The Southern Pine Beetle
Chapter 10: Direct Control
Introduction
The search for practical and effective methods to protect pine resources from the southern pine beetle has challenged scientists and forest managers for many years. Increasing emphasis is now being placed on proper forest management practices (silvicultural control) as the principal means for avoiding beetle problems (see Chapter 9). But outbreaks continue to occur in areas where host and climatic conditions favor SPB population increases. Direct control methods — salvage, cut-and-leave, insecticides, or pile-and-burn — provide the last line of defense for protecting pine resources from excessive losses once infestations are in progress.
Until about 1970, the primary approach for combating the southern pine beetle and other bark beetle pests was to destroy broods in infested trees. With limited knowledge of the insect and a lack of trained personnel, forest managers saw this strategy as the simplest and most efficient way to deal with bark beetles when they became a problem.
Before the 1960’s, we knew little about the underlying causes of southern pine beetle outbreaks and relationships between beetle attacks and host condition. As a result, the SPB was considered a "beetle problem" rather than a "tree problem," to be dealt with in a manner similar to insect pests in agricultural crops. For many years, direct control was the first and only line of defense for protecting valuable timber resources. Conveniently enough, science helped us fight the beetle battle with chemical weapons.
Early methods for killing beetles were varied and imaginative: rapid utilization of infested trees (= salvage) and burning the slabs, tops, and unmerchantable trees; immersing infested logs in water (Hetrick 1949); exposing infested trees to solar heating (= cut-and-leave) (St. George and Beal 1929); and injecting poisons into the sap stream of recently infested trees (Craighead and St. George 1938). But some of these practices also proved inefficient and often impractical.
The search for more efficient means to maximize brood mortality led to the development of toxic chemical sprays. Orthodichlorobenzene in kerosene and later in fuel oil was successfully used on SPB during the 1940’s (Thatcher 1960). Following World War II, a new chlorinated hydrocarbon insecticide — benzene hexachloride (BHC) — became available and was first used in 1950 to combat an SPB outbreak in east Texas (Morris 1951 unpublished).
With BHC and its gamma isomer lindane as weapons, pest control specialists firmly believed that outbreaks could be suppressed and beetle problems solved simply be treating enough beetle-infested trees to eliminate the beetle’s pest status — if not the insect itself. State and Federal pest control agencies and industrial landowners pursued this "brute force" approach diligently. Chemical control had priority over salvage because of the belief that "salvage contributes little or nothing toward control of the beetle populations" (Texas Forest Service 1950).
But the insecticide treatments did have limitations. They were expensive and time-consuming; costs ranged from $1 to $10 per tree (Drake 1970). And chemicals required careful, thorough treatment in order to give high mortality (Anderson 1967). Further, their extensive use was charged with selectively eliminating beneficial insects (Williamson and Vité 1971).
These factors, along with the discouraging fact that beetle outbreaks continued year after year in spite of large-scale chemical control programs, ultimately put an end to their wide use for bark beetle control in the South. The BHC era had ended by 1970, and the search for new and environmentally acceptable alternatives began.
Pest managers are changing their philosophy toward direct control. Control programs that involve both preventive and remedial measures are being encouraged. Depending on the value of the resource threatened, the objectives of the landowner, and the season, the control strategy may be to minimize timber losses, to maximize beetle mortality, or to let nature take its course (Hedden 1979). Indeed, spots may go untreated if the likelihood for additional timber losses is small or if the landowner places little or no value on the threatened trees. New strategies will integrate insect control programs with total resource management and will neither exclude nor rely solely on direct controls (Coster 1977).
At present, forest managers or landowners faced with southern pine beetle infestations can choose from four direct control options: (1) removal and utilization or sale of infested trees (salvage), (2) cut-and-leave or cut-and-top, (3) fell and spray with insecticides, and (4) fell, pile, and burn infested trees. Options (1) and (4) are among the earliest controls recommended for use against SPB (Hopkins 1911).
