Pine bark beetles
Pine bark beetles cost Georgia landowners from $100,000 to more than $25 million each year. Bark beetle populations and their subsequent damage vary tremendously between years and locations (Figure 1). Their attacks are not limited to timber production stands: they also attack and kill many high-value yard, ornamental, seed orchard and naval stores pines.
The first recognized sign of bark beetle attack often is yellowing or reddening tree crowns. Unfortunately, these symptoms usually are not evident until long after the attacks have begun and the trees are already dead. Many people also notice white, sawdust-like "boring dust" at the base of the tree. Ambrosia beetles, which attack the tree after the bark beetles have colonized and weakened it, produce this dust. By the time the crown changes colors, most of the bark beetles have completed their life cycle and emerged from the tree.
Of course, other factors such as disease, herbicide injury or mechanical damage can cause similar symptoms. The presence of bark beetles in the tree(s) should be verified before any action is taken.
All pine bark beetles common in Georgia can attack and kill living trees. Large numbers of beetle adults and/or larvae girdle the tree by feeding under the bark on the phloem tissues. Besides their feeding, some bark beetles carry blue stain fungi on their bodies. Once introduced into a tree, these fungi colonize the sapwood and disrupt the flow of water to the tree crown, killing the tree. These fungi usually cause blue-staining of the sapwood. Some scientists believe that action of these fungi make nutrients more readily available to the beetles, or that the beetles feed on the fungi.
The primary role of bark beetles in nature is to prepare the way for ecological succession in forests by selectively removing mature, senescent, stressed or damaged pines from the forest. However, bark beetle attacks on trees in commercial forests or yards are a serious problem for forest managers and homeowners. They often attack trees of great economic or aesthetic value. Bark beetle outbreaks can be extremely disruptive to forest management practices aimed at wood and fiber production.
Beetle infestations often begin on damaged or severely stressed trees. Odors emitted by trees struck by lightning, damaged by storms, mechanically injured by construction or harvesting equipment, or severely stressed by heavy pruning attract bark beetles from other places. Pines under any kind of stress are susceptible to attack by bark beetles, particularly under beetle outbreak conditions. Colonizing bark beetles attack living trees and (depending on species) may emit specialized chemicals called pheromones. These pheromones combine with volatiles released from the host tree to "call" in large numbers of beetles that "mass attack" a tree. Once on the tree, beetles bore through the bark to the cambium, mate and begin to construct galleries.Healthy trees respond to beetle attacks by exuding copious amounts of pitch or sap. This defensive response can sometimes "pitch-out" the attacking beetles.
The pitch and sap may exude from the entrance holes and harden on the bark surface to form pitch or resin tubes. Where the beetle attacks a tree is characteristic to each species.
Depending on species, either sex can begin the attack. Females that successfully infest trees, mate and lay eggs along the sides of the gallery. Eggs hatch in three to nine days into first-stage larvae. Bark beetle larvae are legless, creamy white and crescent-shaped with glossy, reddish-brown head capsules.
Bark beetles can also give off chemicals that inhibit the arrival of other beetles, preventing overpopulation of the trees by developing brood. Many natural enemies of bark beetles (predators and parasites) also use these chemical cues to locate their prey! Both sexes may reemerge from the tree and fly off to infest other trees. Populations of bark beetles can build up fast under favorable conditions. Depending on species and temperature, brood development time ranges from 25 to 120 days.
How to Recognize Infestations
Bark beetle attacks can be recognized by the presence of boring dust, pitch tubes on the outside of the bark, characteristic galleries under the bark, and beetle adults and larvae in the inner bark. The presence of "pitch tubes" on the bark of trees is one of the best ways to identify bark beetle attack. However, pitch tubes may not form on stressed trees. Instead, attacks are indicated by the presence of brown boring dust. Use a good pair of binoculars to observe the middle and upper trunk and the base of large limbs for pitch tubes, since the initial attack of some bark beetles occurs at mid-trunk height or above. Observing the change in color of tree crowns is not a good method of locating beetle attacks since it may take several weeks or longer after the initial attack for the crowns to "fade" to a light yellowish-green or red.
