Those Boring Beetles
When it comes to the destruction of trees, next to forest fires the most likely runner-up is the beetles that bore into the trunks and major branches of trees. There are several families of beetles that are capable of doing this to live trees, but one family in particular is the cause of an occasional widespread kill of forest or landscape trees, and this is the family called the Bark Beetles, the family Scolytidae. Many other names are given to these small beetles, such as the Turpentine Beetle, Engraver Beetle, Ambrosia Beetle, and Shothole Borer, and these sometimes are descriptive of the kinds of damage they do or evidence they leave. This is the group we will take a look at in the most detail, and offer you some insights on how to deal with them.
Now, there also are many other kinds of beetles whose larvae feed on wood as well, but on dead wood only, and these are not going to be the cause of a tree dying. In fact, they are some of the necessary recyclers of trees that have died, and are the organisms that begin the process of tearing that wood down into the reusable compost the soil needs. Some of these beetles are the huge Long Horned Wood Boring Beetles or the lovely Metallic Wood Boring Beetles, and the evidence that they have been working is only made visible when the adult beetles finally bore their way out of the bark, leaving large holes behind them. You might be concerned some winter night, when some of them emerge from some firewood you bought the summer before, and brought into the house for a week or so. No problem though, they will not attack your house.
The damage from bark beetles begins with the fertilized females, who find a suitable and susceptible tree and land on its trunk. The adult beetles fly, so they can go long distances in their search for food for their babies. The beetle then chews her way into the bark to get under the outer layer of dead wood and into the living tissue just under the bark. This area of the tree is called the cambium layer, and it may be only a few cell-layers thick, with nutrients produced by the leaves moving down to the roots in one area and water and soil nutrients moving up to the leaves in another area. It is in this narrow layer of vital, living tissues that she will lay her eggs, and once their she will chew her way vertically under the bark laying eggs as she goes.
The many eggs now hatch into little tiny larvae that look like small white worms. This will be the destructive stage, as the larvae now move out laterally, or sideways, in the cambium layer, feeding on that important area. They may move through a foot or more of cambium tissue, effectively cutting off the ability for the tree to transport its nutrients either up or down the tree at that area. Since it is very common for many beetles to have laid many, many eggs in the same tree and on all sides of the tree, as all of these larvae do their work they quickly eliminate all ability for the tree to sustain itself, and it dies.
Before we too quickly condemn this wanton destruction of nature we must try to keep it in perspective. While it is agonizing for us to watch our cherished landscape trees die, or see an area of forest lose half of its trees where we love to wander and appreciate nature, in the grand scheme of the needs of the environment the bark beetles are part of the natural process of thinning out older or weakened trees, providing the dead trees that eventually become nutrients back in the soil, and even providing dead trees used by many other animals for roosting or nesting sites. Just as the forest fire provides the impetus that opens some pine cones to release their seeds, so do bark beetles ultimately have a beneficial role. It is just difficult for us to think in terms of the many decades needed for that area of forest to return to “normal”.
An important word I used in the last sentence is one key you have to preventing the invasion of bark beetles into your landscape trees, and that word was “weakened”. Bark Beetle females have a much easier time penetrating to the interior of a tree that is under stress, and that stress may come from disease, malnutrition, physical damage to its trunk, or drought and lack of water. All of these conditions you are likely to have control over in your own landscape, while in a forest environment we do not, and the beetles will do what they can. In California a drought over a two year period created a stress condition for pine trees over wide areas of the Sierra Nevada Mountains, and some large patches of forest near Lake Tahoe lost up to 60% of the trees to bark beetles.
In your landscape then, one of the most important things you can do to protect your trees from attack by bark beetles is to keep them healthy. This begins with planting the right tree in the right place so that it can grow under conditions it needs. If it is a sun-loving tree don’t expect it to thrive in shade. If it likes moisture around its roots then dry soils will weaken it, and if it is susceptible to root rot then perpetually wet soils will harm it. Using California as the example again, many of the huge, spreading native oaks have grown for centuries on the dry hillsides and grasslands, becoming adapted to zero water around their crowns from May to October. When homes are built around these oaks and lawns and landscape installed up to their trunks, we often see the rather sudden death of the majestic tree due to fungus and rot in roots that did not want water.
Determine from your local professional nursery or local University Extension Service what the proper care is for the kinds of trees in your landscape, and then water, prune, fertilize, and otherwise care for it as needed. Water may be a major factor in preventing the intrusion of bark beetles, for the tree that is healthy and with proper water pressure inside it may be able to repel the invader by pushing it back out with sap or water flow. Most plants have evolved defenses against insect attack over their millions of years of coexistence, and trees are no different.
