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The southern pine beetle (SPB) (Dendroctonus frontalis) is the most destructive insect pest of pine forests in the South (Thatcher and Conner 1985). Populations build rapidly during periodic outbreaks and kill large numbers of trees. Average annual losses may exceed 100 million board feet of sawtimber and 20 million cubic feet of growing stock. From 1991 to 1996, total value of trees killed by SPB in the Southern United States was estimated at $493 million (Price and others 1998). However, during endemic periods, SPB populations may be so low that it is difficult to locate a single infested tree or capture beetles in pheromone traps (Thatcher and Barry 1982, Thatcher and others 1980).
The SPB, which attacks all species of pines, prefers loblolly, shortleaf, Virginia, pond, and pitch pines but seldom attacks longleaf pine. Recently, SPB has been observed to successfully infest white and Table Mountain pines. Mature trees in pure, dense stands have long been considered most susceptible to SPB attack (fig. 17.7), but in recent years unthinned pine plantations have increasingly supported SPB infestations. Trees less than 5 years old or 2 inches in diameter are seldom attacked.
During outbreaks, SPB activity peaks in early summer in the Gulf States and in late summer and early fall farther north.
Figure 17.8 shows a summary of SPB outbreaks as reported by Price and others (1998). Since 1960, a SPB outbreak has occurred somewhere in the South almost every year. Outbreaks, which may last 3 to 6 years, have been most severe and persistent in southeast Texas and southwest Louisiana, central Mississippi, the Piedmont of Alabama, Georgia, and South Carolina, and the Coastal Plain of Georgia, and North and South Carolina. Currently a catastrophic infestation of SPB is threatening pines in Virginia, Kentucky, Tennessee, North Carolina, and Georgia. Ridgetop pine ecosystems for which control options are extremely limited are of special concern to ecologists and forest managers.
Natural enemies, including diseases, parasites, and predators, can help maintain beetle populations at low levels. However, they seem to have little effect in preventing periodic outbreaks.
The primary suppression method is to salvage infested trees plus a buffer of green trees to stop spot expansion. Cutting and leaving infested trees, under appropriate conditions, also protects the residual stand (Swain and Remion 1981).
While chemical treatments are available, chemical insecticides are seldom used on a large scale to suppress SPB. They are most often used to prevent attacks of SPB and associated bark beetles on individual trees of high value. A new semiochemical is being tested to protect trees from SPB attack.
The most practical way to minimize timber losses and avoid costly, short-term suppression projects is to maintain forests in a vigorous, healthy condition. Several practical hazard-rating systems have been developed to help managers to prioritize SPB prevention activities. Thinning and harvesting are extremely important prevention tools since outbreaks are generally less likely in actively managed forests, where management is designed to enhance health and vigor of the residual stand.
SPB outbreaks affect pine forests on all ownerships. The severity of loss tends to be greatest on Federal forests due to the preponderance of mature pine sawtimber in dense stands. Areas set aside for wilderness or preserves have proven especially prone to SPB outbreaks, due largely to the advanced age, high density of the stands, and the policy of not controlling SPB on these areas.
In the last five decades, large acreages of pine plantations have been established in the South. Even-aged, single-species plantations become increasingly susceptible to SPB infestations as they age. Precommercial and commercial thinning to promote rapid growth in these plantations should reduce their susceptibility to future SPB outbreaks. Nevertheless, the SPB is expected to play an increasingly important role in the future of the South’s pine forests.
Impacts vary with ownership. Federal land supporting an abundance of overmature loblolly pine forests is expected to be particularly vulnerable to extensive outbreaks. Industrial forests are likely to suffer SPB problems primarily in young, unthinned pine plantations. Short-rotation pine plantations receiving intensive management (periodic thinning, fertilization, etc.) should have minimal problems with SPB. Small private forests will face SPB problems in inverse proportion to management intensity.
