When prolonged drought occurs in the east Texas piney woods, an increase in Ips or pine engraver beetle activity is likely to occur. Engraver beetles are small, brown to black, cylindrical insects that attack and kill pine trees by feeding and laying eggs in the inner bark of the tree. Engraver beetles usually breed harmlessly in fresh logging debris and weakened trees and do not kill a significant number of pine trees to be considered a major pest. However, when trees are weakened or stressed due to drought or other conditions, engraver beetles may attack and kill a significant number of trees. Almost any aged tree may be attacked, but generally, trees less than 10 years old may be killed by drought alone.
Ips beetle activity occurs every year, and attacks are usually very scattered and involve only a few trees in an infestation. The beetles follow this pattern during drought years, too, but they kill a greater number of scattered trees. Some infestation areas containing a thousand or more trees may occur, particularly along the western fringe of the natural range of pine in East Texas. The beetles are responding to the drought-stressed pine trees. Even in these areas though, not all the pine trees are killed.
Prolonged droughts, especially during the growing season, historically have been associated with outbreaks of engraver beetles in North America. Timber growers can expect to experience a significant drought at least once during the life of a pulpwood stand and twice during a sawtimber rotation. Pine trees growing in shallow soils or heavy clay soils are especially subject to moisture stress during droughts. Fire, hail, ice, lightening, wind, standing water, disease, logging, and other factors also may make pine trees more susceptible to engraver beetle attacks.
There are three principal species of engraver beetles that attack and kill southern pines. They are the eastern six-spined engraver (Ips calligraphus) which is about 5 mm long, the eastern five-spined engraver (Ips grandicollis) which is about 4 mm long, and the small southern pine engraver (Ips avulsus) which is about 3 mm long. The six-spined engraver usually is found in large diameter material such as the tree's trunk and large branches. The five-spined engraver usually is found in medium-sized material such as the upper trunk and large branches or in small diameter trees. The small southern pine engraver is almost always confined to branches in the top of the tree. It is not uncommon to find all three species in a single tree. If only the small southern pine engraver attacks a tree, the top portion of the crown may die, while the lower limbs remain alive. However, even these trees succumb to the beetles eventually.
Two other bark beetle species may also be present in beetle-killed pine trees. The southern pine beetle (Dendroctonus frontalis) is the most damaging insect pest in the southern forests of the United States. Besides fire, this beetle is usually the main concern of forest landowners in Texas. It is capable of killing large numbers of trees (even healthy trees) and a single infestation may encompass hundreds or thousands of acres. On the other hand, pine engraver beetles would never kill large areas of timber in a single infestation. Populations of southern pine beetles fluctuate from year to year in east Texas, and fortunately, no infestations of this insect were reported in 1999 and 2000. During the worst year on record (1985), over 15,000 infestations caused by southern pine beetles were reported across east Texas.
The other pine bark beetle of concern in east Texas is the black turpentine beetle (Dendroctonus terebrans). This beetle readily responds to fresh pine sap (resin) associated with injured trees. Like the three species of engraver beetles, the black turpentine beetle is not usually a serious problem because their typical attack pattern is to infest scattered trees. The turpentine beetle is most commonly found in stumps and injured trees associated with logging activity.
It is important to determine which of the five bark beetles mentioned above has attacked a tree. All of the beetles chew holes through the bark and feed and lay eggs in the inner bark or cambium (the area between the bark and the wood). Bark beetles do not bore into the wood of the tree. The three species of engraver beetles construct distinct, vertical-oriented galleries or tunnels generally in the shape of the letter "Y," "H," or "I" as they lay their eggs in the inner bark. The small southern pine engraver usually constructs the unbranched "I-" shaped gallery. As the beetles construct these egg galleries, the pattern is etched on the inside of the bark as well as the outer sapwood of the tree. The presence of the vertical galleries in the shape of "Y," "H," or "I" is the easiest way to identify engraver beetle attacks. These galleries may range from an inch or two to over a foot in length.
By contrast, the southern pine beetle constructs a winding gallery in the shape of the letter "S." The gallery of the black turpentine beetle has no particular shape, but the attacks of this beetle are usually limited to the bottom six to eight feet of the trunk of the pine tree and a large mass of pitch or resin will usually form where they attack. It is a good idea to remove some bark from a recently-attacked tree to look for the distinct gallery pattern made by the adult bark beetle since all five of the pine bark beetles are quite small and can easily be confused with other insects found in dead pine trees.
Although it is often overlooked, the first sign of attack by engraver beetles is the presence of reddish-brown boring dust in the crevices of the bark. When a tree has a good supply of moisture, sufficient pitch or resin will be produced such that a glob of resin, called a pitch tube, will form where the beetle attacks the tree. This pitch tube will often have a reddish-brown appearance because of boring dust mixed with the resin. Pine engraver beetles tend to attack the tree on the flat bark plates and that is where the pitch tube will form. By contrast, the southern pine beetle tends to attack in the crevices between the bark plates and their pitch tubes are generally a creamy white color. Pitch tubes are a sure sign of attack. However, during periods of drought, pitch tubes may not form on the bark of the trees and only the boring dust and small round holes (about the size of a pencil lead) will be visible. Once engraver beetles colonize the tree, it will soon die. The next visible characteristic of attack will be the foliage (needles) of the tree turning from green to yellow to red. During the heat and drought typical of late summer, the tree's foliage will turn from green to red in about three weeks. But, during winter when temperatures are cooler and there is generally more rain, the needles of a beetle-killed pine tree may remain green for 60 or more days after the tree dies. Once the needles turn red, the tree is dead and cannot be saved (remember: if the needles are red, the tree is dead). Pine trees seldom survive when engraver beetles have attacked them. It is important to keep in mind that engraver beetles may have attacked the upper portion of the tree and killed the top, but have yet to attack the lower portion of the bole where a person could reach while standing on the ground.
