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Southern Pine Beetle

Written by G. K. Douce, C. Evans, and D. J. Moorhead for Forest Encyclopedia Network

Figure 1. Adult southern pine beetles. Photo by David T. Almquist, courtesy of forestryimages.org.

The adult southern pine beetle (SPB) is 2 to 4 millimeters in length, has a rounded abdomen, and is brownish-black in color (Figure 1). Males have a frontal groove on top of the head, while the females possess a broad elevated ridge called a mycangium on the anterior pronotum. Both males and females are capable fliers. Larvae are wrinkled, yellowish-white, legless grubs with prominent heads and stout, dark mandibles. Mature larvae are about 7 mm long.

Figure 2. ‘S’-shaped egg galleries produced by female beneath bark of attacked tree, typical of winter season. Note lack of sawyer galleries and blue stain fungus. Photo by Ronald F. Billings, courtesy of forestryimages.org.

Dependent upon temperatures, the beetles complete their life cycle – egg, larva, pupa, and adult – in 35 to 60 days, generally from April to September in the southern Appalachians. As many as seven generations (broods) may be produced annually, but in the northern extremities of its range (North Carolina, Virginia, and Maryland) as few as three generations may occur. A warming climate may increase brood production and, thus, the ability of this bark beetle to reproduce more quickly

After mating, the female excavates the characteristic S – or serpentine-shape gallery (Figure 2) – and lays eggs along both lateral walls of the gallery. Eggs are deposited singly in discrete cavities (egg niches). SPB galleries normally are created by one pair of adults. Eggs hatch in two to nine days, and the larvae enter the cambium layer, enlarging their galleries as they grow. When mature, larvae bore to the outer dead bark, create a cell, and pupate. Generally, emerging adults leave the host tree and aggregate on an adjacent tree or leave the area to find a suitable new host tree. Trees from which the brood have emerged are covered with large numbers of small (~1/16 inch in diameter) emergence holes. Adult emergence may occur over an extended time period of 10 to 32 days.

SPB attacks normally occur on open trunks of trees from the base to the crown, usually attacking first at midtrunk or in the lower crown. Large numbers of beetle adults and/or larvae girdle the tree by feeding under the bark on the phloem tissues. Besides their feeding, these 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 also cause blue staining of the sapwood, which can decrease the salvage value of the wood. Beetles also introduce mutualistic fungi, carried in special pouches, that supply necessary nutrients to beetle larvae.

Figure 3. Pitch tubes from a tree infested by southern pine beetle. Photo by Robert F. Billings, courtesy of forestryimages.org.

Bark beetle attacks can be recognized by the presence of boring dust, pitch tubes on the outside of the bark (Figure 3), 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, stressed trees may not form pitch tubes. Instead, attacks are indicated by the presence of brown boring dust. On severely weakened trees, brown boring dust accumulated in bark crevices or on spider webs is often the only visible sign of early attack (Douce 1993). SPBs rarely attack trees younger than 15 years and less than 2 inches in diameter.

Adapted for eXtension.org by Thomas DeGomez, University of Arizona

References cited:
Douce, G.K. 1993. Pine Bark Beetles. University of Georgia, College of Agriculture & Environmental Science, Cooperative Extension Service, Bulletin 1097: 8 p.

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