Selection of a tactic usually depends on the value of the resource threatened, the scope of the pest problem, spot size and accessibility, the landowner’s management objectives, and the season (Hedden 1979, Swain and Remion 1980). Also, each method has advantages and limitations that may either favor or preclude its use in a particular situation.
Salvage
Throughout the South, prompt salvage and utilization of infested trees has become the preferred method for minimizing the beetle’s economic impacts and simultaneously reducing beetle concentrations.
Although the benefits of salvage seem obvious, it is not appropriate for all southern pine beetle spots (Kucera 1969). Clearly, to be controlled by salvage, a spot must be accessible to heavy logging equipment and contain sufficient merchantable volume to cover harvesting costs. Also, there must be a local market for beetle-killed trees. Because of these constraints, many spots are not suitable for salvage control. Despite these limitations, salvage, when properly and promptly applied, remains the most practical and economic control tactic for treating large, rapidly growing infestations (Swain and Remion 1980).
Cut-and-Leave (Cut-and-Top)
In east Texas a tactic known as cut-and-leave has been increasingly used on spots where salvage is not practical (fig. 10-2). In 1979, the tactic was added to the list of recommended SPB suppression procedures for which Federal cost-sharing funds may be obtained (Harvey Toko personal communication). To date cut-and-leave has been used only on a limited basis in other Southern States.
Procedure
The procedure for cut-and-leave, described in detail elsewhere (Texas Forest Service 1975, Swain and Remion 1980), is similar to salvage treatment, except that felled trees are not removed (fig. 10-3). Only currently infested trees and a buffer strip of uninfested trees are felled, with the crowns pointing toward the center of the spot. Cut-and-top is a variation of cut-and-leave in which the crowns of infested trees are severed from the lower boles. The use of cut-and-top in east Texas largely has been discontinued in favor of the simpler and less time-consuming cut-and-leave method.
Rationale
Cut-and-leave and cut-and-top methods were initially designed to capitalize on biological limitations of developing broods (Ollieu 1969). Low moisture and high temperature in the inner bark area of the felled trees were expected to reduce beetle survival. But experimental tests of cut-and-leave did not demonstrate sufficient beetle mortality to justify its application solely on that basis (Hodges and Thatcher 1976, Palmer and Coster 1978, Hertel and Wallace 1980).
But cut-and-leave continues to see use in Texas, primarily because of another beneficial effect — when a buffer strip is included, it stops the expansion of a spot (Ollieu 1969).
The biological rationale for spot disruption by cut-and-leave is based on our understanding of how individual spots expand during the summer (see Chapter 2). Continuous spot growth requires at least three factors: emerging beetles, nearby pine trees, and a source of secondary attractants (fig. 10-4). Felling the most recently attacked trees eliminates the attractant source (Vité and Crozier 1968), the felled buffer strip eliminates nearby unattacked pines, and beetles emerging from the infested trees tend to disperse in the absence of attractants (Gara 1967).
 Figure 10-4 – Summary of biological and economic rationale for summer treatment to disrupt spot expansion. |
What becomes of the adult beetles that emerge in treated spots? Their fate is unknown, but during the hot summer months, mortality may be high since the beetle’s energy reserves are low (Hedden and Billings 1977).
Could the dispersing beetles aggravate the pest problem by causing new spots? This possibility is difficult to test, but a computer analysis of recent detection and control records for east Texas, where spot disruption tactics are commonly used, suggests that summer control by cut-and-leave or salvage did not contribute to the number of new spots in the area. On the contrary, new spots appeared most frequently in the vicinity of uncontrolled active spots and those controlled after September (Billings and Pase 1979a).
Advantages and Disadvantages
Cut-and-leave provides a simple and practical treatment for preventing spot spread in small to medium-sized spots (< 100 active trees) where salvage is not feasible. It requires little equipment and can be applied by a two- or three-man crew, often at time of first ground check. As a result, the average lag time from detection to control by cut-and-leave is considerably less than that for salvage (Texas Forest Service 1980). This time lag becomes an important consideration when a landowner is faced with a large number of spots to treat.
