Southern Pine Beetle

Dendroctonus frontalis Zimmermann (Coleoptera: Scolytidae)

Contact: Eric Day, Manager, Insect Identification Laboratory

Southern Pine Beetle Factsheet

1997

Eric R. Day, Manager
Insect Identification Laboratory
Entomology Department
Virginia Tech
idlab@vt.edu

ENTOMOLOGY PUBLICATION 444-243


Identification and Life History

Adult southern pine beetles (SPB) are 2 - 4 mm in length and brownish-black in color. They are cylindrical and somewhat stout to elongated in shape. The head is broad and prominent with a distinct longitudinal median groove bordered by a narrow elevation of tubercles (protrusions) on each side. The pronotum (shield behind the head) is slightly narrow at front, broadest at the middle, and about as long as wide. Males and females can be differentiated by the large tubercles of the male and the raised mycangium (anterior portion of the pronotum) of the female. The wings are as wide as and over twice as long as the pronotum.

The beetles may have as few as three generations in Virginia and as many as seven generations in Texas where they may remain active year-round. The SPB overwinters within the bark of trees at all life stages. Generally, in the spring when the dogwoods bloom, adults begin to fly. Females land on host trees, 2 - 9 m above the ground, bore through the bark, and if successful, produce a pheromone that attracts other males and females. Along with host volatiles, pheromones produced by the females, induce other SPB to attack the tree en mass such that they are able to overcome the trees defenses, mate, and reemerge to attack the same tree or an unattacked tree. Within the infestation fresh-attacked trees serve as a center of attraction for beetles emerging from within the same infestation, particularly during the late spring, summer, and fall. As the infestation grows, adjacent trees succumb to attack, resulting in a group of trees with thousands of beetles producing pheromones. Infestations often have multiple heads and expand in more than one direction.

Mating takes place in a special chamber of the inner bark formed by the female. Females lay eggs on alternate sides of s-shaped egg galleries which they build in the phloem tissue of the tree. The eggs are opaque, pearly white, and shiny, measuring about 1.5 mm long by 1 mm wide. They are slightly oblong to oval in shape. Developmental duration from egg to adult generally spans 26 to 54 days, depending upon the season and geographical location. The larva is a wrinkled, legless grub with three thoracic and 10 abdominal segments. The larva is yellowish white, and 2 mm long upon emergence from the egg, developing up to 5 - 7 mm long prior to pupation. Southern pine beetle larvae exhibit four instars or periods between molts. First instar larvae can be found in the phloem tissue of the tree just under the bark. As the larva matures and progresses toward the fourth instar, it slowly moves toward the outer bark layer of the tree. The pupa is also yellowish-white. It has the form of the adult, but the wing pads and legs are folded beneath with the abdominal segments exposed. The pupae range in size from 3 - 4 mm in length. Immature adults that are not ready to emerge from trees are called callow adults and are light brown in color. Pupae and callow adults occur near the outer bark where they complete their development. Click here to see an illustrated example of SPB development from egg to adult.

Distribution and Hosts

SPB occurs in the southern and southeastern United States, extending as far west as Arizona and as far south as Central America. The northern range extends from southern New Jersey and Pennsylvania, west to southern Missouri, south to east Texas, and east into Florida. Generally, in the southern U.S., this insect occurs wherever shortleaf and loblolly pines are grown. The beetle has been reported to attack and kill all pine species in its range, however, outbreaks (significant SPB activity over a 1-2 year period) are uncommon in the northernmost portion of its range except in areas where pine is commercially grown. In the southeastern states, loblolly and shortleaf pine are preferred, yet Virginia, pitch, table-mountain, longleaf, pond, and slash pines are often successfully attacked.

Description of Damage

The SPB is the most destructive forest insect pest in the southeastern U.S. They most commonly mass-attack the trunks of mature or overmature pine trees, but may attack and kill pines as young as five years of age. The beetles bore through the bark and develop s-shaped galleries in the phloem tissue where the eggs are laid and brood develop. They also introduce a blue-stain fungus into the sapwood that eventually inhibits water flow in the tree. The colonization by the beetles and the fungus combined results in tree death only days after mass-attack. It may take weeks or even months (if attacks are in late fall) for the foliage to fade from green to red. Pitch tubes are often present at the entrance holes made by the beetles. In addition, white, or reddish-yellowish boring dust may also be present in bark crevices and around the base of recently- attacked trees. The tree attempts to pitch out the beetles and is sometimes successful. Beetle populations will remain at endemic levels for years until populations build up to epidemic levels for a two-or three-year period. These cycles occur about every 7-10 years. Often infestations that show up in the spring do not continue and will die out. However, when populations are high, infestations can expand almost like wildfire within pure pine and mixed pine-hardwood stands, killing thousands of trees and covering hundreds of acres.

