The Southern Pine Beetle
Chapter 9: Silvicultural Guidelines for Reducing Losses to the Southern Pine Beetle
Introduction
The proper silvicultural treatment of forest stands is an essential element of developing strategies to achieve long-term success in suppressing the southern pine beetle. Intermediate cuttings and regeneration methods provide the means to produce environmental and biological conditions unfavorable to the attack, spread, and population growth of this forest pest. Yet silvicultural practices that promote stand resistance to insect damage are fundamentally lacking in management planning and practice. Severe timber losses and the recent decline of pine resources because of the SPB are evidence of this neglect.
Foresters in the South have long realized the opportunities for reducing insect damage through silviculture. General guidelines were developed to identify areas at high risk for SPB attack and reduce the potential for losses (Bennett 1968, 1971). Recommendations were sound and the message clear: good forest management provides protection against most insect pests. Detailed information was still lacking, however, to develop control tactics and prevention strategies that could be integrated with management operations and objectives. Forest pest managers did not know what stand, site, and host tree characteristics are associated with SPB infestations. Systems and methodology to rank the relative susceptibility of stands to beetle attack were needed. Studies had not been conducted to determine how host-insect relationships differed between and within the Southern Coastal Plain, Piedmont, and Southern Appalachian Mountains. Many of these questions have been answered by the accelerated efforts of the Expanded Southern Pine Beetle Program. Findings from this 6-year program provide the basic information used to develop the silvicultural principles and practices discussed in this chapter.
The purpose of silviculture is to produce and maintain such a forest as will best fulfill the objectives of the owner (Smith 1962). The practice of silviculture involves harvesting, regeneration, tending, and protecting the crop. Treatments are assigned according to economic considerations and management objectives. Cultural practices to promote long-term resistance to SPB attack can be in harmony with these goals. In many instances, "prevention silviculture" may be necessary to obtain the desired forest crops.
Protecting Stands from the SPB
To fulfill management objectives and maintain stable, healthy forest stands, the manager must understand tree physiology, the factors that influence the growth and development of stands, and the importance of site. Attention to these basic principles is required before proper practices can be defined to protect stands from the SPB. They are the keystones to the silvicultural techniques outlined in fig. 9-1.
 Figure 9-1 – Silvicultural guidelines to reduce losses from the SPB. |
Promoting Individual Tree Resistance
Favor Most Resistant Species
Intermediate cuttings and regeneration systems should restrict the composition of the stand to species that are best suited to the site and most resistant to southern pine beetle attack. Slash pine, longleaf pine, Virginia pine, and eastern white pine tend to be more resistant to SPB attack than loblolly pine, shortleaf pine, or pitch pine (Hodges, Elam, and Watson 1977; Belanger, Osgood, and Hatchell 1979b). Differences in susceptibility are related to the physical properties and to the toxicity of the oleoresin system (Coyne and Lott 1976, Hodges et al. 1977). The oleoresins of highly resistant pines are extremely viscous, crystallize slowly, and continue flowing for long periods of time after wounding. Southern pines with a high limonene content may be more resistant to continued beetle attack than trees with a low limonene percentage.
Strains of southern pines highly resistant to SPB infestation are not available for planting. The potential does exist, however, for making genetic gains through selection. Hodges et al. (1977) found that in regard to oleoresin properties, 19 percent of loblolly pine trees studied in central Louisiana resembled longleaf pine or slash pine more closely than the average for loblolly pine.