There is evidence that greater numbers of predators and parasites emerge from SPB-infested trees treated by cut-and-leave or cut-and-top than from standing infested trees during the summer (Baker 1977). This potential benefit deserves further study.
The disadvantages relate primarily to the need for a buffer strip of uninfested green trees. Many landowners are reluctant to sacrifice this buffer if the trees cannot be salvaged. Particularly in small spots (< 20 active trees), more trees may be cut for the buffer than would be killed were no treatment applied (Hertel et al. 1980), since spot growth in small spots may not occur. Expanding spots with more than 100 active trees are more difficult to stop with cut-and-leave, and reinfestations (breakouts) are more likely to occur. Salvage is preferred for such large spots.
Insecticides for Remedial and Preventive Control
Lindane
Since 1950, BHC and lindane have been the standard chemicals recommended for control of the southern pine beetle and other bark beetles (Hetrick and Moses 1953, Coulson et al. 1972b). Both chemicals are effective when applied in fuel oil or in water emulsion to infested logs for destroying broods or in water emulsion to standing trees to prevent SPB attacks. Although BHC is no longer available, lindane is still registered for remedial control and prevention of bark beetles.
In recent years, however, the use of pesticides in forestry — particularly chlorinated hydrocarbon insecticides such as lindane or BHC — has become highly controversial (Koerber 1976). Drawbacks include safety hazards, toxicity to nontarget organisms, and persistence in the environment. In November 1969, the Mrak commission on pesticides and their relationship to environmental health (U.S. Department of Health, Education, and Welfare 1969) recommended that the use of persistent insecticides including lindane, be restricted to essential purposes and be replaced by safer alternatives whenever possible.
Chlorpyrifos
With the goal of developing acceptable and effective chemical substitutes for lindane, researchers have screened numerous alternative insecticides in the laboratory to contact toxicity to SPB adults (Hastings and Jones 1976) and efficacy for reducing brood survival in infested log sections (Ragenovich and Coster 1974). Several proved to be more toxic to SPB adults or developing broods than lindane. One of the most promising was chlorpyrifos (Dursban® 4E), an organophosphate insecticide.
Subsequent field tests have documented the efficacy of chlorpyrifos for remedial control (killing broods in trees). In three independent field tests (Louisiana, Georgia, Mississippi), formulations of 1- and 2-percent chlorpyrifos were found to be equivalent to 0.5-percent lindane in water for reducing numbers of emerging beetles from treated bolts (Ragenovich 1977 unpublished; Brady and Berisford 1977 unpublished, Fitzpatrick, Neel and Lashomb 1979). The same concentrations can be used as a topical spray on standing trees to prevent SPB attack for up to 4 months (Ragenovich 1977 unpublished, Brady and Berisford 1977 unpublished). These efficacy data, when combined with the necessary support information on safety, toxicity to nontarget organisms, and the environment (Hastings, Jones, and Kislow 1977), have enabled the manufacturer to obtain EPA registration for a 1-percent chlorpyrifos spray. It can now be used for the remedial control or prevention of SPB.
Fenitrothion
Another organophosphate insecticide successfully field tested for remedial control or prevention of southern pine beetle is fenitrothion (Sumithion®). Both 1- and 2-percent formulations have proven superior to lindane in water for remedial control (Berisford and Brady 1978 unpublished, Mizell and Neel 1979 unpublished). As a protective spray, a 2-percent solution provided protection up to 3 months in the presence of moderate SPB population pressure. Efforts are currently underway to obtain EPA registration for fenitrothion.
