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Dead ash trees

Dead ash trees are widespread throughout Central Kentucky in 2015

Trees are dying all over Central Kentucky where I live.  The emerald ash borer, a beetle from Asia, has been slowly expanding its population, and has now reached the point of mass destruction. You can see dead and dying trees all over town, most of which have died this spring.  From here on, barring some miraculous change in ash borer populations, the fate of the remaining ash trees is sealed.  This is happening over the entire range of ash trees in North America.

Why is it that invasive pests and pathogens are so successful, and able to defeat our efforts to control them?  The list of  introduced insects and diseases is very long and getting longer.  The growth of international commerce and lax inspection programs are the root cause. Why is it, though, that it is so difficult to eradicate or even control these pests and pathogens?

great curve

Ash mortality curve (idealized) due to emerald ash borer. Arrow at 2015, the beginning of explosive increase in mortality

The answer lies in the mathematics of nature, specifically in the phenomenon known as the Great Curve of Growth. Mathematically, the Great Curve follows a logistic equation.  It works like this:

  1. At first, ash borer numbers are very small and few trees die even as the borer population increases.  This is called the lag period. Typically, the presence of the beetle is not observed for about 10 years after they arrive in an area. This is why eradication programs fail – eradication usually does not begin until the pest is well established.
  2. As beetle numbers increase, an explosion of death takes place, increasing steeply. This is the logarithmic phase. We are early in the logarithmic phase, and each subsequent year will bring approximately a doubling of the number of dead trees.
  3. Eventually, the beetles have killed most of the trees, and only a few live trees remain to be attacked. This is the stationary phase and ends with the elimination of white and green ash.

This is the fundamental challenge of introduced pests and pathogens: we are unlikely to notice them during the lag period as their populations are small and tree mortality is low.  By the time we realize the new pest is present, its populations, and tree mortality, are poised to accelerate.

Ash trees can be saved if treated prior to infestation, but this is not practical for most trees.

Ash trees can be saved if treated prior to infestation, but this is not practical for most trees.

We can conclude from this analysis that the next few years in Kentucky will see an explosion of dead trees. Is there anything that can stop this explosion of tree death?  Perhaps.

  1. Weather changes, especially extremely cold winters, could slow the beetle. However this beetle has thrived in the cold of Michigan, so this is unlikely to happen
  2. Predators and pests of emerald ash borer could increase in populations and lower the population of beetles. This would slow the rate of death of trees.  There are programs to releasepredatory wasps, but they are probably too little, too late.
  3. Treatment of trees is a known method to preserve important trees.  The cost of treatment has gone down and treatment is affordable for more landowners. Contact a certified arborist for more information.  In spite of the success of treatment, it is not possible to treat the majority of trees even in urban areas.  Treatment should be focused on large or important trees.
  4. Blue ash trees (Fraxinus quadrangulata) appear to be resistant. Our large population of blue ash trees are probably, but not certainly, safe.

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