Identification, Biology and Management
Pine bark beetles belong to the Order Coleoptera, Family Scolytidae. The important pine bark beetles in Georgia are:
- The black turpentine beetle (BTB), Dendroctonus terebrans (Olivier)
- The southern pine beetle (SPB), Dendroctonus frontalis Zimmermann, and
- Four species of Ips beetles:
The top of the abdomen (back end) on adult BTB and SPB are rounded (Figure 6). In contrast, the rear end of adult Ips beetles are concave (scooped-out in appearance) with four to six pairs of spines on either side of the concaved areas (Figure 7).
Proper identification must be made of all suspect bark beetles. If you are unsure of your beetle identiftication, take bark and beetle samples to your local county Extension or Forestry Commission office for identification.
What can you do to reduce the likelihood of bark beetle infestations?
- Encourage vigorous tree growth. Susceptibility to beetle attack increases with stand age and slow diameter growth. Shorten pine rotations in areas where beetles have historically caused considerable timber losses.
- Delay harvest and thinning operations or severe pruning until winter when beetle activity has declined if bark beetles are active in your area.
- Remove storm- or lightning-damaged pine trees as quickly as possible. Damaged pines are ideal sites for the start of bark beetle infestations.
- Minimize mechanical tree damage such as skinning of the trunks, partial pushovers, etc. during construction or harvesting operations. Tree damage from recent logging activities favors all kinds of bark beetles.
- Remove and salvage skinned trees next to skid trails, logging roads and loading decks.
- Build barricades around trees to prevent mechanical damage by equipment in yard and landscape environments. Try to minimize root damage by keeping trenching and digging to a minimum.
- Maintain final soil level around tree trunks and roots at the same height as it was before construction during landscaping operations.
- Maintain recommended stocking rates for commercial forest sites. Overstocking results in reduced diameter growth and creates conditions favorable to beetle infestations. Thinning of overstocked stands improves tree growth and vigor and reduces the chances of bark beetle attack. Thinning intensity depends on the age of the stand, site index, total stand density and management objectives. In Georgia, basal areas of 80-100 feet-squared per acare are recommended to reduce the potential for a beetle attack. Pine stands in the northern half of Georgia are susceptible to ice damage if thinned to heavily. Planting around 600 pines per acre will reduce the number of thinnings required, thus lessening the effects of ice.
If there are active beetle infestations in your area, what should you do?
Contact the Georgia Forestry Commission county office if you are a commercial timber producer. The Commission routinely conducts aerial surveys for bark beetle detection. From the air, Commission personnel identify and locate possible beetle activity and mark these spots on maps for follow-up ground verification. These aerial detection surveys provide important information for bark beetle prevention and suppression programs. The surveys are available to you and can help you locate beetle spots.
Be alert for possible bark beetle infestations in the vicinity of your property . If you see spots of dead or dying trees, ask your county Extension agent, Georgia Forestry Commission personnel or city arborist to investigate the spot or check it yourself. These spots may have occurred since the last aerial survey was completed, or the landowner may not have been notified of the infestation. If trees are dying as a result of an active bark beetle infestation, the landowner/homeowner should be told about the infestation, or county/state government officials should be notified of the infestation. Hopefully, the landowner will take action against the infestation before it spreads to your property.
If beetles are active on your property, what can you do?
Look for the signs of recent beetle attack (eg. pitch tubes, boring dust, etc.) on your trees, not just changes in crown color. Identify the beetles that are causing the problem by removing bark and examining adults and/or gallery patterns present. Locate and mark all trees that have been attacked and identify all trees with developing brood.
Determine and begin appropriate management action(s) immediately. Current methods for dealing with bark beetle infestations are summarized below. Articles listed in the reference section can provide you with greater detail on management strategies. Contact your county Extension agent, Georgia Forestry Commission office or city arborist for assistance if you need help. Many tree surgeons or pest control companies also have the expertise and equipment to help you in dealing with bark beetle.
Before starting any management tactics, remember:
- If SPB or one of the Ips beetles have successfully constructed egg and larval galleries in a tree, the tree will die either as a result of the girdling of the tree by larval feeding, or the introduction and subsequent proliferation of blue stain fungi.
- If BTB are infesting the tree, it may be possible to save it if larval feeding has not completely girdled it and you take prompt action.