Advantages and Limitations of Insecticides
Because of high cost and increasing Federal restrictions, we will probably never again see the large-scale use of toxic chemicals for control of bark beetles in the South that prevailed in the 1950’s and 1960’s. Nevertheless, there remains a need for fast-acting, effective tactics to reduce bark beetle concentrations or to prevent tree mortality in high-value or special use situations. To date, only insecticide sprays can assure this level of protection (Swain 1976, Roettgering et al. 1976). In commercial forests, insecticides are useful for treating small spots during the winter or those inaccessible to salvage equipment. Also, insecticides provide an effective means to protect high-value trees from bark beetle attack in pine seed orchards, naval stores, or urban, home, and recreational areas (Thatcher, Coster, and Payne 1978).
Cost, the need to fell and spray all surfaces of infested trees, safety precautions, and toxicity to nontarget organisms are expected to limit the use of the new insecticides in forests. Chlorpyrifos, a cholinesterase-inhibiting insecticide, is labeled for restricted use to be applied only by or under the supervision of pest control operators or other trained personnel. This chemical also is more costly to apply than lindane or fenitrothion.
Pile-and-Burn
The practice of felling, piling, and burning infested trees to destroy the developing broods represents one of the earliest approaches to bark beetle control. This method is still recognized as an option for SPB, and broods can be destroyed if all infested bark is completely burned. This practice has been largely abandoned as an operational method, however, because of the labor and logistical problems involved. In most cases, heavy equipment is required to pile the trees. In wet areas, burning felled trees becomes difficult if not impossible. In dry areas, the procedure increases the chances of wildfire, and burning as a control is necessarily restricted to the seasons when fire danger is low. Nevertheless, fire is still available as a control tool for whoever prefers this mechanical method of destroying beetles.
Potential Control Methods Evaluated
Scientists have continued the search for new methods of direct control. Among possibilities explored by the ESPBRAP have been fertilization of infested stands, use of systemic insecticides, and manipulation of beetle populations with synthetic behavioral chemicals (pheromones).
Fertilization of Infested Stands
Fertilization of forest stands promotes rapid growth and presumably increases tree vigor. Moore and Layman (1978) conducted tests in North Carolina to determine the extent to which fertilizers increase resistance of pines to bark beetle attack. A 9- to 11-year-old loblolly pine plantation infested with SPB and black turpentine beetle was treated with a summer application of 10-10-10 fertilizer at a rate of 1,000 lb/acre. Over an 80-day study period, beetles continued to kill similar numbers of trees in both fertilized and unfertilized plots. The investigators speculated that fertilizers applied in the spring might prove more successful. The trees would have more time for nutrient uptake and response before beetle activity increased during the summer. Because of collapsing beetle populations, no additional fertilizer tests have been conducted to date.
Systemic Insecticides
A systemic insecticide applied aerially to the foliage of beetle-endangered trees would be a useful tool for direct control of the southern pine beetle. To be effective, such a chemical would need to be absorbed into the needles and rapidly transported through the inner bark (phloem) in concentrations sufficient to kill beetles where they feed. One insecticide that showed early promise as a phloem-moblie systemic was acephate (Orthene®). Extensive field evaluations of acephate for reducing within-tree populations of SPB have been conducted (Crisp, Richmond, and Shea 1979 unpublished). Foliage applications at two different rates prior to beetle attack reduced survival of larvae. But the treatment had no effect on eggs, pupae, or callow or parent adults. The investigators concluded that systemic insecticides will need to be more phloem-mobile, more toxic to all life stages, and more persistent than acephate if this approach is to succeed.
Behavioral Chemicals
Most bark beetle species select and colonize suitable hosts by using species-specific systems of chemical communication. For SPB, host selection involves both attractants and inhibitors (see Chapter 2). The concept of controlling SPB by exploiting its own behavioral chemicals has captured the interest and imagination of entomologists for many years.
Pioneering field experiments conducted in 1963 demonstrated that the southern pine beetle’s aggregating pheromone could be used to concentrate and subsequently decimate a beetle population on resistant trees (Gara, Vité, and Cramer 1965). These early studies utilized log sections infested with virgin females as the source of attractant because the pheromone had yet to be identified.