For trees that are killed, loggers try to salvage the wood as quickly as possible to recover the dead trees and minimize the amount of degrade. The blue-stain fungus is apparent in the sapwood, reducing the price at which sawtimber and pulpwood are sold.

Control

Prevention is the best form of control. Forest stands should be thinned two to three times during the rotation. Thinning keeps trees healthy and vigorous. Overmature stands or dense, unthinned pine plantations are most susceptible to attack. Therefore, a plan for periodic thinning and harvesting trees at rotation age (25-40 years) should be followed. To obtain information about a thinning plan for your stand, call your county or state forester.

It is very important to conduct aerial surveys for SPB activity in the spring and periodically in the summer. Groups of dead and dying pines with discolored foliage are plotted onto maps or aerial photographs. This must be followed up by immediate ground checking of suspected infestations. Quick action to protect a stand of trees is essential in trying to suppress the infestation. Expanding infestations can be halted using several cultural tactics. One such method known as cut-and-remove aims to salvage all old and new attacks, plus a buffer strip of unattacked trees in the direction the infestation is spreading. Size of buffer strips should range between 6 to 35 meters, depending on the size of the infestation and number of fresh-attacked trees present. Another option, referred to as cut-and-leave, is often applied in pre-commercial timber stands or in areas with difficult access. Under this treatment, a 6-35 m horseshoe-shaped buffer of uninfested trees is marked in the direction of infestation spread. All currently infested tree, plus those uninfested trees within the buffer, are felled toward the center of the infestation. Many of the brood in the bark of the felled trees will die from direct solar heating at the forest floor. Tree felling opens the stand to increased light and air currents which helps to disperse emerging beetles by dissipating the aggregation pheromones produced by surviving beetles. Brood that emerges from felled trees may disperse out of the infested area without initiating new infestations, due to the absence of aggregating pheromones. The cut-and-leave treatment is most effective for halting the spread of small infestations (< 100 trees) during summer months.

Knowledge of the complex chemical communication system used by this bark beetle species has stimulated scientists to develop a new suppression tactic based on disrupting the aggregation of adult beetles on host trees. While the tactic is not yet operational, it should provide an important alternative in the near future.

Insecticides are available for application; however, they are only feasible for situations where the owner is interested in protecting individual high-value trees. Please consult the Pest Management Guide for Horticultural and Forest Crops, Virginia Cooperative Extension Publication 456-017.

Further Reading

Billings, R. F. and C. Doggett. 1980. An Aerial Observer's Guide to Recognizing and Reporting Southern Pine Beetle Spots. U.S. Dep. Agric. Comb. For. Pest Res. Dev. Prog. Agric. Handbook No. 560. 19 p.

Billings, R. F. and H. A. Pase. 1983. A Field Guide for Ground Checking Southern Pine Beetle Spots. U.S. Dep. Agric. Comb. For. Pest Res. Dev. Prog. Agric. Handbook No. 558. 19 p.

Drooz, A.T. Insects of Eastern Forests. 1985. U.S.D.A. Forest Service. Miscellaneous Publication No. 1426. 608 pp.

Flamm, R.O., R.N. Coulson, and T.L. Payne. 1989. The southern pine beetle. pp. 531-553. In A.A. Berryman (ed.). Dynamics of Forest Insect Populations. Plenum Publ. Co.

Swain, K. M. and M. C. Remion. 1981. Direct Control Methods for the Southern Pine Beetle. U.S. Dep. Agric. Comb. For. Pest Res. Dev. Prog. Agric. Handbook No. 575. 15 p.

Thatcher, R.C. and P.J. Barry. 1982. Southern pine beetle. U.S.D.A. Forest Service Forest Insect & Disease Leaflet 49.

Thatcher, R.C., J.L. Searcy, J.E. Coster, and G.D. Hertel. 1980. The southern pine beetle. U.S.D.A. Forest Service Technical Bulletin 1631.

Prepared by S.M. Salom, Department of Entomology, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0319.