Northern oak wilt? Examining vectors and spread risk across upper Michigan and Canada

Currently known in 24 states in the U.S., oak wilt, Bretziella fagacearum, has not yet been detected in symptomatic trees in Canada. The disease has been present throughout southern Michigan and near the border with Wisconsin for decades with no apparent expansion into the rest of northern Michigan or Canada. In a cooperative effort, four sites across the Upper Peninsula, Michigan and eight partner sites across Canada (Manitoba, Ontario, New Brunswick) were monitored for nitidulid (Nitidulidae: Coleoptera) vector activity and climate variables from April through August 2021-2022. Each site utilized two methods of trapping beetles: artificially wounding trees, and pheromone and yeast baited windvane traps. In addition, site specific canopy and light conditions, woody debris, soil moisture and temperature, air temperature, relative humidity, and precipitation were collected as known factors that influence nitidulid activity. Eleven species of nitidulids were collected from artificial wounds in oaks in Michigan and in Canada, including known oak wilt vector species at sites without the disease. Flight traps 200 m away collected an additional 9 species in Michigan and 19 additional species in Canada. In 2021, nitidulid attraction to wounded oaks (20–27 May, 190-degree days) occurred 3 to 6 weeks after flight started (6 April – 8 May) and continued until early August. As we continue to evaluate timing and environmental factors with hosts and vectors this year, we anticipate better understanding the potential vector populations across Upper Michigan and Canada and aim to better understand the risk of oak wilt expanding northwards.