The primary component of the aggregation pheromone produced by attacking southern pine beetle females has since been identified as frontalin (Kinzer et al. 1969, Payne et al 1978). Frontalure, a synthetic attractant composed of frontalin in a D-pinene, is available for experimental use, Behavioral chemicals that repel beetles, such as endo-brevicomin (Silverstein et al. 1968) and verbenone (Renwick 1967), are also being evaluated for control purposes.
Frontalure might be used in several ways for southern pine beetle control. One technique, based on a knowledge of SPB attack behavior and the effects of the herbicide cacodylic acid (Vité 1970), was tested on a limited basis on private lands in east Texas from 1970 to 1973. The procedure involved baiting uninfested trees with frontalure on the periphery of active spots and simultaneously treating the baited trees with cacodylic acid. Emerging beetles were thereby induced to attack and colonize the "trap trees" and most of the broods failed to develop due to the excessively high inner bark moisture resulting from the herbicide treatment (Ollieu 1969). A similar procedure was successfully tested in Virginia for controlling spring-emerging populations of SPB during outbreak years (Copony and Morris 1972). The tactic, however, has not been adopted operationally. Apparently its success was dependent upon too many unanticipated and uncontrollable variables for the method to be applied by nonprofessional personnel (Coulson et al. 1973a and b, and 1975b).
The transfer of frontalure from experimental to operational use for southern pine beetle control has been slow, but potential applications are still under investigation. An experimental attempt to "confuse" emerging SPB in a summer infestation by permeating the active front with frontalure applied from the air proved unsuccessful (Vité, Hughes, and Renwick 1976). The frontalure intensified the infestation rather than disrupting it, presumably by preventing dispersal losses and attracting beetles from surrounding stands. These investigators concluded that inhibitory compounds may hold more promise than attractants for future aerial permeation experiments.
In a more recent test in east Texas, frontalure applied from the ground to nonhost trees and pines containing late brood stages behind the front of an expanding spot successfully interrupted the natural process of spot growth. No additional trees were killed in the spot until the frontalure was removed 6 days later (Richerson, McCarty, and Payne 1980).
The test was repeated in Georgia in 1979. After 50 days, no additional spot growth occurred in a previously expanding spot following similar application of frontalure to trees containing developing brood. The treated spot eventually went inactive, while untreated spots nearby continued to expand during the course of the experiment (T.L. Payne personal communication).
The treatment presumably disrupts spot growth by preventing emerging beetles from responding to natural, but more distant, pheromone sources at the spot’s active head. Further tests are planned to evaluate the effectiveness and practicality of this application as an operational control tactic.
Recent tests on protecting individual trees within an active spot from attack with various inhibitory pheromones also have proved encouraging. Although the inhibitor-treated trees were killed by a combination of SPB and Ips avulsus, a mixture of endo-brevicomin and verbenone reduced SPB landing on traps by 84 percent and egg deposition by 88 percent during the 17-day treatment period (Richerson and Payne 1979). Reductions in both number and length of beetle galleries in treated trees suggest that this treatment could significantly reduce brood production and lead to the early disruption of spots.
It is possible that permeating an area with these compounds to inhibit beetles from perceiving and/or responding to attractants may be more successful and practical than baiting individual trees. The necessary tests and evaluations are currently underway to determine if these inhibitors can be developed into practical pest management tactics (T.L. Payne personal communication). Progress has also been made toward the development of sustained-release delivery systems for bark beetle pheromones (Payne, Coster, and Johnson 1977).
Since behavioral chemicals are not yet available for operational use, a discussion of their advantages and disadvantages for southern pine beetle control is largely speculative. Among advantages, behavioral chemicals are natural, nontoxic compounds that are considered nonhazardous to the environment. Unlike current control methods, the application of behavioral chemicals does not require felling trees. The chemicals could be formulated for easy application by nonprofessional field crews or small landowners. The use of frontalure for spot containment shows promise as a simple and viable substitute for cut-and-leave that eliminates the need to sacrifice unattacked trees for a buffer. Conceivably, the same approach could be combined with salvage to prevent further tree mortality until all brood trees were removed from the spot. Aerial applications of inhibitors may permit the treatment of a large number of spots within a short period. This capability would place operational controls for the first time in the hands of a few highly trained pest control specialists.