- Insecticides can be used to protect high-value, healthy trees and/or kill brood within infested trees if used properly. Before or when using insecticides, it is important to:
- Correctly identify the beetles causing the damage
- Carefully identify trees needing treatment,
- Use appropriate chemicals and application methods,
- Apply the chemicals in a timely manner,
- Cover each tree thoroughly, and
- Treat all trees that contain developing brood.
A Preventive Chemical treatment can be used when the cost of the treatment is more than offset by the potential losses that could occur should beetles attack and kill the tree(s). Preventive chemical applications protect trees from beetle attack for periods of several months up to a year if properly applied. This method is frequently used to protect high value trees in landscape, yard plantings and nursery stands when there has been beetle activity nearby. Don't apply insecticides to trees infested by SPBs or Ips beetles, unless the tree cannot be cut down and removed before the beetle brood emerges. Don't apply insecticides to trees from which bark beetle broods have already emerged. Spray trees damaged by construction equipment with an approved insecticide as a preventive measure against attack by BTBs, particularly if beetle activity has been high in the surrounding area. Contact your county Extension or Georgia Forestry Commission office for pesticide information.
In Landscape Trees:
If SPB or Ips beetle infestations are present, immediately remove all trees still infested or showing signs of attack, as well as any dead trees. An insecticide application to healthy trees in the vicinity of an infestation is advisable as a preventative measure against attack. Thoroughly saturate the bark of the entire trunk and the base of all large limbs on trees being treated. This will require specialized equipment. Check all pines in the area often with binoculars for the next several months for any signs of new beetle attack.
If BTB are infesting the tree(s) and the crown has not yet begun to fade, immediately apply an approved insecticide to the bottom eight to 10 feet of the trunk. Thorough coverage and wetting of the bark and exposed parts of the base of the tree are essential for BTB treatments. It would be advisable to apply a preventative insecticide treatment to the lower portion of adjacent non-infested pines. Make sure the attacked trees do not undergo any undue stress over the next few months. Do not trench, dig around the roots, etc. Of the attacked trees.
In Commercial Stands:
All trees that have developing beetle brood should be cut and removed from the site, if possible. In some cases, large numbers of developing insect predators and parasites may have not yet emerged from the tree. These insects often have longer life cycles than their bark beetle prey or appeared at the tree after the initial infestation by bark beetles. If you have verified that the bark beetles have already left the tree and the tree is not a hazard, consider leaving it standing to allow these beneficial insects to emerge to help control the future bark beetle populations.
If BTB are infesting the tree(s), it is sometimes feasible and cost effective to treat a limited number of infested trees with an insecticide to stop expansion of the spot (see above section under "In Landscape Trees").
If Ips beetles are infesting the tree(s), salvage removal, chemical treatment, cut and spray or pile and burn are the only effective management options available. If SPB are infesting the trees, select the management scheme best suited for your situation from those presented below.
- Chemical Applications - generally are used in commercial stands only on small numbers of trees. Chemical applications for bark beetle control usually are not practical, cost effective or environmentally wise when large numbers of trees are involved.
- Salvage Removal - is usually only feasible when arelatively large volume of wood is available and makes the operation cost effective for a logger. Prompt action is necessary to prevent significant degradation of the wood to allow for some economic return and to prevent further expansion of the bark beetle infestation. The first step is to remove a 50-100 foot (at least one tree height) buffer strip of green noninfested trees around the most recently attacked trees. Second, remove newly attacked trees and any trees containing beetle brood. Next, remove older standing trees from which the brood have already emerged.
- Cut and Leave - is best used for controlling small spots (10-50 trees) when salvage is not practical or cost effective. Fell attacked trees and a border of healthy trees toward the center of the beetle spot. It may be necessary to cut limbs on the underside of the felled trees to ensure that the tree trunks are lying on the ground. It is believed that the increase in mortality caused by the high temperatures on the bark of the trees, increased humidity on the underside of the trunks, and increased predation may combine with the disruption of emergence and attack patterns to lessen further infestation of standing trees. Cut-and-leave is practical, relatively inexpensive, and requires minimal manpower, equipment and training. Unfortunately, the owner/manager realizes no cost recovery.
