Origin of Emerald Ash Borer

The emerald ash beetle, also known as EAB or Agrilus planipennis, is the bane of American ash trees of all species. But this bright green and seriously detrimental pest did not originate in the United States. Rather, this wood boring insect is native to various parts of Asia, including parts of China, Japan and Korea. So how did a population of insects, each smaller in length than the width of a penny, travel the great distance from Asia to the U.S.? As with many types of organisms that cause infections, these insects were passed from infected host wood imported into the country to uninfected native ash tree populations. Several key aspects of the emerald ash beetles habits have made it possible for this transfer to take place.

Adept at remaining hidden for most of its life cycle, the emerald ash borer is only visible on the surface of a tree for the few weeks of its short adult life span. During this time, mating and egg laying occur. The eggs are placed in the crevices of the tree bark. Once the larvae hatch, they immediately burrow deep under the bark and into the growth and transport tissues of the tree called the phloem, xylem and cambium. Depending on the health of the tree and its geographic location, the developing larvae may remain hidden beneath the host tree bark for one to two years, exiting only once they have fully developed into a mature adult.

With most of its natural life span spent enclosed in tree tissues, it may not always be easy to spot a tree that is infected with emerald ash beetles. This was especially true back in the 1990’s when it is believed that the first transmission of this species to the U.S. occurred. Not much was known about these insects, including the symptoms of infection displayed by the affected tree, such as branch and leaf loss in the crown of the tree and tiny capital D shaped holes bored into the bark. Practically invisible during shipping inspections, the emerald ash borer migrated to the U.S. with little difficulty.

Another trick of the emerald ash borer that veiled its true threat and that is common of invasive non-native species is that the insects are not devastating to healthy trees in their regions of origin.
As with many native American beetle species, the emerald ash beetle only attacks and kills stressed and diseased trees in its native Asian habitat. The true threat of the emerald ash beetle was not able to be realized until the insect species established its population in the United States and began to decimate the native ash tree populations in the Midwest and on the East coast. If this fact had been known, stricter shipping regulations may have been created and enforced.

Today there is much interest in the emerald ash beetle and research is being conducted on a wide range of topics related to its life cycle, the differences in its native and non-native habits, and methods of slowing or halting population expansion. While the main focus on the emerald ash beetle must be control of the population in order to save the remaining ash tree species in America, research on the life cycle and native versus non-native activities of these tiny, yet largely destructive, green pests may help to prevent other invasive species migrating into the U.S.