SPB will continue to play a major role in the health of the southern forest. Catastrophic population buildup will continue to occur periodically. When this occurs survivor species will assume a higher profile in the residual forest. In some cases, total loss of the pine component in the forest may result.
Although the southern pine beetle is the most damaging insect in southern pine forests, it is only one of five species of pine bark beetles of concern for forest managers in the South. The other species are the six-spined engraver (Ips calligraphus), the southern pine engraver (Ips grandicollis), the small southern pine engraver (Ips avulsus), and the black turpentine beetle (BTB) (Dendroctonus terebrans). These beetles are usually considered secondary pests because they usually infest only stressed, weakened, damaged, or downed pines. They also colonize pines that have been attacked by SPB or another species of bark beetle. Host species in the South include loblolly, shortleaf, slash, longleaf, pitch, sand, eastern white, and Virginia pines. Both pure pine and mixed pine-hardwood stands may be affected (Conner and Wilkinson 1983, Smith 1972, U.S. Department of Agriculture Forest Service 1985a).
Adult BTBs are the largest of the southern pine bark beetles. Although BTB attacks may continue for several months, infestation is not always fatal, and multiple attacks around the entire circumference of the tree are required to cause mortality (Smith 1972, U.S. Department of Agriculture Forest Service 1985a).
The small southern pine engraver is the smallest of the Ips spp. in the South; the southern pine engraver is midsize; and the six-spined engraver is the largest. The small southern pine engraver and the six-spined engraver are the most aggressive and may kill small groups of trees. Losses may be extensive during periods of drought (Conner and Wilkinson 1983, U.S. Department of Agriculture Forest Service 1985a).
In the past, the secondary bark beetles played a vital role in shaping forest structure. They attacked individual weakened or severely stressed trees, or older trees reaching senescence. Large infestations developed only occasionally, usually in the aftermath of widespread environmental stress, drought, storm damage, or wildfire. Overall, their action served to thin the pine forests, reducing competition, leaving the stronger trees, and decreasing the risk of SPB outbreaks. Over time, they may have had a greater impact on regulating pine stands than SPB (Clarke and others 2000, Paine and others 1981, Thatcher 1960a).
Today, the impact of these other bark beetles depends largely on management activities (Coulson and others 1986). On unmanaged land they function much as they did in the past, attacking single trees or small groups of pines, and reducing pine basal area. They provide openings for pine reproduction or for established hardwoods to grow. The effects are often not noticeable except during periods of extended drought, after storm damage, or at the end of SPB epidemics.
On managed land, outbreaks of secondary bark beetles occur infrequently, and primarily impact dense, unthinned young pine stands. Infestations temporarily increase after burning or thinning. Increases in beetle activity are usually short-lived, and the long-term benefits of thinning and prescribed burning outweigh the temporary, negative effects. Black turpentine beetles may attack pines scored for the production of naval stores. Ips bark beetles quickly infest pines downed by storms, and often introduce blue stain fungi that invade the wood.
Secondary bark beetles are important killers of individual, high-value pines in urban or recreation areas. There they create hazard trees that are expensive to remove.
In the past, secondary bark beetle infestations were often aggressively controlled, usually by felling and then spraying the affected trees with insecticides. This tactic was expensive and killed the natural enemies of the beetles. It was determined that such treatments were generally not cost effective, and today few infestations are controlled. When large infestations develop after drought or wildfire, prompt salvage of the currently infested trees may limit the spread of the beetles and allow time for uninfested, stressed trees to recover. Populations of secondary bark beetles infesting storm- or fire-damaged pines rarely move into healthy trees.
Prevention is the key to reducing losses to secondary bark beetles. Maintaining healthy pine stands and minimizing damage during management activities keep impacts low. If infested trees in high-value areas cannot be removed, the at-risk pines may be sprayed with insecticides to prevent attacks. Only the lower bole should be sprayed for BTB, but the entire bole must be treated to keep out Ips bark beetles.
