (ESA), held Nov. 11-14, 2012 in Knoxville, Tenn. “Her poster reached a very interested target audience because Knoxville is in the heart of the distribution of eastern black walnut trees and in the center of the current distribution of TCD in the eastern U.S.,” Seybold said. “In a very creative touch, Kristina sculpted a replica of the female walnut twig beetle (through her participation in the UC Davis Art/Science Fusion Program) and attached the sculpture directly to her poster. This elicited quite a response at the national meeting and led to a news story released by UC Davis.”
The poster also drew attention at the arts exhibit at the 24th Annual UC Davis Undergraduate Research, Scholarship and Creative Activities Conference, held April 26, 2013.
In addition, Tatiossian delivered an oral presentation on her research at the 97th Annual Meeting of the Pacific Branch of the ESA in South Lake Tahoe, Nev.
Her poster, now on permanent display at Briggs Hall, credits Seybold; Extension entomologist Mary Louise Flint, associate director for Urban and Community IPM, UC Statewide Integrated Pest Program; entomology graduate student Stacy Hishinuma, and postdoctoral researcher Yigen Chen of the UC Davis Department of Entomology. Robin Schmidt of UC Davis Molecular and Cellular Biology mounted the unusual poster with the ceramic beetle.