The evidence you are likely to see first, when a tree is infected with bark beetles, is small holes suddenly appearing all over the trunk or on the major limbs. Unfortunately, once you see these there probably already has been considerable damage inside, and it may even be too late to save the tree. If this is the case though, consult with a professional tree care or pest control company to see what options you have. At this time there is no guaranteed method for killing the larvae of beetles that are working under the bark, even though some kinds of chemical applications do seem to be effective once in awhile. The problem is that of getting the chemical into the tissue of the tree the bark beetle larvae are feeding on, and even soil injection of systemic chemicals that are taken up by the roots of the tree do not appear to be widely effective, as they pass by that area on their way to the leaves and the beetle larvae are not exposed to them.
Professional companies also have a few options for applying preventive chemicals to the outside of the tree trunk, and most homeowners have neither these chemicals nor the specialized equipment necessary for large trees. The material should be applied just prior to the time when the adult beetles would be active, and if applied thoroughly it is hoped that the beetles will be killed before they have entered the tree and laid eggs. The pesticides available are not going to be effective for more than a few months, so applications are needed every year, or perhaps more often, and they are effective only at preventing infestation, not curing an existing one.
Some of the most susceptible trees are conifers, and this includes not only pine, cedar, fir and spruce, but also the large, tree-like junipers and cypress. In landscape there are also Shothole Borers that make short work of trees such as plum, cherry, apricot, or other “stone-fruit” trees. Once you see the extensive array of holes from the emerging adults you might take a screwdriver and peel back a bit of the bark. By now it probably will come away very easily, and reveal to you the thin, wandering chambers the beetle larvae ate their way through as they grew. This will confirm to you the presence of bark beetles.
Bark Beetles are also responsible for a terrible disease spread to elm trees, called Dutch Elm Disease, a tree disease that has virtually wiped out elm trees in many parts of the country. We will cover this is a different BugInfo article, but the short version is that the female beetle carries the fungus on her body, and as she penetrates to the cambium of the tree she infects that tree with the disease. The standard “control” for Dutch Elm Disease, at this time, is to remove the tree and burn it.
The last area I would like to cover here in “Those Boring Beetles” is a few species of beetles other than the bark beetles that have begun to cause widespread destruction. These are recent imports to the United States, and continuing with my theme on the hazards of importing “exotic” animals we are now witnessing the environmental terror exotic insects can cause in an environment where they do not belong. These beetles are in the family Cerambycidae, and they are called the Long-horned Wood Boring Beetles, due to the very long antennae on the adult insects. They are truly beautiful beetles, so it is unfortunate that they cause so much damage.
The first is called the Eucalyptus Long Horn, and it appeared in the western United States in the late 1980’s, as a hitchhiker from its native Australia. If we were cynics we might propose that this beetle was brought in deliberately by those who would like to see the west cleared of eucalyptus trees, for these trees are not natives either. Biologically they are labeled as “weeds”, having been brought in as landscape or windbreak plantings many decades ago, and in some areas they have caused environmental damage by their invasive growth and competition with native plants. Eucalyptus trees are native to the Indo-Australian region of the world, and when the Eucalyptus Long Horn found its way to California it had a ready-made food supply. This large, shiny, black and yellow beetle has caused the death of many eucalyptus trees.
The second long-horned beetle is the Asian Long-horn, and this large, shiny blue beetle has been in the U.S. for only a few years, apparently having snuck in by infesting wooden packing crates brought into the eastern U.S. on board ships. It found its new home here in the United States appropriate for its survival and set about finding a food source, which is virtually any of the hardwood trees that constitute so much of the eastern and Midwestern forests in this country. Already it has cause widespread destruction of forests in the New England area, and one forest expert has predicted that this beetle may cause more damage to our environment than the Gypsy Moth has. The ability to control it has not yet been found.
While there are a few other minor players in this family of Long-horned beetles, that are capable of causing some damage to living trees, the great majority of the species will only feed in dead wood, and for this we thank them. They are doing a great service in the smooth operation of a forest, and since they cannot harm us and are beautiful to look at it’s nice to have them around. Once in awhile some of the larger kinds even get built into homes by being present in the 2 x 4 lumber used for wall studs, and we get that unwanted close-up look at them when they come through the walls and into our living room or bedroom. However, this is a rare occurrence, and little damage is actually done.
So, what are our lessons in the area of beetles that attack trees? First, most of them are in dead trees only and are beneficial. Second, good plant health is the best deterrent to keep the destructive kinds out of our landscape trees. Third, we must be very careful what we bring into this country, for the hitch-hiking insects may have the potential to cause widespread destruction in their new home.