The operational use of behavioral chemicals for southern pine beetle control must await EPA registration and development of safe and practical delivery systems. Costs may prohibit use over extensive forest areas. Perhaps most importantly, we must keep in mind that success under experimental conditions does not assure that a new tactic will become operational. But treatment efficacy is essential and represents the first step along the path to eventual application.
Practical Considerations and Complexities of Areawide Control Programs
In nature, southern pine beetle populations occur at three different levels of "organizational complexity" (Coulson 1979c; see also Chapter 5): beetles in trees, beetles in spots (groups of infested trees plus attacking adults), and beetles in areas (groups of spots plus dispersing adults). Control efforts, in turn, may be directed at any one of these three levels. Experience has shown that efficacy at the first level (high beetle mortality in treated trees) or the second (spot disruption) doesn’t necessarily imply equal success at the third level (suppressing outbreaks). In fact, the desired goal of control efforts, choice of tactics, and measures of treatment efficacy may vary, depending upon which level is addressed (Hedden 1979).
Suppression of beetle outbreaks over wide areas by means of direct control action has seldom been achieved, with one possible exception (Lorio and Bennett 1974). The apparent failure to curb outbreaks in progress is often blamed on deficiencies in available control tactics. In reality, to achieve effective control at the area level requires prompt treatment of all active spots over a broad area within a relatively short timespan. This goal is seldom attainable due to various practical constraints that enter the control picture at the area level. These obstacles include the multitude of land ownerships involved and seasonal influences of the insect, host trees, and man. Also, until the recent development of population estimation techniques, we have been unable to evaluate the efficacy of areawide control tactics. For a more complete discussion of this subject, see Chapter 6 and Coster and Searcy 1979.
practiced, spreading directly by spot growth (fig. 10-5) or indirectly through beetle dispersal to adjacent lands, where losses may be less acceptable. The very fact that SPB infestations do not generate the same level of concern among all landowners severely complicates efforts to suppress populations on an areawide basis.
In certain States, laws have been passed requiring landowners to control SPB infestations promptly. In some cases, State forest agencies have legal authority to control spots on private lands in situations where landowners are reluctant to do so. Originally intended to aid areawide control programs, such laws have proven difficult to enforce. State forest agencies simply do not have the manpower, equipment, or money to assume responsibility for all spots that develop during SPB outbreaks.
Seasonal and Regional Constraints
Seasonal limitations also complicate direct control efforts, particularly in the Gulf Coast region. What we know about the reproductive potential and seasonal habits of the SPB clearly suggests that, to reduce beetle populations, direct control should be applied primarily during the winter (Hopkins 1911, Thatcher and Pickard 1964, Franklin 1970a). In practice, however, control programs along the gulf coast are applied primarily during the summer (fig. 10-6), because detection of new infestations is seasonally dependent. From 45 to 75 percent of all multiple-tree spots are reported from May through July (Coulson et al. 1972b). Also, access to spots is often hampered during fall, winter, and spring months by wet ground conditions, particularly in lowland areas.
 Figure 10-6 – Typical patterns of control application in relation to seasonal trends in the attack: emergence ratio for seven overlapping generations of SPB in the Gulf Coast region. |
When outbreaks occur, new spots are reported faster than available resources can be marshalled for direct control action. The 1979 outbreak in Georgia is an excellent example. Over 11,000 spots averaging 50 trees in size were detected over a 69-county area by August (Price and Thomas 1979). But due to rapid beetle development, many summertime spots may already be inactive (abandoned by SPB) by the time they are ground checked, and ground crews must devote much time to checking spots that may no longer contain beetles.
