History of Southern Pine Beetle Control

The first attempts to control bark beetles were probably European and involved Ips spp. Disastrous bark beetle outbreaks occurred in Germany during the seventeenth and eighteenth centuries. So severe was the problem that a special prayer for the protection of forests from wind and insects was included in a prayer book printed in 1705. Gmelin (1787) reported that over a million-and-a-half trees were killed in the Hercynian Mountains alone between 1781 and 1787. Gmelin collected data from these seventeenth and eighteenth century outbreaks and in 1787 published a treatise on bark beetles. In addition to biological data, the treatise contained comprehensive detection and control recommendations. As a first step, Gmelin recommended an intensive survey to locate infested trees.

His major recommendation for beetle control was prompt salvage or burning of infested trees. Emphasis was placed on selecting trees still containing brood and ignoring trees from which beetles had already emerged. After trees were salvaged, bark removed from trees during the milling process was burned.

Gmelin also detailed the use of trap trees as a control measure. This consisted of cutting healthy trees at specified intervals. After the trees were attacked by beetles, they were burned to eliminate the brood.

In addition to direct control measures, Gmelin recommended thinning and sanitation measures to prevent attack. He also suggested that careless logging and weather and soil condition may predispose stands to attack.

He astutely attempted to correlate resin flow of individual trees with attack success and suggested that seed from resistant trees be used to propagate future beetle resistant stands.

Gmelin reported that seventeenth and eighteenth century attempts to control beetles with chemicals were generally unsuccessful and were considered dangerous because of the available chemicals: arsenic smoke of heather, sulfur and straw. He also toyed with the notion of using electricity for beetle control.

Gmelin looked at reasons for population collapse and attributed collapses to weather or to “the increasing number of enemies which limits unusual and tremendous overpopulation of the beetle.” Although Gmelin’s recommendations were made for and European species, we will see the same basic suggestions appear in American literature on southern pine beetle.

After the German control measures for bark beetles, the next attempt and probably the first in the United States was instigated by the Moravians in piedmont North Carolina (Fries et al., 1922). In 1797 they made a concerted attempt to salvage dead and dying beetle-attacked timber. Their salvage program appears to have been aimed more at loss minimization than at beetle control.

Hopkins (1909) observed an extensive southern pine beetle outbreak in Virginia and West Virginia in 1891-1892. He recommended salvage with subsequent destruction of bark by burning as a control measure. He believed that control action would be most effective during the winter months when beetle development is slow. He suggested water immersion of bark as an alternative to burning.

Hopkins also made sanitation recommendations designed to minimize beetle problems. These included removal of lightning-struck trees and restricting cutting to winter months in areas of known occurrence.

During an epidemic which occurred in North and South Carolina in 1911-1912, Hopkins’ recommendations were used in organized control projects in Mecklenburg and Gaston counties, North Carolina (Pratt, 1912). In 1912 the U.S. Bureau of Entomology established a branch office in Spartanburg, South Carolina to supply technical expertise for support of the SPB control projects (Pratt, 1911).

The use of chemicals for SPB control has been investigated since the first quarter of the twentieth century. Surprisingly, a major investigation was made of systemic chemicals by U.S. Forest Service researchers in the 1920’s and 30’s. St. George and Caird (1929) and St. George and Huckenpahlyer (1933) injected a wide range of chemicals into SPB infested trees hoping to kill the insect brood. They found that denatured alcohol, wood alcohol, carbon bisulphide, ammonium fluoride, and hydrocyanic gas provided adequate brood control. Mercuris chloride, zinc chloride and zinc meta arsenite injections not only killed beetle brood but were found to be good wood preservatives.

Chemically pure nicotine injected into recently infested trees by U.S. Forest Service researchers in 1933 (anon., 1933) was found to kill SPB without causing tree mortality. Eleven other materials were found either to kill host trees or were not effective agents for beetle control. Although several of the systemic chemicals appeared effective, subsequent research revealed that the chemicals must be applied within five to seven days of attack to be successfully translocated (Craighead and St. George, 1938). After this time period the blue stain fungus blocks chemical movement. This information led to the abandonment of systemic use in the Southeast at that time.

The same research group used several chemicals to control SPB in logs. Stainless creosote, pine oil (termex), and a mixture of one part orthodichlorobenzene to ten parts kerosene were found to control brood. Spraying recently attacked standing trees failed to increase survival rates of the infested trees. St. George (1932) attempted to apply both kerosene and orthodiclorobenzene as a prophylactic measure. He hoped that these materials would repel attacks. While he thought that the orthdichlorobenzene treatment was effective, the kerosene was a failure.

