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Forest Insect
& Disease
Leaflet 49
Revised April 2009
U.S. Department of Agriculture • Forest Service
The southern pine beetle, Dendroctonus frontalis Zimmermann, is one of the most destructive insect pests of pines. Its range covers the southeastern United States from Pennsylvania and New Jersey to Texas, and from Arizona and New Mexico through Mexico to Nicara- gua (Fig. 1). Loblolly, shortleaf, pitch, pond, and Virginia pines are the favored hosts in the southeast U.S., while Pinus
(^1) Forest Entomologist, USDA Forest Service, Forest Health Protection, Southern Region,
Lufkin, TX. (^2) SPB Prevention Program Coordinator, USDA Forest Service, Forest Health Protection,
Southern Region, Asheville, NC.
Southern Pine Beetle
Stephen R. Clarke¹ and J. T. Nowak²
oocarpa and P. caribaea are preferred hosts in Mexico and Central America. During outbreaks, the southern pine beetle may infest all pine species, and even marginal hosts such as spruce and hemlock may be killed.
Populations often are concentrated in infestations or “spots.” Periodically beetle numbers may rapidly increase to outbreak levels, and healthy, vigorous pines may be at- tacked and killed as infesta- tions expand. The southern pine beetle generally is in outbreak status every year somewhere within its range. Average annual tree mortal- ity in the U.S. often exceeds 100 million board feet of sawtimber and 30 million cubic feet of pulpwood. From 1999-2002, an outbreak in the eastern U.S. caused in excess of one billion dollars in timber losses. Over 225, acres of pine forests in Central America were killed over that same period.
Figure 1. Range map for southern pine beetle; Mexico distribution adapted from Salinas-Moreno et al. 2004.
Life Stages and Attack
Sequence
The southern pine beetle has four life stages: egg, larva, pupa, and adult. The adult is dark red brown to black in color and 2-4 mm in length (Fig. 2). The rear end is rounded and the head is visible from above. The egg is pearly white (Fig. 3). The crescent-shaped larva is white with a reddish brown head (Fig. 3). There are four larval stag- es, with the fourth stage averaging 3 mm in length. The pupa is white (Fig.
- and develops into a callow adult, which is soft and amber-colored prior to darkening and hardening.
The female initiates the attack. Once a suitable host is located, the female re- leases aggregation pheromones, primar- ily frontalin. Frontalin, in combination with host odors, attracts a male for mat- ing plus additional males and females. Arriving males also release phero- mones, including endo -brevicomin, which may increase aggregation. If suf- ficient numbers of beetles are attracted, host resistance is overcome and the tree is successfully colonized. Verbenone is a pheromone produced essentially by males, and at high concentrations it can inhibit landing and cause beetles to switch attacks to adjacent pines.
Attacking females bore through the bark into the cambial layer. These at- tacks usually occur in the bark crevices. During initial attacks, the tree produces resin and the beetles may be “pitched out.” The male and female may work together to clear away the resin and suc- cessfully enter the bark. As a result, trees attacked by southern pine beetles are characterized by pitch tubes in the bark crevices, which, dependent on tree species, may be white or yellow and resemble popcorn or may be reddish (Figs. 4a, 4b).
Once beneath the bark, females begin constructing S-shaped egg galleries in the cambium (Fig. 5). Eggs are laid in niches cut in the wall of the gallery. The male follows behind the female, packing the galleries behind them tight-
Figure 2. Adult southern pine beetle.
Figure 3. Left-right: Southern pine beetle egg, early instar larva, late instar larva, pupa, callow adult, mature adult.
presence of reddish-white boring dust, tight bark with white phloem, and green crowns. Adult southern pine beetles may be observed working in the pitch tubes, and clerid beetles, Thanasimus dubius , often are present (Fig. 7). Once egg gallery construction is complete and the larvae hatch and begin feed- ing, the foliage begins to fade in color. A fading crown is symptomatic of a successfully colonized pine and usu- ally indicates that southern pine beetle brood is present. When a section of bark is removed from fading trees, the S-shaped egg galleries characteristic of the southern pine beetle are clearly vis- ible and the surface of the sapwood is brown in color. Adult clerid beetles are no longer present on these brood trees. Trees vacated by the brood adults have numerous exit holes in the bark and the needles are either red or have fallen off. The bark is very loose and peels away easily. White sawdust produced by wood-boring ambrosia beetles often is abundant at the base of vacated trees (Fig. 8).