Physical properties of oleoresins are under strong genetic control. Squillace and Gansel (1968) have developed procedures for assessing the potential oleoresin yield of pines at 2.5 years from seed. Peters
(1971) showed it is possible to select for oleoresin properties while at the same time selecting for form and vigor. An early evaluation of the relative susceptibility of selected individuals or established genetic strains to SPB attack appears possible without sacrificing other desirable traits. Remove High-Risk Trees Trees damaged by lightning, wind, ice, and other destructive agents increase the chances of attack and buildup of troublesome insects and should be removed from the stand. Pines struck by lightning (fig. 9-2) offer a favorable environment for the SPB and Ips beetles (Hodges and Pickard 1971). Stands damaged by wind, ice, or hail are more susceptible to bark beetle attack than undisturbed stands (see Chapter 4). Salvage cuttings should be completed as soon as possible after injury has occurred. Sanitation cutting fells or removes damaged, infested, and high-risk trees to prevent the establishment, spread, or proliferation of the southern pine beetle. Cut-and-leave (Chapter 10) is recommended for infestations that are too small and scattered for practical salvage removal. Treatments appear most successful when applied to spots with 10 to 50 active trees (Texas Forest Service 1975). Often, spots with fewer than 10 active trees become inactive and require no treatment. Large, active infestations are difficult to control regardless of treatment. Cutting infested trees into an opening may increase mortality of developing broods, especially if the entire log is exposed to direct sunlight (Hodges and Thatcher 1976). |
 Figure 9-2 – Pines struck by lightning are highly susceptible to beetle attack. |
Promoting Stand Resistance
Maintain Proper Density
High-risk stands are characterized by slow radial growth (see fig. 8-2). Intermediate cuttings are a means of promoting and maintaining rapid growth of trees in young stands and reducing losses from the SPB (fig. 9-3). Thinnings in North Carolina (Maki, Hazel, and Hall 1978 unpublished) reduced the average spot size from almost 6 acres per infestation to less than 3 acres and appeared to reduce the incidence of attack. Low thinning or "thinning from below" is recommended in natural stands and plantations to reduce competition and remove the types of trees most subject to SPB attack. The poorer crown classes — suppressed and intermediate trees — are cut first. These small, slow-growing trees are more susceptible to beetle attack than healthy trees (Haines, Haines, and Liles 1976; Ku, Sweeney, and Shelburne 1976). Dominant and codominant trees with large live crown/height ratios and desirable phenotypic traits should be favored as crop trees. They are best suited to respond with increased growth after release and have the most potential for high-value products.
 Figure 9-3 – Intermediate cuttings promote the rapid growth and vigor of young stands. |
 Figure 9-4 – Prescribed burning can be used to eliminate small, high-risk trees from stands. |
Prescribed burning (fig. 9-4) can be used to eliminate suppressed, high-risk trees from overstocked stands (McNab 1977). Site-stand studies have shown no relationship between burning and SPB attack. The role of fire in pest management strategies deserves additional study.
Thinning schedules depend on the close relationships between site quality, age of the stand, stand density, live crown ratio of individual trees, and rate of growth. Root and crown competition among individual trees usually occurs at ages 10 to 15 years on good sites, later on poor sites. Thinning at this time is advised to maintain rapid tree growth. Delay will reduce live crown ratios and tree vigor to levels unfavorable for growth but attractive to the SPB.
Degree of thinning to reduce the risk of southern pine beetle attack is determined by the intensity of management, the kind of product desired, available markets, and natural conditions particular to each location. Reduction of basal areas to 80 to 100 ft2/acre is recommended for overstocked stands of immature trees. The risk of beetle attack in most instances will increase considerably at basal areas greater than 100 ft2/acre. Wide spacing distributes growth on a selected number of desired stems, resulting in the production of small saw logs by age 35 to 40 years on good sites. Heavy thinning should be avoided, however, in areas subject to severe wind and ice storms (Belanger and Brender 1968). Dense stocking is recommended if the management objective is high yields of wood fiber. Light thinnings may need to be repeated at short intervals to maintain vigorous growth.
Thinning on high-hazard annosus root rot sites (> 70 percent sand in topsoil) can lead to spread of the disease and severe infection followed by a reduction in radial growth and attack by the southern pine beetle (Skelly, Powers, and Morris 1974). Precautions can be taken to reduce the danger of annosus infection. Thinning should be scheduled during summer, when fewer annosus spores are produced and high temperatures kill those that are produced. Treating stumps with borax or Peniophora spores minimizes spread. Prescribed burning before and after thinning also reduces severity of annosus root rot in the South (Froelich, Hodges, and Sackett 1978).