As the engraver beetles construct their egg galleries underneath the bark, the female beetles will lay eggs along the sides of the gallery. From these eggs hatch small, white grubs that feed and then pupate in the inner bark. When the pupae mature, they transform into new adult beetles. The new adults then chew a small, pencil-sized round hole in the bark, emerge, and fly in search of another tree to attack. During the summer months when daily high temperatures are in the 90s, the time from egg to new adult beetle (one generation) may be as short as 21 days. Overlapping generations of the beetles occur such that all stages of the beetle are present at all times. When temperatures drop into the 50s, very little beetle development and activity will occur.
Maintaining healthy trees is a landowner's best policy for preventing engraver beetle attacks. In a forest situation, good forest management practices are also good beetle prevention practices. If direct control is needed for an infestation of engraver beetles in a forest situation, cutting and removing the infested trees is the best course of action to follow. If only a few trees are involved, doing nothing is often a good choice. Felling trees and leaving them on site is of no value for controlling Ips. Cutting a buffer of green, uninfested trees around an area of Ips-killed trees is not recommended either.
In yard situations, watering trees (slow and deep) may be a homeowner's best action to prevent engraver beetle attacks. Prompt removal of visibly infested trees (trees that contain some life stage of the beetle -- egg, larva, pupa, adult) is recommended. If a beetle-killed tree is cut, care should be taken to avoid damage to residual, uninfested pine trees. Damaged trees may be more susceptible to attack by pine beetles. Adjacent uninfested pines can be sprayed with a preventive insecticide registered for pine bark beetles (such as Astro; traditional bark beetle insecticides like Lindane and Dursban are probably unavailable). However, spraying is only recommended for extremely high value trees and in many cases is not practical to apply in urban settings. Homeowners need to consider the cost of spraying, especially when the tree might not be attacked anyway (remember, Ips beetles tend to attack scattered trees). Also, if the bark surface from the ground to the crown of the tree is not covered with insecticide, the tree has not been completely protected. Then there is the environmental concern of spray drift to non-target areas when the insecticide is sprayed into the top of large trees. A neighbor may not appreciate your insecticide drifting onto his property.
If insecticides are used for controlling engraver beetles or protecting individual trees, a topical spray is preferred over injection of systemic insecticides. Systemic insecticide injection sounds like a great solution to the Ips problem, but this technique has met with certain problems. First, injectors tend to fill with pine resin or pitch rather than the insecticide going into the tree. Second, if the insecticide is successfully injected, most of the material is translocated to the top of the tree rather than to the inner bark area where pine bark beetles feed. Other novel techniques for controlling pine bark beetles have been suggested. One method involved connecting a 12-volt car battery to a pine tree to electrocute pine bark beetles or their larvae. This is of no value for control.
If Ips-killed trees occur in an area where they could eventually fall on structures, roads, power lines, fences, etc., the trees should be considered a hazard and removed. Generally, 6-10 months after a pine tree dies, it will have decayed to the extent that large limbs, the top, or the trunk could break.
During the fall, it is natural for pine trees to develop small "flags" of yellow and red needles scattered through the crown of the tree. Also, discoloration of the second year needles (needles away from the end of a branch) occurs in the fall and during drought stress. These needles may persist on the branches, and needles that drop from upper branches may lodge on lower branches. This tends to give the tree's foliage an off-color or unhealthy appearance. In addition, an entire branch in the lower crown (usually a bottom branch) may die in the fall of the year. This is normal and does not indicate that pine bark beetles are beginning to attack the tree. However, the casual observer may MISTAKENLY think the tree is succumbing to pine bark beetles. On the other hand, if the upper third or half of the tree's crown turns red, or all the needles have turned red, it is likely to have been attacked by pine engraver beetles. Because of the sometimes-confusing color pattern of pine needles in the fall, landowners should be cautious about assuming they have an epidemic of engraver beetles in their trees or timber.
In a discussion of pine engraver beetles, there is need to mention at least two insects that are commonly associated with any of the pine bark beetles. One associate is the ambrosia beetle. This insect is responsible for the fluffy, white sawdust that is often seen around the base of a dead or dying pine tree. These small beetles (about 3 mm long) bore into the sapwood of the tree. Ambrosia beetles are not responsible for killing the tree, but when their white sawdust is present around the base of the tree, the tree is probably dead. Another associate of pine bark beetles is a round-headed borer commonly called a sawyer. These beetles are attracted to pine trees that are very weak or recently dead. The adult beetle is about ½-inch long, has a mottled brown color pattern, and has antennae at least as long as the body. The female will lay her eggs in a small cone-shaped pit she chews in the bark. The larvae (or grub) will feed between the bark and the wood for a period of time and then bore into the sapwood of the tree. On a still, warm day, it is not uncommon to hear a rhythmic chewing or crunching sound made by the larvae of the sawyer as they feed. Mature larvae may reach a length of two inches.
Ips bark beetle activity can be widespread in East Texas during times of drought, and many landowners can be impacted. Landowners are encouraged to closely watch their trees and their pine stands during periods of drought. If engraver beetles attack, direct control may be warranted. Incorrect information about engraver beetles tends to abound in drought situations. Landowners may be told they need to cut all their pine trees before the pine beetles kill the trees. Only in rare cases would this be true. Property owners are urged to exercise caution before cutting pine trees because of pine engraver beetle attacks. For additional information, please contact your nearest Texas A&M Forest Service office or Forest Health, P.O. Box 310, Lufkin, TX 75902.