Emerald ash borer (EAB) (Agrilus planipennis Fairmaire; Coleoptera: Buprestidae), a phloem-feeding beetle native to Asia, was identified in 2002 as the cause of widespread ash (Fraxinus spp.) mortality in southeastern Michigan and Windsor, Ontario. We used dendrochronological analyses to reconstruct where this invasive insect originally became established and how it spread throughout southeastern Michigan. The area that we sampled was approximately 15,000 km² in size and encompassed the original 6-county EAB quarantine that was established in 2002. Two to 4 increment cores and/or cross-sections from EAB-killed green ash (F. pennsylvanica) were preferentially collected over declining or non-stressed ash trees on at least a 4.8 × 4.8 km sampling grid (2.4 × 2.4 km sampling grid throughout the heart of the core infestation). Samples were dried, mounted and surfaced in the laboratory prior to measuring annual growth rings to the nearest 0.01 mm using a Velmex measuring system. Skeleton-plots depicting annual relative growth rates for each sample were generated and used to visually crossdate samples to a known master chronology compiled from ash trees surrounding the sample area.

Results/Conclusions

Preliminary crossdating analyses of ash trees in the sample area suggest that EAB initially became established and began to kill trees in the greater Westland-Garden City vicinity by 1997-1998. Additional analyses are currently in progress to verify the accuracy of the preliminary crossdating analyses. In related research conducted at several EAB outlier sites, we have found that an area is typically infested for 3 to 5 years before tree mortality occurs. In turn, this suggests that EAB was introduced and became established in southeastern Michigan in the early to mid-1990’s.  Preliminary measurements of the reconstructed spread of EAB in southeastern Michigan indicate that the EAB population exhibited a biphasic expansion following an initial establishment phase. This type of expansion is characteristic of invasive species in which nearby expanding satellite colonies coalesce with their primary core infestation. The core EAB infestation initially radiated from the epicenter by about 6.5 km each year, then increased to 30 km per year as nearby satellite EAB colonies started to coalesce. Jump distances of new satellite colonies of EAB averaged 20 km from the nearest edge of the core infestation (95% C.I. = 15 to 24 km). In 5 years (1998 to 2003), the area occupied by the core EAB infestation increased 170-fold.