The emerald ash borer, or Agrilus planipennis, kills ash trees through a slow process of strangulation of the trees’ circulatory systems. Vital water and nutrients that provide the limbs and leaves with hydration and energy are blocked by the colonization and growth of these tiny green pests. Likewise, nourishing sugar supplies are cut off from their ultimate destination in the roots. With dwindling nutritional support from below, the branches and leaves slowly begin to die off. With less and less of the sugars produced in photosynthesis to feed the tree roots, they begin to die as well. To gain a clearer understanding of the problem, it is important to look at the basic function and structure of a tree as well as the life cycle of the emerald ash borer to see how the habits of the ash borer interfere with the ash tree’s vital physiological processes. Another important consideration is that the Asian pest’s habits also lead to easy and often unwitting spread of infection and general ash tree population decline.
All trees have several layers of tissues in their trunks that have unique functions. A cross section through the body of a tree trunk, starting from the outside and traveling inward, would reveal the bark (made mainly of cork and cork cambium), phloem, cambium, xylem and finally the woody core. The bark is the older cork tissues that get pushed outward as new cork is produced. This origin is responsible for the cracked appearance of bark, whose dried and tough surface serves to shield the tree from outside forces.
The phloem itself transfers newly made sugars from the leaves at the tips of the branches in the canopy, all the way down to the tree roots deep underground. Conversely, the xylem draws up water and nutrients from deep below ground at the tips of the most microscopic root hairs all the way up the trunk and into the canopy to feed the tree’s growing and food producing appendages. Between the phloem and the xylem, the cambium is the generative tissue that produces the new tissues of both the food and water transporting systems, growing one type of tissue in each direction simultaneously. The inner core of the trunk is made of the growth tissues of past years the tree has outgrown and so have slowly gone dormant. These inner core layers are the rings of the tree that most people are familiar with and also make up the woody portion of the tree used for furniture and raw construction materials.
At the start of the ash beetle’s life cycle, the spaces in the cracks of the ash tree’s bark are used as housing for the eggs. Once the larvae hatch, they dig into the bark and make there way into the growth tissues beneath. The time spent in the larval stage seems to vary depending on the health of the tree and also the geographic location. Whether for one or two years, the larvae live comfortably in the growth tissues of the tree as they derive protection from the bark and constant food and water from the phloem, cambium and xylem tissues.
Eventually mature beetles will bore paths back out of the tree, exiting the bark through distinctive holes shaped like tiny capital D’s.
That period of time when the larvae infest the growth tissues of the tree and disrupt nutrient and water flow is when the emerald ash borer life cycle detrimentally affects the health of the ash tree. As the larvae eat these tissues, they wind there way through the tree trunk, making a distinct wavy or s-shaped pattern where the growth layers are eaten away. As the larvae mature and as the overall infestation of the tree progresses, more and more of the food and water transporting tissues are removed or disrupted and branches and leaves gradually and continuously fade. With the tree’s circulatory system first hindered and eventually cut off, the infested tree dies, often within about three years.
Besides the affect of the emerald ash borer on a single ash tree, these non-native invasive beetles have caused quite a sizable amount of devastation to ash trees in large geographical areas. Because the largest portion of the insects life cycle is spent under the bark, because the decline of the tree is incremental, and because a number of other tiny green insects also tend to inhabit the bark of trees, infection can be difficult to notice or even purposefully detect in the earlier stages.
While the general public and local and national agencies are very conscious of the issue today, when knowledge of the symptoms of emerald ash borer infestation and the danger posed by the ash borer were not so commonly understood, infected timber was transported to uninfected areas and even different geographic regions as firewood and raw construction material. The ease of introduction of the invasive beetle to previously uninfected areas has resulted in widespread presence of the species in the Midwest and along the East coast. Millions of trees have been killed since monitoring of the issue in 2002, and some authorities seem to believe the entire ash tree population of the United States is at risk.
The emerald ash borer’s life cycle and affinity for ash trees, combined with the ash tree’s natural defenselessness against the infectious intruders has resulted in an uncertain future for all ash species in the United States and Canada.