Evaluation and monitoring of ash trees that have survived long-term exposure to Agrilus planipennis in southeast Michigan

Agrilus planipennis, the emerald ash borer, (Coleoptera: Buprestidae) was detected in the US near Detroit, Michigan in 2002, but is estimated to have become established approximately 10 years earlier. This invasive species has caused the mortality of millions of ash trees in the genus Fraxinus and is known to occur in 14 US states and two Canadian provinces. Previous reports indicate that all green ash (Fraxinus pennsylvanica) and white ash (F. americana) are susceptible. However, in the Huron-Clinton Metroparks surrounding Detroit, 220 surviving ash trees have been identified. One-third of these trees do not show any symptoms or signs of infestation. The main objectives of this study are to 1) monitor these surviving trees to determine their fate and 2) evaluate the stage(s) of infestation that is/are interrupted by the trees. Adult landing rates were monitored on all trees, while egg surveys were conducted on a subsample of trees to evaluate where oviposition had occurred and whether the trees were infested. Another subsample was challenged by placing eggs on them and monitoring the success of larvae. This study will enable us to evaluate whether some of these trees are apparently resistant to A. planipennis, and whether some of the trees tolerate periodic attack by this insect. While the overall percent of trees surviving the A. planipennis invasion is extremely low, if these trees are resistant to, or tolerant of this pest, they may be useful in management efforts by providing material from which propagation can occur.