Ash and Oak Borer Beetles
Since European immigrants made their way to the “New World” and brought along their domesticated animals there has been a regular highway of other exotic invaders. As commerce has increased from one country to another this influx of non-native organisms has accelerated, and with the increasing level of importation of products from Asia in the past few decades we are finding more and more living organisms native to those countries finding their way to this continent. Even species that are native to North America have recently expanded their range, and when entering new geographic regions they also may pose problems. The two most recent insect invaders we will focus on in this BugInfo article are the Emerald Ash Borer – Agrilus planipennis – and the Goldspotted Oak Borer – Agrilus coxalis, both beetles in the family Buprestidae, a family of lovely beetles called the Metallic Wood Boring Beetles.
While many of the species of beetles in the Buprestidae are important in the decomposition and recycling of dead wood, some species also feed on living trees and may result in the death of those trees. Similar to the girdling action caused by Bark Beetles as they feed under the bark in the cambium layer of a tree, the presence of these beetles can result in a tree’s inability to transport nutrients and water, and that tree’s death in a very short period of time. The Emerald Ash Borer (EAB) is native to China and other surrounding countries of Asia, and the Gold Spotted Oak Borer (GSOB) appears to be native to Central America and southern Arizona. Since these beetles have no ability to fly across an ocean, how is it the EAB found its way to North America.
The EAB was first discovered in the United States in 2002 in Michigan, where it was identified as the cause of the deaths of many ash trees in southeast Michigan. It is believed to have been brought into that state in wooden packing crates from China. Since that first discovery this destructive beetle has become established throughout most of the northeast United States and southeast Canada, and it has the ability to kill virtually any and all ash trees. On its own the EAB is capable of flying, but no more than ½ mile in a year, so the rapid spread throughout the northeast must be due to some other means of transport, and it is believed that this has been in firewood, infested nursery stock, and in unprocessed lumber products of ash. Government agencies now have put out a desperate plea to the general public to not move any wood products out of their own area, in the hope of stopping the spread while a management program is devised.
As discussed in another BugInfo article on Exotic Invaders, in its native habitat an insect like the EAB may be kept in check by other forces there. Parasites or predators native to that habitat may feed on the beetle, or through millions of years of association between the beetle and the trees there may have evolved varieties of the trees that can now tolerate the beetle to some extent. When the EAB reached the United States it found no competition and no enemies, and all native ash tree varieties are susceptible to the attack and destruction by the beetle.
The Gold Spotted Oak Borer did not have so far to travel, but in 2004 it was discovered for the first time in San Diego County in southern California, and by 2008 it was found to be attacking and killing several species of native oak trees in that area. Coupled with the Sudden Oak Death fungus disease that struck California’s majestic oaks several years ago, these trees face an uncertain future. Exactly why this beetle has suddenly appeared in California after being a resident just one state to the east for hundreds of years is not known for certain, but there is evidence that it, like the EAB, has hitchhiked in firewood, in this case oak firewood brought in from Mexico.
The effect on the trees that are infested with GSOB begins as visible staining or black areas with red blisters on the bark, and with sap oozing from the bark. As a generation of the adult beetles emerges from the tree they leave small oval holes through the bark. Woodpeckers detect the presence of the beetle larvae living just beneath the bark and they will peck at the bark to remove it and expose the larvae, now leaving bare patches on the sides of the tree. On close examination the meandering feeding channels of the beetles can be seen, evidence that they have destroyed the tree’s cambium layer at that point. The stains on the bark represent these dead areas beneath, which cannot repair themselves. It is the cambium layer of living cells in which transport of water and nutrients between roots and foliage occurs, and once this area dies the tree no longer can sustain itself. Within just 2 or 3 years of the initial attack by the beetles there will be dieback of foliage and outer limbs, and ultimately overall death of the tree.
The Emerald Ash Borer attacks in a similar manner. Each female beetle will deposit around 60 eggs on the bark of an ash tree, and within a week the eggs hatch and the larvae bore through the bark and into the cambium layer beneath. On its own the feeding of a single larva may not threaten the tree, but when many dozens of them feed in one tree they cause mortal damage. Each generation may take 2 years to complete, but heavily infested trees will die after about 4 years of attack, and it is almost certain that an infested tree will die. Once attacked the tree becomes stressed due to loss of foliage and branches, and a stressed tree now becomes more attractive to other female beetles who deposit eggs on it as well, accelerating the pace at which the tree’s death occurs.
So, what can the homeowner do to help stop the spread of these kinds of threats to our forests? To begin with, we must not move cut wood from place to place when a known insect problem such as these occurs in our area. There may be no outward sign on the firewood that the beetles are living within, but as soon as that wood warms up it can trigger the emergence of the adult beetles, which now will mate and deposit eggs of their own. If you are in an area known to be infested with such imported pests you should keep an eye on the susceptible trees around you, and notify your local Department of Agriculture should you suspect a tree is showing the signs of an infestation, or if you find any of the beetles themselves that resemble these species. Mulching and chipping of cut wood to pieces smaller than 1 inch may kill any larvae or pupae that are in it. Very important is your awareness of the nature around you and the potential problems trees face.
And very important is to listen to the information coming from your local forestry, agriculture, or natural resources agencies, and heed their instructions and advice. The profound and negative impact many of these exotic pests can have on our lives can be minimized if we play our part.