Secondary bark beetle activity and damage are expected to continue at natural levels into the future. Periodic significant outbreaks will also continue to occur.
The pales weevil (Hylobius pales) and pitch-eating weevil (Pachylobius picivorus) are two of the most serious insect pests of pine seedlings in the Eastern United States. In the South, they are found wherever pine occurs (fig. 17.1). Adult weevils of both species are attracted to freshly harvested pines, where they breed in logging slash, stumps, and old root systems. Seedlings planted in freshly cut areas are injured or killed by adult weevils that feed on bark. It is common to have 30 to 60 percent weevil-caused mortality among first-year seedlings in the South, and mortality of 90 percent or more has been recorded (Thatcher 1960b). A third species, the eastern pine weevil is generally less common but is known to kill terminal and lateral branches and to girdle the stems of small trees (Doggett and others 1977, Nord and others 1984).
In the South, pales weevils prefer loblolly, shortleaf, pitch, and white pines and almost never attack longleaf pine. Rare instances of pales weevil feeding on hardwoods also have been recorded. The pitch-eating weevil is reported to feed on similar hosts, whereas the eastern pine weevil prefers cedar but will also attack most southern yellow pines. Pales and eastern pine weevils may serve as vectors of various pathogenic fungi.
In the South, weevil control is unnecessary after winter or spring cuts because all weevils are gone before the next winter’s planting. On the other hand, after summer or fall cuts, control will probably be necessary because the weevils remain onsite and attack newly planted seedlings during the spring (Corneil and Wilson 1980, Grosman and others 1999, Speers 1974). Weevils are not a problem when plantations are established on areas formerly covered with nonconiferous vegetation (for example, old fields and hardwoods) or on land where stands are allowed to regenerate naturally.
Only a few biological control agents that affect reproduction weevils have been reported. Very little is known about their effect in regulating field populations. Silvicultural and chemical strategies are available to reduce losses to reproduction weevils. A hazard rating system is available and should be used before scheduling pine planting.
Forest managers who harvest, prepare the site, and plant on a schedule that allows stumps to stale after cutting and prior to planting do not often experience high weevil-caused seedling mortality. In contrast, nonindustrial private landowners who often plant during the spring after late-year harvests often experience greater than 20 percent weevil-caused seedling mortality (Grosman and others 1999).
Reproduction weevil impacts may increase in the future. Current trends suggest that forest industry will continue to shorten rotations and may be less willing in the future to delay replanting to avoid the weevils. This trend could lead to an increased risk of weevil-caused damage or an increased need for proactive control strategies. Informed land managers can effectively reduce or eliminate the risk of weevil-caused damage, so education is a key to future prevention of this problem.
The Nantucket pine tip moth (Rhyacionia frustrana) is one of the most common forest insects in the Southeast (Berisford 1988). Although it is usually considered a southern pest, its range includes most of the eastern half of the United States.
Most hard pines are susceptible to attack by the Nantucket pine tip moth, but there are considerable differences in relative susceptibility. Among the southern pines, shortleaf, loblolly, and Virginia pine are highly susceptible, while slash and longleaf pine (with the exception of very young nursery seedlings) are highly resistant.
Damage, while potentially serious, is normally transitory or negligible in forest stands. Tip moth damage (loss of growth and deformation) is most severe on seedlings and saplings, usually under 5 years old. Deformation is particularly important on ornamentals and Christmas trees, which may become virtually worthless if tip moth attacks are not controlled. Experts disagree about the long-term impact of Nantucket pine tip moth attacks.
The abundance of the Nantucket pine tip moth is strongly affected by the availability of preferred hosts that are in susceptible age classes. Colonization of pine plantations is often rapid (Clarke 1982). Highest tip moth populations and damage tend to occur in even-aged, low-diversity stands (Berisford and Kulman 1967). Intensive stand management techniques including mechanical site preparation, or the application of herbicides or fertilizer, increase tree growth, but often favor increased tip moth damage (Nowak and Berisford 2000). The primary effect of ownership on this disease is a secondary effect of choice of management intensity. Naturally regenerated stands or plantations that are not managed intensively generally do not suffer enough damage to offset the cost of control.