By winter, much of the beetle population in the Gulf Coast region has become distributed in single trees and small new spots scattered throughout the forest. Because new infestations during the fall, winter, and early spring are difficult to detect (Billings 1979), as many as four consecutive beetle generations may effectively escape control pressure. And this is the very season when the attack: emergence ratio of the insect is at its highest (fig. 10-6). It is possible for beetle populations to recover to outbreak levels the following year, largely counteracting control efforts the summer before. The cycle repeats itself as long as such factors as favorable climatic and host conditions prevail.
Control during the winter is less problematic in the northern part of the beetle’s range (Tennessee, the Carolinas, and Virginia). Cold winters tend to restrict beetles to the multiple-tree spots they occupied during the fall; this fact simplifies winter detection. Since the beetles complete only three to five generations per year in this area, overwintering broods seldom emerge before late April or May. This schedule allows more time for control prior to beetle dispersal. It is not unusual for landowners to control as many SPB infestations during the winter in these States as during the summer (Coleman A. Doggett personal communication).
Finally, the recurrence of southern pine beetle infestations in an area from one year to the next should not necessarily be attributed to "ineffective" control tactics. Recurrence may also be a symptom of high-hazard stand conditions (Hedden 1978b). Such stand management problems remain largely unaltered by direct control treatments.
Learning to Cope with SPB Outbreaks
In recent years, pest managers have accumulated much historical information from southern pine beetle outbreaks and direct control programs (Price and Doggett 1978, Pase and Fagala 1980). With this information and recent research on SPB population dynamics, patterns of infestation development, and susceptibility of forests, we are better prepared to suggest how forest managers can more efficiently confront SPB outbreaks.
We now recognize that once SPB populations reach outbreak levels (e.g., more than one multiple-tree spot/1,000 acres host type), it is probably too late to change drastically the course of events (i.e., total number of spots). But we can reduce considerably the ultimate size of spots and their economic impact by setting realistic priorities and establishing "action thresholds" (Chant 1966) at each level of operation: detection, ground check, and control. By recognizing that all spots do not pose the same threat, we can focus control efforts on those spots likely to have the greatest impact on resources if left unattended.
Improving Detection and Ground Operations
We now have enough empirical evidence to approximate the action threshold level for detection and control of southern pine beetle spots. Experience has shown, for example, that it is impractical to detect single-tree spots during the summer for purposes of control. Most State agencies in the South restrict detection to those spots containing a minimum of 5 to 10 red- and yellow-crowned trees. This action threshold has been set because smaller spots are mostly inactive at the time of ground check (fig. 10-7) or are the result of causes other than SPB (Billings 1974 and 1979).
 Figure 10-7 – Relationship of initial spot size as observed by survey crews and persistence of SPB infestations. |
The efficiency of ground-check operations can be further increased if aerial observers estimate the size of each new spot they report and assign each a ground-check priority (Billings and Doggett 1980). When the size of individual spots is estimated from the air, unavoidable errors occur (Mayyasi et al. 1975). The extent of the error varies with spot size, season, and the experience of the observer. Nevertheless, an aerial estimate of beetle activity based on the number of trees with discolored foliage is useful for deciding which spots to ground check. During outbreaks, spots assigned a low priority by aerial observers may not warrant immediate ground checking. These spots, together with others that are not controlled promptly, should be reevaluated on a subsequent flight. At the time of the second visit, spots that my have enlarged since first detection can be assigned a higher ground-check priority and spot size estimate to reflect the need for immediate attention by ground crews. On the other hand, spots which no longer contain yellow-crowned trees can be designated as nonexpanding spots, requiring no control action during summer months (Billings 1979).
Improving Control Operations
Even with a multiple-tree reporting threshold, many newly detected spots will be inactive during summer months — approximately 30 to 40 percent during most years in Texas, for example. Efficiency of control operations could be improved by concentrating efforts only on those spots most likely to expand.