Manage Species Composition
Favoring beetle-resistant species of pine and removing high-risk trees lower the hazard index of stands. Stand composition of pines and hardwoods also promotes resistance to attack and the spread of endemic beetle populations (Belanger et al. 1979b; Belanger, Porterfield, and Rowell 1980 unpublished). The SPB prefers host types that are uniform and continuous. Spread of infestations is greatest in dense pine stands (Gara and Coster 1968, Hedden and Billings 1979). Hardwoods limit these conditions by disrupting continuity between host trees. Intermediate cuttings should favor hardwoods that are suited to the site and are compatible with long-term management objectives. BA of the pine component should be maintained at < 100ft2/acre.
Minimize Logging Damage
Logging activity has a tendency to increase southern pine beetle attack (Porterfield and Rowell 1980). Careless cutting, skidding, and hauling often cause severe mechanical injury to above- and below-ground portions of residual trees. These disturbances are attractive courts for the black turpentine beetle, Ips species, and SPB. The more recent the logging damage, the more susceptible the stand is to attack.
Increased beetle activity resulting from poor logging practices can quickly offset the benefits from silvicultural treatments. Harvesting systems should be developed that minimize damage to the stand and site. Use of small harvesting equipment and removal of short roundwood are recommended for intermediate cuttings. Heavy equipment and tree-length logging generally increase the amount of damage to residual trees. Equipment operators and ground crews can be trained to minimize damage to residual trees and promote stand conditions less vulnerable to insect attack.
Regulate Age Classes
Susceptibility of stands to SPB attack increases with age throughout most of the South (Lorio 1978, Belanger et al. 1979b, Coster and Searcy 1980). Overmature stands are characterized by slow radial growth, flat-topped crowns, and thin foliage. Trees in these advanced stages of decline are seldom able to respond to treatment. The best option is to regenerate these stands. Excellent guidelines exist that bring together information on methods of regenerating the southern pines (U.S. Department of Agriculture Forest Service 1973; Society of American Foresters 1981). The manager has the option of planting or natural regeneration. Quality trees of the most resistant host species should be retained as seed trees if the stand is to be renewed naturally. Planting provides choice of species and desired spacing. Close initial spacings will require intermediate cuttings at an earlier age than wide spacings in order to reduce risk of SPB attack.
Many overmature stands of pines are intentionally being preserved throughout the South for esthetic reasons, ecological study, and wildlife. Overmature pines add to the visual attractiveness of woodlands and are the required habitat of some wildlife species. Overmature stands are extremely susceptible to SPB attack and need to be monitored closely to prevent buildup of epidemic populations and loss of the entire pine component. Host trees attacked by bark beetles should be removed as soon as possible.
Regional Recommendations
Unhealthy stands are highly susceptible to attack by the southern pine beetle. This familiar and basic principle applies regardless of region or pine type. Although silvicultural practices can prevent or reduce losses from beetles by increasing the resistances of host trees, no "standard" recommendation will apply to all situations. Each forest condition and locality presents different management problems; each might require a different combination of methods to increase resistance to insect attack.
Southern Coastal Plain
The Southern Coastal Plain includes the seaboard extending from Maryland and Delaware along the Atlantic Coastal Plain and Gulf Coastal Plain to Texas. The region can be further subdivided into the lower and upper Coastal Plain. The lower Coastal Plain consists of the low elevation "flatlands" and wetlands. Slash pine, longleaf pine, and planted loblolly pine are the principal SPB host species. The upper Coastal Plain lies inland from the flatlands. The topography is slightly rolling and the soils have good drainage. Between one-half and three-fourths of the area is forested. The principal pine species in the upper Coastal Plain are loblolly and shortleaf.
Natural stands most susceptible to southern pine beetle attack in the Coastal Plain are characterized by high stand densities (fig. 9-6), a large proportion of pine sawtimber, and declining radial growth (Coster and Searcy 1980; Chapter 4). Poorly drained soils and low-lying areas are also indicators of high-risk sites. Trees on dry or droughty soils are less commonly attacked. Timely cultural treatments can prevent or reduce stress conditions that favor attack and spread of the SPB. |
 Figure 9-6 – Dense stands are subject to attack by the SPB. |
Intermediate cuttings in heavily stocked plantations and natural stands will reduce competition between trees and reduce the probability for southern pine beetle attack. Initial cuttings should be early — not later than 20 years — to anticipate rather than relieve the adverse effects of severe crowding. Thinning may be required sooner on good sites than on poor ones, and possibly sooner in the Gulf Coastal Plain than on the Atlantic seaboard (Wahlenberg 1960). Overstocked stands should be thinned to a BA of 80 ft2/acre to increase growth rate and vigor (Hicks et al. 1979, Toko and Landgraf 1979). Residual BA can be slightly higher as age and site index increase. Intermediate cuttings should be rescheduled as BA approaches 120 ft2/acre.