Researchers at the Southern Forest Experiment Station tested benzene hexachloride (BHC), orthodichlorobenzene, chlordane, and DDT against SPB. BHC proved to be most effective and 0.5 percent BHC in fuel oil became the standard chemical for SPB control in the South. BHC was first recommended for SPB to combat a 1950 outbreak in east Texas (Billings, 1989). BHC was further tested in 1955 (Speers et al., 1955) and was found to be more effective than either ethylene dibromide or orthodichlorobenzene for beetle control. This further reinforced the use of BHC as the predominant chemical control agent in the southeast. Accordingly, BHC mixed as a 0.5 percent active ingredient in fuel oil was the principal, direct control method used throughout the South from 1959 through 1970.

Interest in systemics resurfaced when Ollieu (Ollieu 1969) investigated the use of cacodylic acid, a fast-acting herbicide, and found successful brood reduction. From 1963-1974, Texas forest industry leaders organized and founded the Southern Forest Research Institute, under the direction of Dr. J. P. Vite. This Institute studied SPB attack behavior and infestation dynabics (Billing, 1989) and eventually isolated and identified several SPB behavioral chemicals, including frontalin, trans-vebenol and verbenone (Kinzer et al., 1969; Renwick, 1967). Alpha pinene and frontalin were subsequently mixed to form an attractant called frontalure. This was placed on cacodylic acid-treated trees in an attempt to trap and kill beetles in a single operation. A widespread test of the technique in Texas in 1970 met with variable success (Coulson et al., 1975) and the technique is no longer used. Research is still continuing toward developing new control tactics using SPB behavioral chemicals. In recent tests in several southern states, the beetle-produced inhibitor verbenone has been effectively used to halt spot growth without need for felling uninfested tress (Payne and Billing, 1989; Billings, 1990).

After comprehensive testing, the chemicals chlorpyrifos (Dursban 4E) and fenitrothion (Pestroy) were registered with the EPA in 1979 for both prophylactic and remedial treatment. These chemicals, along with lindane, are the chemicals currently registered (199) for SPB control.

In addition to chemical control, mechanical control has undergone an evolution since Gmelin recommended salvage and burning of infested material and Hopkins added water immersion.

During an outbreak in Texas in 1938-39, control consisted of cutting a half-mile swath around the infested areas (Billing, 1989). By 1945, the recommendation for swath width had been reduced to a quarter-mile. By the early 1960’s, mechanical control recommendations consisted of salvage of actively infested trees plus a buffer strip to ensure that recently attacked trees would not be overlooked in the salvage operation. Thatcher, et al. (1982) summarized current salvage recommendations. Salvage remains the most recommended direct control method for treating SPB infestations (Swain and Remion, 1981).

In addition to salvage control, a second mechanical option is cut-and-leave (Billing, 1980). An early version of the cut-and-leave treatment was described by Patterson (1930) as the solar heat method. Originally, control consisted of felling limbing trees. The boles were then exposed to the sun for a few days to kill brood and then the boles were rolled to expose the other side to the sun’s rays. By 1969, Texas personnel had modified the technique (Ollieu, 1969) to take advantage of known limitations in SPB attack behavior. Actively-infested trees, along with a 40-60 foot wide green buffer strip, were simply felled and left in the forest. The treatment eliminates natural sources of attraction (pheromone production), causing emerging beetles to disperse (Billing, 1980). This was found to halt effectively spot growth, particularly when small spots (10-100 trees) were treated. Treatment of active SPB infestations by salvage or cut-and-leave during summer months in East Texas also was found to reduce the frequency of new spot proliferation in the vicinity of treated spots (Billings and Pase, 1979b). An analysis of cut and leave in the Georgia Piedmont in 1980 was conducted by the Georgia Forestry Commission. Treatment effects were evaluated for ten replicates established in eight infestations. Nine of ten replicates showed a mean net reduction in brood production. Spot proliferation did not occur following cut and leave but SPB populations were clearly on the decline (GFC, 1980).

Although the individual tactics currently used for direct control of SPB have been around for many decades, the rationale or general approach to suppression has been revised in recent decades. During the era of chemical insecticides (1950-1970), the goal of most state and federal forestry agencies in the South was to detect and chemically treat each and every suspected SPB infestation, regardless of its size. Clearly, the ultimate goal was to solve the pest problem by eradicating the insect, if at all possible. The Georgia Forestry Commission cut and sprayed over 1 million SPB infested trees in 1962 (GFC Internal Report, 1963). Despite thousands of dollars of chemicals and countless manhours dedicated to suppression activities, the SPB declined in counties where control had not been investigated as well as in counties receiving control.

Large-scale insecticide control was voluntarily discontinued around 1970 due to the increasing cost of materials and persistence of the pest population. In addition, research findings by the Southern Forest Research Institute (Williamson and Vite, 1971) provided evidence that use of chemical treatments in East Texas may have contributed to the unprecedented 20-year SPB outbreak selectively eliminating populations of natural enemies. Since 1970, mechanical control methods (salvage removal and cut-and-leave) have largely replaced insecticides in operational control programs.

