Pine Bark Beetles
Cooperative Extension Service, College of Agricultural and Environmental Sciences, The University of Georgia Bulletin 1097, October 1993, 8 pp.
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.
|Figure 1. Losses and Costs of Control Due
to Pine BarkBeetles in Georgia, 1972-1991.
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 (Figure 2). 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.
|Figure 2. Blue Stain Fungi on Cross-Section of Infected Pine Truck.|
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 (Figure 3).
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 (Figure 4).
|Figure 3. Pine Beetle "Pitched-Out" on Pine Tree.||Figure 4. Major Trunk Areas Usually Attacked
by the Three Common Types of Bark Beetles.
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 (Figure 5).
|Figure 5. Life Cycle of Pine Bark Beetles;
the Southern Pine Beetle Life Cycle Shown.
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.
With the information contained in this publication the landowner, forest manager or homeowner can:
- Recognize bark beetle infestations
- Identify the bark beetle(s) causing the problem
- Understand basic biological information about each bark beetle species
- Learn what techniques are available to deal with bark beetles
- Find other sources of help and information.