- Cut and Spray - is a variation of cut-and-leave. Trees with developing brood are cut and all surfaces of the trunks and major limbs are sprayed with an approved insecticide using a small backpack sprayer to ensure that all developing beetle brood are killed. The felled trees are left in place in the forest. Trees adjacent to those cut should be checked frequently as a safeguard against possible- attack in the event that the original infestation was not completely eliminated.
- Pile and Burn - Trees with live brood are felled, piled and burned. A distinct disadvantage to this technique is the need for heavy equipment to pile the trees so that they can be burned. Infestations in pre-commercial stands can be controlled by knocking down all the trees and burning.
- Behavioral Modifying Techniques - Projects are under way in which researchers are trying to develop SPB control tactics by manipulating the various chemicals the beetles (or their natural enemies) use to orient, attack or disperse their populations. To date, these projects have met with limited success. Some advantages of these promising methods are:
- they are natural, non-toxic compounds
- they may be able to be formulated for easy application by non-professional field crews or small landowners and
- they do not require the felling of trees. Perhaps in the next few years these techniques will be available for use by timber and land managers.
- Belanger, R. P. And B. F. Malac. 1980. Silviculture can reduce losses from the southern pine beetle. USDA Agric. Comb. For. Pest Res. Develop. Prog. Handbook No. 576. 17pp.
- Berisford, C. W. , U. E. Brady, V. R. Coleman, L. K. Kudon, T. S. Price, J. W. Taylor and G. D. Walker. 1983. Bark beetles of southern pines - identification and control. The Georgia Forestry Commission, Macon, GA. 14pp.
- Billings, R. F. And H. A. Pase, III. 1979. A field guide for ground checking southern pine beetle spots. USDA Agric. Comb. For. Pest Res. Develop. Prog. Handbook No. 558. 19pp.
- Coulson, R. N. And J. A. Witter. 1984 Forest Entomology - Ecology and Management. John Wiley and Sons, Inc. 669pp.
- Douce, G.K. Pine Bark Beetles. The University of Georgia Bulletin 1097, October 1993, 8 pp.
- Goyer, R. A., G. J. Lenhard, T. E. Nebeker and L. D. Jarrard. 1981. How to identify common insect associates of the southern pine beetle. USDA Agric., Comb. For. Pest Res. Develop. Prog. Handbook No. 563. 31 pp.
- Karpinski, C., Jr., R. L. Hedden, R. P. Belanger and T. S. Price. 1984. Guidelines for managing pine bark beetles in Georgia. The Georgia Forestry Commission, Macon, GA. 61 pp.
- McPherson, R. M. and G. K. Douce. 1992. Summary of losses from insect damage and costs of control in Georgia , 1991. Univ. of GA. Coll. Of Agr. Exp. Stn. Special Pub. No. 81. 65 pp. (Contains 1991 information. Data for previous years contained in reports referenced in the 1992 publication.)
- Price, T. S. 1988. A guide to common forest pests in Georgia. The Georgia Forestry Commission, Macon, GA. 40 pp.
- Price, T. S., C. Doggett, J. M. Pye and T. P. Holmes, eds. 1992. A history of southern pine beetle outbreaks in the southeastern United States. Sponsored by the Southern Forest Insect Work Conference. The Georgia Forestry Commission, Macon, GA. 65 pp.
- SFIWC. Slides from series developed and maintained by the Southern Forest Insect Work Conference.
- Swain, K. M., Sr. and M. C. Remion. 1981. Direct control methods for the southern pine beetle. USDA Agric. Comb. For. Pest Res. Develop. Prog. Handbook No. 575 15 pp.
- Thatcher, R. C. and P. J. Barry. 1982. Southern pine beetle. USDA Agric. Forest Serv., Forest and Disease Leaflet No. 49. 7 pp.
- Thatcher, R. C., J. E. Coster and T. L. Payne. 1978. Southern pine beetles can kill your ornamental pine. USDA Agric. Comb. For. Pest Res. Develop. Prog. Home and Garden Bull. No. 226 15 pp.
- Thatcher, R. C., J. L. Searcy, J. E. Coster and G. D. Hertel (Eds.). 1980. The Southern Pine Beetle. USDA Forest Serv. Technical Bulletin 1631. 267 pp.