Seasonal Patterns
Most multiple-tree infestations are initiated in the spring. Overwintering beetles emerge and disperse in search of suitable host trees. Lightning-struck
or severely-stressed pines with com- promised defense systems are often the target for these “pioneer” beetles. Once a susceptible host is located, the attack sequence and pheromone release de- scribed above occur. As a tree becomes fully colonized, attacks switch to ad- jacent pines. When beetle populations within an area are high, an expanding infestation can develop. However, only about ¼ of trees attacked by dispersing beetles in the spring develop into ex- panding spots of 20 or more trees.
Generations overlap in the late spring and summer, and eventually an expand- ing infestation will contain trees with all beetle life stages. Small infesta- tions usually have one area with trees under attack, called the spot head (Fig. 9). As infestations enlarge, additional spot heads may develop. The continual emergence of brood and parent adults, coupled with pheromone produc- tion of attacking beetles at the spot head(s), sus- tains infestation growth. Large infestations may spread at a rate of > ft/day, and satellite in- festations may develop nearby.
During the warm sum- Figure 8. Boring dust of ambrosia beetles around the base of a mer months disper- tree vacated by southern pine beetles.
Figure 7. Adult of Thanasimus dubius , a clerid beetle and predator of the southern pine beetle.
sal is limited, and most beetles are located within expanding spots. As temperatures cool in the fall, beetles may remain in spots or disperse to indi- vidual trees for the winter. Beetles may overwinter in all stages within a tree. Development slows during the winter, but there is no diapause. During warm winter periods, development continues and some emergence may occur. Win- ter-emerging adults may colonize unoc- cupied portions of the same trees from which they emerged. A few infestations may remain active throughout the win- ter when temperatures are favorable.
Natural Control
Natural enemies, including predators, parasitoids, and diseases, can maintain or reduce population levels. Thanasimus dubius is a major predator of both adult and larval southern pine beetles. However, there is no evidence that natural enemies can stem the development of outbreaks. Ips bark beetles and borers may compete for reproductive space and food resources within the bark. Woodpeckers may feed on
Figure 9. Infestation “spot” head with fading trees.
beetle larvae, pupae and callow adults, often completely stripping the bark from the main stems of infested trees during the winter. Persistent, freezing temperatures can lead to brood mortality, particularly eggs and early larvae. Continuous high daily temperatures in excess of 95o^ F also may kill broods.
Population Patterns
Southern pine beetle populations within an area can range from undetectable to outbreak levels. An outbreak is defined as one or more multiple-tree southern pine beetle infestations per 1000 acres of susceptible host type. In the past 50 years in the Gulf Coastal Plain, outbreaks have occurred on a 6-10 year cycle, though recently patterns have varied. The causes for the onset of outbreaks are unknown, though outbreaks have been linked to extremely wet conditions in low-lying loblolly pine stands in the Western Gulf Coastal Plain. Population collapses have been attributed to lack of susceptible hosts, unfavorable environmental conditions, high numbers of natural enemies, and/or competition for habitat beneath the bark.
Prevention. Direct tactics to suppress southern pine beetle populations have proven successful, as discussed above. However, forest managers and forest health specialists commonly believe that the most effective method of managing southern pine beetle is through preventing outbreak populations and creating forest conditions that lessen impacts once outbreaks occur. Stand density is thought to be one of the most critical factors in determining the chances of spot initiation and the rate of spot expansion within a stand. Thinning is the preferred forest management tool used to attain desired stand densities, and it is widely recommended that stands with a basal area greater than 120 ft^2 per acre should be thinned below 80 ft^2 per acre. Thinning reduces the likelihood that expanding infestations will become established by increasing tree vigor and by changing the stand’s microenvironment. Increased tree vigor increases the likelihood that beetles will be “pitched-out” due to higher resin flows. The changes in the stand’s microenvironment include increased wind flow beneath the tree canopy and the greater potential for disrupting the pheromone communication system of the
southern pine beetle. In urban settings, it is recommended that the spacing between pines should be >20 feet.