Infestations in the Coastal Plains are more frequent on wet and waterlogged sites than on well-drained soils. There are 20 million acres of excessively wet, swampy sites in the Atlantic and Gulf Coastal regions (Zobel 1979). Trees on poorly drained sites are often deficient in mycorrhizal roots and are therefore subject to severe physiological stress during periods of stress during periods of drought (Lorio, Howe, and Martin 1972). Drainage systems designed to remove surplus water from low-lying areas will curtail the damage from rootlet pathogens and stem the decline of host pines (Bennett 1971). Drainage improvements have already bettered over 2 million acres of forest lands in the South. Logging damage on low-lying areas of fine-textured soils can be avoided by diverting operations to sandy soils during wet periods and using harvesting equipment of low bearing pressure (Hatchell, Ralston, and Foil 1970).
Wet sites should be regenerated with more beetle-resistant species, such as slash pine or hardwoods (Hicks et al. 1979). "Wet site" loblolly pine and pond pine grow well on wetlands (Zobel 1979), but their relative resistance to SPB attack is unknown. On high-risk sites — conditions too wet or too dry — resistance to pests may be more important than tree growth alone.
Potential for growth and yield in the southern Coastal Plain appears directly related to potential for beetle problems. Infestations occur more frequently on moist, high-quality sites than on poor sites (Lorio 1978, Kushmaul et al. 1979). Consequently, a disproportionate amount of silvicultural attention can justifiably be directed toward stands growing on good sites. Intermediate cuttings are required sooner and more frequently on quality sites than poor sites to reduce competition from hardwoods and understory vegetation, maintain rapid radial growth, and develop a species composition unfavorable to beetle attack. Quality sites have the potential for high yields of forest products. Extra care and protection may be necessary measures to meet these goals.
The southern Coastal Plain has a high frequency of thunderstorms, tropical storms, and glaze storms. These types of climatic damage to trees promote bark beetle attack and spot proliferation. Spot spread depends on size and vigor of the insect population and interrelationships of different bark beetles present within the stand. Damaged and dying trees should be salvaged promptly to reduce the numbers of focal points for infestations. If needed, other intermediate cuttings could be conducted at the same time to reduce stand density and the probability of spot expansion. The forester can do little to guard against increases in beetle activity associated with extended periods of drought or flooding (King 1972, Kalkstein 1976). Frequent surveillance during periods of extreme climatic stress makes prompt detection of insect attack possible and assists the manager in deciding what measures are needed to control the pest.
Piedmont
The Piedmont extends from Virginia through North Carolina, South Carolina, Georgia, and into Alabama. At one time or another most of this land was in clean-cultivated crop production. Soil depletion following continuous cropping and severe erosion resulted in widespread land abandonment. Forests quickly reclaimed the fields. Loblolly pine and shortleaf pine were the predominant pioneer tree species in the southern portion of the Piedmont; shortleaf pine and Virginia pine spread throughout the northern Piedmont.
About three-fifths of the Piedmont is now in forests, mostly farm woodlands. Approximately half of these forests are in natural and planted pines and the other half in mixed hardwoods. Destructive losses from the SPB are accelerating the natural succession from pine to climax hardwood species (see Chapter 5). Species composition in the Piedmont is widely varied, and forests are interspersed with agricultural lands. Ownerships and management objectives are numerous. Though these conditions complicate forest management, they need to be considered individually and collectively in developing silvicultural systems to prevent or reduce SPB-caused losses.
attack than shortleaf, is often characterized by poor form and persistent branches. These faults present difficulties in harvesting and processing for solid wood products. Virginia pine is recommended for hazard areas where high yields of wood fiber are the primary management objective. Shortleaf pine is recommended for solid wood products.
