Scientists appear to have pinpointed the epicenter of the emerald ash borer (EAB) population in the United States and from this origin they are able to track the spread and growth of this insidious insect community. The U.S. Forest Service and Michigan State University used samples from ashes killed by EAB to reveal a progression of tree death in southeast Michigan dating back to 1997. Using dendrochronology; the science of matching wide and narrow growth rings between trees, this research team was able to determine an exact date of death for each tree in the study. This chronological reconstruction revealed an enormous capacity for EAB population growth and spread.
Normally this invasive insect takes 3 to 5 years to kill a healthy ash tree, so they suspect the introduction must have taken place no later than the mid-1990s. Observing and tracking invasive insect populations is difficult, with conditions to do so rarely favorable. However, EAB leaves distinctive trademarks; recognizable larval galleries, low host resistance and relatively rapid mortality of host trees, which allows for the unique opportunity to utilize tree ring analysis methodologies to discover the historical dynamics of the invasion.
It is estimated that the current EAB invasion spread slowly from 1998 to 2001, reaching only about 2 miles in a given year. However, as the population grew the rate of spread increased, moving outward and joining with isolated satellite colonies, from the epicenter the area of infestation grew out 8 miles from 2001 to 2003. Approximately 7 satellite colonies materialized per year between the years of 1998 and 2002, appearing about 15 miles from the edge of the main infestation site. The beetle’s natural dispersal ability combined with human assistance in the way of moving firewood, transporting nursery trees and lumber was likely behind the increase in formation of satellite colonies and population spread. Transporting firewood long distances will exponentially increase the spread and likelihood of invasion of EAB, but regulating firewood can be very difficult.
Emerald ash borer quickly became one of the most destructive and expensive pests ever introduced into the United States, it has already millions of trees across 22 states, as well as Quebec and Ontario. It now threatens all members of the ash family for the entire continent. This study has implications for management specifically of EAB but also touts the importance of early detection, regulation of wood products and the role that satellite colonies can play in the accelerating the spread of invasive species.