Acknowledgements
This publication is a revision of the previous version by R. C. Thatcher and P. J. Barry (1982). Their work provided valuable guidance in the preparation of this update. The authors also thank R. Hofstetter, T. Rogers, J. Meeker, and R. Billings for their input on the range map, and the latter two for their review of this leaflet.
Illustrations: Fig.2- Erich G. Vallery, USDA Forest Service - SRS-4552, Bugwood.org; fig. 3- USDA Forest Service - Region 8 Archive, USDA Forest Service, Bugwood. org; fig.4a- Southern Forest Insect Work Conference Archive, Bugwood.org; figs. 4b, 5, 8 & 9- Ronald F. Billings, Texas Forest Service, Bugwood.org; fig.6- Lacy L. Hyche, Auburn University, Bugwood.org; fig.7- John Moser, USDA Forest Service, Bugwood.org; fig.10- E.Taylor, Texas Cooperative Extension, and R. Billings, Texas Forest Service.
Figure 10. Diagram of cut-and-leave treatment illustrating buffer around currently-infested trees.
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References
Belanger, R.P., and B.F. Malac. 1980. Silviculture can reduce losses from the southern pine beetle. USDA Comb. For. Res. and Dev. Prog. Agric. Handbook. No. 576. 17 p.
Belanger, R.P., R.L. Hedden, and P.L. Lorio, Jr.
- Management strategies to reduce losses from the southern pine beetle. South. J. Appl. For. 17: 150-154.
Billings, R.F.,and C. Doggett. 1980. An aerial observer’s guide to recognizing and reporting southern pine beetle spots. Agriculture Handbook No. 560, Washington, D.C.: USDA Forest Service. 19 p.
Billings, R.F., and H.A. Pase III. 1979. A field guide for ground checking southern pine beetle spots. Agriculture Handbook No. 558, Washington, D.C.: USDA Forest Service. 19 p.
Billings, R.F., and J.D. Ward. 1984. How to conduct a southern pine beetle aerial detection survey. Texas Forest Service Circular 267. Lufkin, TX. 19 p.
Clarke, S.R. 2001. Review of the operational IPM program for the southern pine beetle. Integrated Pest Management Reviews 6: 293-301.
Nebeker, T.E., J.D. Hodges, C.A. Blanche, C.R. Honea, and R.A. Tisdale. 1992. Variation in the constitutive defensive system of loblolly pine in relation to bark beetle attack. For. Sci. 38: 457-466.
Nowak, J., C. Asaro, K. Klepzig, and R. Billings.
- The southern pine beetle prevention initiative: working for healthier forests. J. of For. 106: 261-267.
Pye, J.M., T.S. Price, S.R. Clarke, and R.J. Huggett, Jr. 2005. A history of southern pine beetle outbreaks in the southeastern United States through
- http://www.srs.fs.usda.gov/econ/data/spb/
Salinas-Moreno, Y., M.G. Mendoza, M.A. Barrios, R. Cisneros, J. Macías-Sámano, and G. Zúñiga.
- Areography of the genus Dendroctonus (Coleoptera: Curculionidae: Scolytinae) in Mexico. J. of Biogeography 31: 1163–1177.
Swain, K.M., and M.C. Remion. 1981. Direct control methods for the southern pine beetle. Agriculture Handbook No. 575, Washington, D.C. 15 p.
Thatcher, R.C., and P.J. Barry. 1982. Southern pine beetle. Forest Insect and Disease Leafl. 49. Washington, D.C.: USDA Forest Service. 7 p.
Thatcher, R.C., J.L. Searcy, J.E. Coster, and G.D. Hertel (eds.). 1980. The Southern pine beetle. USDA Forest Service Technical Bulletin 1631. 267 p.
published by: USDA Forest Service Pacific Northwest Region (R6) Portland, Oregon
FS-R6-RO-FIDL#49/007-
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