The seven billion ash trees of North America are facing a very serious threat; already tens of millions have been killed. Unlike their Asian cousins, they have no biochemical defenses against the actions of the larvae (young) of Agrilus planipennis Fairmaire; given the common name of emerald ash borer or emerald ash borer beetle, since their discovery near Detroit in southeastern Michigan during the summer of 2002.
The emerald ash borer originates in northeastern Asia, so has little or no natural opposition in North America. Without natural predators, parasites or pathogens (disease causing microbes), A. planipennis populations are growing exponentially. Through their comparatively long, natural dispersal flight range and inadvertent transportation by people, these “metallic” beetles have expanded across 10 states centered on Michigan within the United States, as well as spreading into Ontario and Quebec in Canada.
The adult beetle lives on the outside of ash trees (Fraxinus spp.) eating the foliage, and is relatively harmless unless the tree’s population is extreme. The primary problem lies in this insect’s reproductive process; in particular where the larval stages live, what they feed upon and the way they do it.
The emerald ash borer beetle becomes sexually active just 15 to 25 days after eclosing as an adult, or in other words, after emerging from the pupal case where it transforms from a larva into an adult beetle. They stay in the breached pupal case for the first eight to fifteen days while their exoskeletons harden, before burrowing out through the bark and making a dispersal flight that can be up to several kilometers to find a new tree to call home.
Seven to ten days later, the females will mate with several males, increasing the genetic diversity of the 68 to 90 eggs they will oviposit (lay), thus optimizing the likelihood that some of their young will survive through to adulthood. The fertilized eggs mature within seven to nine days and are then oviposited in crevices in the bark of their mother’s ash tree home, usually on the side that receives the most sunlight. The eggs hatch from seven to nine days later; the hatchling is called a “first-instar larva” and the first thing it does is burrow through the bark into the bast, campium and outer sapwood of the tree.
The bast is a layer of the tree just under the bark that is also called the phloem. It is the food-transporting tissue of the tree, carrying the organic molecules produced by photosynthesis in the leaves to all other parts of the plant. The sapwood is composed of xylem, this is the tissue that carries water and nutrients up from the root system. The campium is a lateral meristem, the plant equivalent of stem cells, that is situated between the sapwood and the bast. It produces new xylem tissue on its inner surface and new phloem tissue on its outer surface, thus causing the trunk and branches to grow thicker.
The larvae go through a series of instar stages, each slightly bigger than the previous, eating the phloem, campium and xylem plant tissue for food and leaving s-shaped tunnels filled with sawdust and frass (larval excrement or dung) behind them. The larger they get the deeper they penetrate into the sapwood. Their tunnels disrupt or “short-circuit” the water and nutrient supply to the tree’s canopy (upper) levels, by connecting the upward-flow tubes in the xylem to the downward-flow tubes in the phloem.
The water and nutrients are normally drawn up through the xylem tubes by transpiration, the evaporation occurring on the leaf surfaces. This generates a suction much like a person sucking a drink through a straw. The tunnels created by the larvae have much the same effect as pinpricks in a drinking straw; instead of flowing up to the leaves, much of the water and nutrients in the xylem tubes passes through the larval tunnels into the phloem instead. A sufficiently heavy infestation can result in a top-down dieback from the lack of water and nutrients within a year or two, less concentrated larval populations will take longer to kill the tree but, almost certainly, will still do so eventually.
Having left their natural predators, parasites and pathogens back in Asia, and with the North American ash species not producing biochemicals that inhibit them, the emerald ash borer beetle population is exploding. Having a longer than normal, for Agrilus species, dispersal flight range and the unintended assistance of human transportation, that expanding population is spreading quite rapidly. Strict quarantines are in place to try to at least eliminate that human assistance, while studies and research continues apace to try and find a means of controlling or limiting this infestation, so deadly to North America’s ash trees.