Most Virginia pine stands in the Piedmont resulted from natural seeding on abandoned agricultural lands during the 1930’s and 1940’s. Stocking is usually dense, stems are small, and radial growth is slow. Spread and proliferation of the SPB in these stands can be extreme once infestations occur. Intermediate cutting does little to increase stand resistance under these conditions. Height and diameter growth in mature Virginia pine stands do not respond to thinning as rapidly as loblolly pine or shortleaf pine (Belanger and Bramlett 1979). Mature, slow-growing stands should be harvested and regenerated naturally to Virginia pine or planted to loblolly pine.
The best management strategy to reduce losses from the southern pine beetle is to thin Virginia pine before age 15, with subsequent thinnings as needed to maintain rapid growth. Basal areas should be maintained at about 100 ft2/acre for high yields of wood fiber.
in the Piedmont (Belanger et al. 1980 unpublished). Sweetgum (Liquidambar styraciflua L.) can be reproduced and managed with pine on upland sites. Yellow poplar (Liriodendron tulipifera L.), red oaks (Quercus spp.), sycamore (Platanus occidentalis L.), and sweetgum are suited for lower slopes and bottomland sites. Mixing stand components my be suited to owners of small, nonindustrial woodlands managing for products other than timber. Mixed stands often support diverse and dense wildlife populations, can be esthetically pleasing, and contribute toward a gradual improvement of poor sites.
Little can be done to make immediate or significant long-term improvements to soil and site conditions characteristically associated with high-risk stands in the Piedmont. These soils have a high erosion potential and require careful management to prevent further deterioration of the site. Intensive site preparation and cultivation with heavy equipment are recommended only where soils and slopes are suited for these practices. The application of intensive site preparation methods should be avoided on slopes greater than 10 percent. Burning appears to be a preferable alternative to intensive mechanical preparation from the standpoint of soil erosion (Nutter and Douglass 1978). Herbicides offer the greatest opportunity to control competing vegetation with minimum impact on soil physical properties. Logging practices and methods of site preparation need to be specific to site conditions. Further abuse of already fragile sites in the Piedmont will only intensify SPB problems in the future.
Southern Appalachians
During the last century, removal of large volumes of timber due to mortality from chestnut blight, domestic use of timber, damage from grazing, and woods burning has produced a general forest condition of low stocking and poor quality in the Southern Appalachians (Brender and Merrick 1950). Most of the forest types are mixed hardwood. In the mountains, SPB host types are primarily Virginia pine, shortleaf pine, eastern white pine, and pitch pine. Many pine stands resulted from natural seeding on remote and scattered farms that were sold or abandoned. Today most of this land is managed by Federal agencies for multiple uses. The accessibility of stands often determines the intensity of management and specific objectives. Private, nonindustrial holdings are mostly in the valleys, where topography is suited for farming and settlement.
The southern pine beetle has been a problem for decades in the Southern Appalachians. Several outbreaks have been reported since 1920 (King 1972, Price and Doggett 1978), the most recent activity having occurred from 1972 through 1977. Studies in the mountains of Georgia, North Carolina, South Carolina, and Tennessee showed that stands severely attacked by the SPB were characterized by dense stocking, slow radial growth, and a large proportion of overmature pine sawtimber (Belanger et al. 1979b). Shortleaf and pitch pines are more susceptible to beetle attack than Virginia pine or eastern white pine. Findings indicate that management and silvicultural techniques offer means of reducing SPB-caused pine mortality in the Southern Appalachians.
Managing species composition deserves consideration as a method of reducing such losses. Managers should favor eastern white pine and Virginia pine over shortleaf pine or pitch pine whenever possible. The ability of vigorous-growing white pine and Virginia pine to "pitch out" SPB may account for the low rates of mortality of these species. White pine, economically one of the most important sawtimber species in the Southern Appalachians, can be regenerated by clearcutting and planting, clearcutting with seed in place (Moyers 1979), or planting under a residual hardwood overstory (Van Lear and Cox 1979). Seedlings should be released the third growing season if competition from hardwood sprouts and brush is severe. Intermediate cutting can be scheduled at ages 20 to 30 to obtain high-value products and maintain rapid growth. Harvest should be scheduled at 40 to 50 years. Older trees are susceptible to beetle attack (Belanger et al. 1979b).
