ENGRAVER BEETLE ACTIVITY ON THE INCREASE IN EAST TEXAS
With the prolonged drought that many areas of the east Texas piney woods are experiencing, an increase in Ips or engraver beetle activity is occurring. 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). This has been the case in east Texas this summer.
Ips beetle activity occurs every year, and attacks are usually quite scattered and involve only a few trees in an infestation. That pattern has continued this summer, but there are a greater number of scattered trees being killed. Some infestations containing 20-75 trees have been reported. It is believed that the beetles are responding to the drought-stressed pine trees. For example, I recently observed engraver beetles in an eight-year old loblolly pine plantation in San Augustine County.
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 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. 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.
Two other bark beetle species may also be present in beetle-killed pine trees. The southern pine beetle (Dendroctonus frontalis) is the most serious insect pest in the southern forests of the United States. Besides fire, this beetle is usually the main concern of forest landowners. It is capable of killing large numbers of trees (even healthy trees) and a single infestation may encompass hundreds of acres. Populations of southern pine beetles fluctuate from year to year in east Texas, and 1998 is proving to be a very low year for this insect. During the worst year on record (1985), over 15,000 infestations caused by southern pine beetles were reported in 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 engraver beetles, the black turpentine beetles are not usually a serious problem. They are most commonly found in stumps and injured residual trees following logging.
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 (the area between the bark and the wood). The three species of engraver beetles construct distinct galleries in the shape of a "Y," "H," or "I" as they lay their eggs in the inner bark. The small southern pine engraver usually constructs the "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 "Y-," "H-," or "I-" shaped gallery pattern that tends to be oriented vertically up and down the tree is the easiest way to identify engraver beetle attacks.
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 is healthy and 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. During periods of drought, pitch tubes may not form on the bark of the trees and only the boring dust will be visible. Once the tree becomes colonized by engraver beetles, 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 extreme heat this summer, the tree's foliage will turn from green to red in about three weeks. Pine trees seldom survive when they have been attacked by engraver beetles. 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 which feed and soon pupate in the inner bark. When the pupae mature, they transform into new adult beetles. The new adults then chew a small, 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 upper 90s (or higher), 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 below about 59EF., very little beetle development will occur.
Maintaining a healthy stand of trees is a landowner's best policy for preventing engraver beetle attacks. 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 about the only feasible course of action to follow. In yard situations, prompt removal of visibly infested trees is recommended.