The walnut twig beetle (Pityophthorus juglandis) or WTB, is native to California, the southwestern U.S., and Mexico, where its original hosts were western black walnut trees. However, the WTB is quite happy to be hosted by eastern black walnut (Juglans nigra) and other black walnut hybrids and it’s brought along a very unwelcome guest, a destructive fungus (Geosmithia morbida) that hitch-hikes on the surface of the beetles’ wings. As the beetles penetrate the tree to form galleries, the fungal spores create an infection in the trees’ tissues resulting in lesions, or cankers, of outwardly radiating dead tissue. The death of these tissues impairs the trees’ ability to circulate nutrients and water causing the tree to eventually starve.
Tree decline is typically first noticed in the crown (or top) and moves downward as beetles continue to colonize the tree. Although the WTB is smaller than a grain of rice, they pack a punch by traveling as an entourage. Hence, the disease name, “thousand cankers disease” or TCD, refers to the eventual overlap of many, maybe thousands, of galleries and infected cankers which girdle the tree causing decline and eventually death. Sadly, once a tree becomes infested, no control is available, so it is critically important that forest owners, practicing arborists, landscapers, and all tree lovers be aware of this threat. To prevent the spread to healthy trees, infected trees must be removed, and the material destroyed by grinding or, preferably, burning on-site.
There’s still some good news! The walnut twig beetle has not yet been found in Michigan! So please do what you can to prevent an invasion – don’t move or ship freshly-cut walnut branches, logs, or burls; don’t move firewood; and check out the links below to learn more!
Find more info on WTB and TCD, including photos, identification help, and treatment options, at the sources below:
- Minnesota Department of Natural Resources “Walnut twig beetle – Early Detection Species” https://www.dnr.state.mn.us/invasives/terrestrialanimals/walnuttwigbeetle.html
- MSUE’s “Thousand cankers disease” https://www.canr.msu.edu/news/thousand_cankers_disease
- UC IPM’s “Walnut Twig Beetle and Thousand Cankers Disease: Field Identification Guide”
http://ipm.ucanr.edu/PDF/MISC/thousand_cankers_field_guide.pdf - Midwest Invasive Species Network (MISIN)’s “Walnut twig beetle” identification page http://www.misin.msu.edu/facts/detail/?project=misin&id=236&cname=Walnut%20twig%20beetle
Find info to answer those burning questions on firewood, including what certified, heat-treated firewood is and where to buy it at The Nature Conservancy’s “Don’t Move Firewood” website: dontmovefirewood.org
Find more invasive species information or talk to a local expert at Jackson, Lenawee and Washtenaw Cooperative Invasive Species Management Area (JLW CISMA): jlwcisma.weebly.com or contact Dr. Shikha Singh via Email: shikha.singh@macd.org or Phone: (517)395-2089
by Summer Roberts, WCCD Community Forester