The current control strategy no longer attempts to eradicate the beetle by treating all infestations, but focuses on those infestations likely to expand and cause the greatest resource losses. Accordingly, only multiple-tree infestations are recorded by aerial observers. Each spot that exceeds a detection threshold (5 – 10 trees) is assigned a ground-check priority, based on the presence and abundance of trees with freshly-fading crowns (Billings and Doggett, 1979). To aid ground-check crews, a field guide (Billings and Pase, 1979a) was developed for rating individual SPB infestations and assigning a control priority, based on the potential for expansion (Billings, 1979). For use in critical situations, spot growth models are now available to predict actual tree losses that will occur if no control is applied (Billings and Hynum, 1980; Stephen and Lih, 1985). Small, non-expanding spots are monitored from the ground or air until they go inactive, without need for control (Billings, 1979). This approach has greatly reduced workloads of control crews and increased the efficacy of control efforts.

Area-wide SPB control efforts have long been hampered by such factors as the multitude of small landowners, poor access, lack of markets for beetle-killed timber, and landowner apathy (Billings, 1980). In addition, new constraints have developed during the last decade to further limit the extent to which area-wide SPB outbreaks can be prevented or controlled. The establishment of wilderness areas in various southern states in recent years hinders area-wide control efforts. No direct control or preventive treatments are allowed in these areas unless the infestation occurs within one-fourth mile of the boundary, endangered species are threatened, and/or several other specific criteria are met. As a result, these unmanaged areas have become increasingly prone to severe and persistent SPB outbreaks and threaten to become breeding grounds for perennial SPB populations.

Control efforts on certain National Forests are now routinely hampered by environmental activists who effectively use legal appeals and lawsuits to halt or delay suppression activities. The Four Notch experience in East Texas provides testimony to the destructive potential of SPB if no control is taken. Due to actions by environmentalists that caused delays in direct control, SPB infestations on this proposed wilderness area killed more than 2,000 acres of sawtimber in less than one year, drastically increased the frequency and severity of timber losses on adjacent commercial forest lands, and eliminated several colonies of the endangered red-cockaded woodpecker (Miles, 1987).

The 1988 court-mandated requirement to manage National Forest lands so as to promote survival of the red-cockaded woodpecker may serve to aggravate the SPB problem. Rotation ages have been extended and hardwood mid-story trees eliminated in foraging areas and in colony sites; these manipulations may increase susceptibility to SPB infestations in the long run. Direct control may thus be required more frequently to protect cavity trees and critical foraging areas form SPB infestations.

Silvicultural methods have been recommended to prevent SPB damage. Beal and Massey (1945) recommended fire prevention, slash disposal, thinning and regulating stand composition and density as beetle reduction measures. They also suggested shorter rotation lengths as a measure to avoid beetle problems. Bennett (1971) made comprehensive silvicultural recommendations. These included increasing the resistance of stands by promoting rapid growth, avoiding unnecessary site and stand disturbance, sanitation cutting, particularly when lightning struck trees are involved and drainage to relive soil moisture stress.

The Expanded Southern Pine Beetle Research and Applications Program (1974-1980) developed several hazard rating systems for SBP and identified further silvicultural recommendations to minimize beetle damage (Thatcher et al., 1980). The latter included favoring resistant species (slash, longleaf, Virginia and white pines over loblolly, shortleaf, or pitch), sanitation, maintaining rapid radial growth, promoting mixed hardwood-pine stands, minimizing logging damage, harvesting over-mature stands, and site protection.

There has long been interest in biological control of bark beetles. Gmelin (1787) recognized the importance of natural control agents in the cyclic nature of bark beetle infestations. Although he indicated that “we may become suspicious that the reduction of such enemies…may be one of the causes of the tremendous overpopulations of bark beetle,” he apparently did not try to supplement biological control factors.

Hopkins (1899 was a strong supporter of biological control of SPB. During an outbreak in Virginia, West Virginia, and Maryland in the latter part of the nineteenth century, he attempted biological control of the insect. He traveled to Germany and imported over 3,000 living specimens of a clerid beetle (Clerus fromicarius) which he hoped would function as a biological control agent. These were released at a number of SPB spots in West Virginia in 1892-1894. As with many other studies, shortly after Hopkins introduced this imported clerid, the SPB population collapsed. However, there is no evidence that this clerid became established as a result of these introductions. It is of interest that this collection of predators was largely financially supported by the timber companies in the stricken areas (as was the Southern Forest Research Institute in East Texas).

Although a substantial body of research exists on natural enemies of SPB, there has been surprisingly little research done on utilization of these natural control measures since Hopkins’ early work. Some of the direct control measures currently used are timed to minimize impact of natural control factors, but otherwise there appears to be little interest in this potentially valuable area. The fact that SPB is a native insect has discouraged entomologists from pursuing this approach.

Although outbreaks of the southern pine beetle have been reported for several hundred years and extensive research and control efforts have been aimed at this small insect, it continues to be one of the most destructive pests of southern forests.

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