- Author: Ben Faber
- Author: Kathy Keatley Garvey
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."
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.
“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, who is researching an ensemble of behavioral chemicals that repel the walnut twig beetle from landing on English walnut trees, contributed photos of dead and dying walnut trees in the Davis area. Audley conducts his research in a commercial orchard near Winters. UC Davis doctoral student Corwin Parker and Hishinuma also provided images of deteriorating walnut trees.
“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.
[From the December 2014 issue of the UC IPM Green Bulletin]
Known hosts include walnuts (Juglans species) and wingnuts (Pterocarya species). In California the disease has been observed on both native species of black walnut, Juglans californica and J. hindsii, and both are quite susceptible. English walnut, J. regia, is affected to a lesser degree but is typically grown grafted on a hybrid rootstock, which is susceptible, so symptoms of the disease may be observed in walnut orchards as well. The eastern black walnut, Juglans nigra, is highly susceptible. Although of limited importance and distribution in California, this species is grown in eastern North America for its excellent wood and in much of western North America as a shade tree. Therefore, TCD represents a threat throughout most of the United States. Eastern black walnut appears to have been the key host species that provided for the range expansion of the disease in most of the western states.Wingnut, a less frequently planted tree in California landscapes, is also susceptible to the disease.
Symptoms and Signs
How is It Controlled?
Currently, no insecticides or fungicides have been shown to save trees affected with TCD. Thus, it is important that infested wood is not moved off site. Infested trees should be taken down and ground or burned (where allowed) on site. Because the beetles are very small and difficult to detect, it is important that freshly cut walnut branches, logs, or burls not be moved or shipped from infested areas, not even for woodworking purposes. Seasoning wood on site for 2-3 years should allow walnut twig beetles and other woodborers time to emerge at the site of infestation, but it is prudent to have all wood inspected by a knowledgeable entomologist or cooperative extension specialist prior to movement of the material from the site, even when properly seasoned and de-barked.
For more information, visit the UC IPM web page on Thousand Cankers disease./span>
- Author: Mary Louise Flint
[From the April 2014 issue of the UC IPM Green Bulletin newsletter]
Over the last several decades dozens of exotic pests have invaded California landscapes, causing at least temporary havoc and sometimes severely affecting the aesthetic value of plants or even killing them. Giant whitefly, hackberry woolly aphid, eucalyptus red gum lerp psyllid, Diaprepes root weevil, myoporum thrips, light brown apple moth, spotted wing Drosophila, and olive fruit fly are just a few now established pests that were unknown in the state 25 years ago.
These invaders have come from all over the globe—Asia, Africa, Europe, Australia, Central and South America, and parts of North America. Many new pests arrived on nursery stock; others were imported with shipments of wood, produce, or packing material. Some pests were inadvertently brought in on vehicles or with travelers. Many safeguards including quarantine programs, border inspections, careful procedures at plant nurseries, and outreach programs to educate the public about not moving wood, plants, or produce into the state have had a significant effect in reducing the spread of invasive pests. However, despite these efforts, there is little doubt that new species will continue to arrive.
Five of the newest invaders of concern to landscapers are described in the following paragraphs. For information on these and other exotic pests see the web sites of the UC Riverside Center for Invasive Species Research, the California Department of Food and Agriculture, or the UC Statewide IPM Program.
Goldspotted oak borer. First identified in eastern San Diego County in
Polyphagous shot hole borer. Like the walnut twig beetle, this tiny
Brown Marmorated Stink Bug. A native of Asia, the brown marmorated
This article was originally published in the April 2014 issue of the UC IPM Green Bulletin. See this and other articles at http://www.ipm.ucdavis.edu/greenbulletin/index.html.