Management recommendations for Virginia pine in the Southern Appalachians are similar to those for the Piedmont: some form of clearcutting for regeneration, early intermediate cuttings to maintain rapid growth, and harvesting at ages 40 to 50 years. Excellent stands of Virginia pine have been regenerated in the mountains by clearcutting, burning to reduce heavy slash and hardwood competition, and planting with superior stock.
Another preventive measure where nontimber values are an objective is managing pine and hardwoods in mixture. Hardwoods are a component of most pine stands and the climax species in the mountains. Favoring hardwoods during intermediate cutting will reduce the incidence of beetle attack as well as improve the quality of stands and sites.
A combination of thinning, improvement cuts, and salvage cuts may be used to reduce the BA of overstocked stands. Approximately 80 to 90 ft2/acre of leave BA is recommended for these types of stands (U.S. Department of Agriculture Forest Service n.d.). Overmature, slow-growing trees should be cut to improve the age distribution and growth potential of the residual stand. It may be necessary to carry out two or three intermediate cuts to obtain the desired results. Overmature stands should be harvested and regenerated to species most resistant to SPB attack.
Intensive culture of forest stands is difficult in the Southern Appalachians. The most common deterrents to silvicultural treatments are the inaccessibility of stands, poor market conditions, and a lack of logging and planting contractors. Where pines contribute significantly to management objectives, these obstacles need to be overcome. Neglect of the pine component could increase the risk of attack and probability of severe losses should SPB outbreaks occur.
Discussion
In recent years SPB activity has increased in frequency and severity throughout the South. The problem appears closely related to changes in forest composition and stocking conditions (Hedden 1978b). Much of the present southern pine forest resulted from natural seeding and planting on abandoned agricultural lands from 1930 through 1950. Young stands grew rapidly with little or no management. Insect and disease problems developed as stands became crowded and vigor declined. Problems intensified as stand age, stocking, and sawtimber volume increased. Proper management can reverse this trend. Information and ranking systems are now available (see Chapters 4 and 8) to identify highly susceptible stands. Studies have shown that silvicultural treatments have been effective in reducing losses from the SPB (Morris and Copony 1974, Maki et al. 1978 unpublished).
Natural stands and plantations can be equally susceptible to SPB attack when mismanaged; they can be equally resistant to the beetle when treated properly. Planting offers the best opportunities to control seedling quality, species composition, stocking, and culture of the stand for high yields of wood products. Natural regeneration provides protection to the site, has low establishment costs, and can satisfy a number of forest management goals. Methods and intensity of silviculture depend on objectives of ownership.
Systems developed to determine the relative susceptibility of stands to southern pine beetle attack can be used to evaluate the effectiveness of silvicultural treatments. Intermediate cuttings to reduce stocking levels and stimulate radial growth will lower the probability of attack in most high-risk stands. Response to cutting will not be immediate after treatment. A period of 3 to 5 years may be necessary before roots and crowns of released trees can support rapid growth. Response time will increase with age of the stand and poor site quality. Overmature stands are usually past the point of physiological improvement and should be regenerated.
Cultural treatments are also needed in young stands and low-risk stands to maintain rapid growing conditions. Competition in the early life of the stand usually occurs at ages 10 to 15 years, depending on site quality and initial spacing. Initial thinning is recommended shortly after crown closure. The purpose of treating young stands is to prevent rather than remedy high-risk conditions. Periodic thinning and improvement cuts will maintain stand vigor.
Careful tending of host types is necessary to obtain effective and lasting management of the southern pine beetle. Most forest agencies and industries are experienced and respond quickly with the salvage of infested stands and control measures during epidemic beetle conditions. But silvicultural activities related to forest pest management generally lag during periods of low beetle activity. When SPB populations are down, planning and application of prevention strategies should be accelerated. High-risk stands can be identified and treated to reduce the susceptibility of beetle attack and probability of spot spread. Low-risk stands can be tended to maintain vigor and rapid growth. Stands and forests that are highly resistant to SPB attack should be a primary objective of management. Silvicultural practices offer the most practical and long-lasting means of achieving this goal.
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