A recent study of spot growth (Hedden and Billings 1979) has provided useful information for establishing a more realistic control threshold. This team found that summer spots containing less than 10 active trees seldom became larger after detection. Ten-tree spots that did expand exhibited declining rates of growth (i.e., the number of active trees declined over time), and the spots soon went inactive. Additional timber losses in these spots were insufficient to justify immediate control.
In medium-sized spots (11 to 50 active trees) growth was largely dependent on prevailing stand conditions, with the number of active trees increasing only in stands of high basal area. A field guide illustrating how to evaluate the potential for spot growth as a basis for setting control priorities has been prepared (Billings and Pase 1979b).
Large spots (50 or more active trees) demand highest priority for control. Even though such spots are relatively uncommon, they account for over 60 percent of the timber volume killed each year (fig 10-8). Large spots almost invariably grow larger, with rates of spread increasing in direct relation to the initial number of active trees and density of host trees (fig. 10-9). If they are left unattended, excessive timber losses will accrue from both rapid spot growth and the eventual proliferation of new spots as winter approaches.
Studies in North and South Carolina (Moore 1978) suggest that attack: emergence ratio may be more reliable for summer predictions of spot growth in that region than criteria of number of active trees and stand density. Moore accurately predicted population trends of several spots in Atlantic Coast States over time periods of 8 to 12 months, using the attack: emergence ratio, plus five secondary factors to improve the accuracy of predictions on static spots. This method appears practical and simple enough to be used by trained personnel in Atlantic Coast and Piedmont States, where beetles have fewer generations per year and individual spots may persist from one summer to the next. The reliability of the attack: emergence ratio for prediction purposes has been questioned (see Chapter 5).
Selecting a Control Tactic
Among factors to be considered in selecting the control option best suited for a particular spot are the season of the year and the size of the spot. Salvage remains the preferred choice at all seasons if the spot is accessible, sufficient timber volume is involved to cover the costs, and there is a market for the beetle-killed trees. Cut-and leave treatment to disrupt spot growth is recommended for use in summer and fall only — the seasons when spots are most likely to expand. Similarly, cut-and-leave or registered insecticides (lindane or chlorpyrifos) are most appropriate for medium-size spots that are a threat to enlarge but are not suited to prompt salvage. Small spots (< 10 active trees) may need no control during the summer. In the winter, insecticides may offer the best means to reduce overwintering beetle populations in spots not accessible or suitable for salvage. Cut-and-top and pile-and-burn remain of questionable value as control tactics, even during winter months.
Direct control options currently available to the land manager remain few and relatively unchanged from previous years. Prompt salvage and utilization of infested trees is still the preferred direct control procedure for most spots. Yet we have progressed substantially in our understanding of the insect and in our approach to direct control. For example, the SPB is no longer considered an unavoidable pest to be eliminated wherever and whenever it appears. Foresters are recognizing that preventive (silvicultural) treatment to increase host resistance is the best way to assure long-term protection from beetle problems. By applying proper silvicultural practices to those unmanaged and overstocked stands most favored by SPB, we can markedly reduce both the occurrence and severity of infestations.
Once spots develop, however, prompt salvage or disruption by cut-and-leave assures that timber losses are kept to a minimum. In high-value situations, lindane and the newly registered insecticide chlorpyrifos are available for remedial control or prevention. A third chemical, fenitrothion, has proved effective against SPB but has yet to be registered.
Meanwhile, progress has been made toward the eventual application of synthetic pheromones for direct control. Once techniques are developed, the use of the bark beetle’s own chemical attractants or inhibitors may eliminate the need to fell trees in order to disrupt spot expansion.
Perhaps most important, we now realize that all infestations need not be treated by direct control in order to cope with the beetle. Some infestations pose more of a threat than others. By considering the initial size of the spots, prevailing stand conditions, value of the timber, and season, pest managers can set realistic detection, ground-check, and control priorities. These procedures will optimize available manpower and equipment use during outbreak periods. In this manner, we make the best of a very complex pest problem while foresters strive to correct the stand conditions that predispose our forest to SPB outbreaks.
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