Reliable sampling methods have been developed for determination of tip moth populations. However, the necessary links between population estimates and damage predictions have not been established.
The biology of the Nantucket pine tip moth as it relates to control is described in a variety of publications (Berisford 1974, Fettig and Berisford 1999, Haugen and Stephen 1983). Nantucket pine tip moth has a significant complement of natural biocontrol agents (Eikenbary and Fox 1965, 1968; Warren 1985). While several are being evaluated for use, none are commercially available. Insecticidal control can be used if damage is severe. There are a number of insecticides registered for tip moth control and for aerial application.
Tip moth infestations in loblolly pine stands are generally regarded as inevitable. However, as the acreage of intensively managed pine plantations is predicted to increase, this tip moth should become a more common pest problem in the future.
The baldcypress leafroller, Archips goyerana, periodically defoliates baldcypress in Louisiana. It has also recently been found in Mississippi. Kruse’s publication (2000) describes the baldcypress leafroller, summarizes its biology and its effects on baldcypress, and lists relevant publications.
The baldcypress leafroller was first recorded in 1983 in Louisiana, where it feeds almost exclusively on baldcypress. Since 1983, it annually has defoliated the baldcypress component of the bottomland hardwood/cypress forest (about 35,000 acres).
While this insect is mainly a pest of flooded baldcypress, it can move into drier upland and urban settings during periods of heavy infestation. Baldcypress trees of all sizes display canopy dieback and significant reductions in diameter growth because of repeated annual defoliation. Pole-sized to small sawtimber-sized baldcypress trees growing on forest edges or in dense stands are most severely affected. In areas where chronic saltwater intrusion is a problem, trees die after as little as 2 consecutive years of defoliation.
Most defoliation caused by baldcypress leafroller occurs on unmanaged private, nonindustrial wetlands. Although several parasitoids and predators attack A. goyerana, the general lack of natural enemies in forested wetlands leads to persistent high populations of this leafroller. Lacking economic incentives, little or no direct control is applied. A bacterial spray is available, but is seldom used. Starvation is the major factor causing local reductions in caterpillar populations. One potential future control tactic involves planting genotypes of baldcypress, cultured originally for salt tolerance, which may minimize caterpillar development and limit female fecundity.
High populations of A. goyerana are expected to continue in the forested wetlands of southern Louisiana and Mississippi. The insect may spread and become a problem in other areas of the Gulf Coast, but movement has been slowed by breaks in the baldcypress forest type (mapped as oak-gum-cypress) and the obstacles presented by large bodies of water. Dieback and mortality of baldcypress trees will increase.
The Texas leaf-cutting ant, Atta texana, is a serious pest in first- and second-year pine plantations in east Texas and west-central Louisiana. In areas where the ants are abundant, it is nearly impossible to establish pine plantations. Pine seedling mortality due to the Texas leaf-cutting ant occurs on nearly 12,000 acres annually and control and seedling replacement costs average $2.3 million per year (Cherret 1986, Texas Forest Service 1982).
The Texas leaf-cutting ant is generally confined to well-drained, deep sandy soils (Moser 1984, Vilela 1986). Figure 17.9 shows the range of the Texas leaf-cutting ant in Texas and Louisiana.
The impact of this insect appears to be unaffected by management intensity or ownership (Waller 1986).
Currently, only one chemical is registered to control Texas leaf-cutting ants, and it is scheduled for phase-out by the year 2005. A new baited formulation containing a slow-acting insecticide has been highly effective in field trials but is not yet registered for use.
Untreated colonies will remain a source of reinfestation and future losses.
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content: James Denny Ward and Paul A. Mistretta |
created: 4-OCT-2002 |