Temporal and spatial patterns of ash mortality and emerald ash borer populations

Background/Question/Methods

Emerald ash borer (EAB) (Agrilus planipennis), an introduced insect pest, has killed millions of ash (Fraxinusspp.) trees in the Midwest and is spreading rapidly.  In each of 60 forest stands across Ohio and 38 stands in southeast Michigan, encompassing different forest habitats and stand ages, and including all five ash species native to our region, monitoring plots were established to understand the impact of EAB on forest ecosystems.  We have recorded the health of ash trees, symptoms of EAB including exit holes, and dead ash tree breakup and fall in three to five monitoring plots in each stand. Yearly data collection began in 2004, when the first sites were starting to exhibit symptoms.  In 2008, we began yearly measurements of EAB population density in a subset of the sites using purple panel traps.  ArcGIS was used to examine spatial patterns of ash decline and mortality over time.  Survival analysis was used to determine the factors related to the survival rate of individual trees. Surviving ash trees in long-infested areas were surveyed and monitored since 2010.

Results/Conclusions

Nearly 100% mortality of mature ash trees was evident in all sites infested long enough for the infestation to run its course.  Ash seedlings and saplings too small to be infested, as well as <1% of the larger ash trees, remain alive.  The small number of surviving ash trees includes many trees with healthy crowns that remain healthy over time.  Sites progressed from healthy to >95% mortality within 1 to 6 years after the first EAB exit holes appeared on the lower part of the trunk, and dead trees fell quickly.  Both site factors and tree factors are related to the survival rate of individual ash trees.  In particular, forests with a low density of ash trees experience very rapid mortality.  Mortality is slow at first then increases rapidly as the infestation progresses.  Maps of ash mortality over time show the rapid progression of ash mortality, first appearing in Michigan and northwest Ohio then spreading to central Ohio.  EAB populations start small, rapidly increase, then crash and persist at low levels after the food source is eliminated.  The nearly complete elimination of an entire genus of trees from forest overstories is occurring as EAB spreads.