Suppression of Bark Beetles and Protection of Pines in the Urban Environment: a Case Study

Southern pine beetles (SPB), and associated bark beetles, have long been recognized as major pests of southern forests. Tactics used for controlling infestations in conventional forest settings have not proven effective at achieving area-wide control, nor are they suitable for the control of infestations in high-value stands such as homesites or wildlife habitat areas. Limited options exist for protecting highrisk uninfested pines of urban forests and often pose undesirable risks. One of the most promising areas in bark beetle research currently being experimentally tested on a largescale is the use of deterrent behavioral chemicals (semiochemicals), produced by the insects or their host trees, as biopesticides to disrupt or inhibit infestations. In addition to traditional suppression tactics instituted in an unprecedented SPB outbreak in Gainesville, Florida, a semiochemical, 4allylanisole (4-AA), was successfully tested as a protectant of pines in residential areas. 4-AA is a host-produced compound with repellent properties to many species of coniferfeeding bark beetles. The “freak” SPB outbreak in this urban environment and successful actions taken to mitigate damage are discussed. Bark beetles (Coieoptera: Scolytidae) are by far the most destructive pests of pines in the southeastern United States. The 5 predominant species of indigenous bark beetles responsible for killing millions of pines annually throughout the south include the southern pine beetle (SPB), Dendroctcnus front&; the black turpentine beetle (BTB), Dendroctonus ferebrens; and 3 pine engraver (Ips) species (Ips at&us, 1. calligraphus, and 1 . grandiculus). Of these, SPB is much more pernicious than BTB or Ips. BTB has relatively few generations per year (23), is attracted to weakened or wounded pines, and typically attacks fresh stumps or live trees near their bases. Trees often survive successful attacks; infestations generally result in limited damage to healthy trees and stands. Ips infestations are even less threatening to healthy, undamaged trees than are BTB. Ips attacks are typically restricted to stressed, weakened, and damaged trees and freshly felled host material. Ips attacks may occur along any portion of the bole or stem as wellas in limbs of crowns, and are generally lethal when successful. SPB, on the other hand, is notorious for its ability to overcome numerous healthy and vigorous trees in rapid succession during outbreaks. This highly destructive nature of SPB is due, at least in part, to a unique combination of life history characteristics. These characteristics include a congregative mass attack process, the ability to attack more than one host per generation, and multiple generations per year (up to 8). SPB tends to initiate attacks on living trees at the mid-bole and can infest the entire bole from near ground level up into the lower crown (upwards of 18 m). The female SPB init iates attack and bores through the bark to the vascular layer of the bole. During this process she emits a chemical signal into.the air, a pheromone, that attracts additional beetles of both sexes in mass numbers. These beetles bore into and begin to reproduce in the host pine. The tree’s immediate defense is the outflow of resin at an attack site, which forms what is referred to as a pitch tube. This resin flow may result in the “pitching out” of the beetle. However, under heavy attack, even the healthiest tree may be overwhelmed. As the beetles and their offspring bore, successfully colonizing a tree, they typically introduce fur@into the tree. This combination of beetle boring and fungal growth inevitably causes tree mortality. During low bark beetle populations, trees under some physiological 1. Florida Division of Forestry, 1911 SW 34th Street, Gainesville, FL 32614-7100 2. Dept. of Entomology and Nematology, University of Florida, Gainesville, FL 32611-0620 3. Southern Research Station, 2500 Shreveport Highway, Pineville, LA 71360 68 Hayes et al.: Suppression of Bark Beetles stress, such as from lightning, wounding, windstorms, severe drought, or flooding, appear to be most vulnerable. When bark beetle populations are high, even healthy trees may succumb in rapid succession to the mass attack and colonization by beetles. SPB outbreaks throughout the south have occurred with cyclic frequency at approximately 7to 1 O-year intervals and tend to persist for 2 to 5 years. In a recent historical review, SPB damage to pine forests over the last 30 years has been estimated at $900 million (13). Not included in this impressive figure are pine losses suffered each year by homeowners and by communities with tree-lined streets and parks (4). These trees are valued not for their timber but for their contributions to a community’s air and water quality, real estate values, energy conservation, wildlife habitat, and aesthetics. As in campgrounds and other scenic areas, these trees are prominent features of the landscape and therefore irreplaceable. As trees mature and increase in size and value, they become more vulnerable to bark beetle attack. Protection of these highly prized trees, particularly in the urban, suburban, and rural forest interface areas, is of great importance. Bark Beetle Management Tactics in Forested Areas In conventional forest settings, SPB suppression tactics focus on reducing beetle populations and creating barriers to the spread of individual infestations using “cut-and-remove,” cut-andleave,” or “cut-and-handspray” techniques (14). In each of these tactics, all infested trees are felled along with a buffer strip of uninfested trees surrounding the active front or “head” of the infestation for cut-and-remove and cut-and-leave. These tactics rely on disruption of the typical pattern of infestation growth by eliminating immediately available, acceptable host material and reducing the overall number of attacking individuals. While essential for limiting infestation growth, these tactics have not proven effective at achieving areawide control, nor are they suitable for the control of infestations in high-value stands such as homesites or wildlife habitat areas. In addition, none of these methods provides direct protection for highrisk uninfested pines of urban forests. Three traditional pesticides are registered for use in control and prevention of SPB infestations: lindane, chlorpyrifos, and fenitrothion (2,3, reviewed in 7). The forestry research community has intensified its efforts, particularly in the last decade, to develop and transfer biorational technology as an alternative to pesticide use. Widescale use of traditional pesticides for bark beetle population suppression is limited. Aerial applications are impractical because the active ingredient cannot penetrate the tree canopy to reach the targeted attack area of the bole. Biorational technology emphasizes methods and materials that effectively control only target pest species while reducing adverse impacts to nontarget organisms or the environment. Biological control, the use of one organism to regulate the numbers of another, is one of the most common forms of biorational technology. Others include semiochemical technology, which involves manipulation of insect populations through strategic use of the insect’s airborne chemical communication system to effect control of behaviors. One promising area in bark beetle research currently being tested on a large scale is the use of the insects’ own communication signals (semiochemicals) as biopesticides to disrupt or inhibit infestations. In addition to the chemical attraction signal emitted by females in the initial stages of colonization, there is a chemical inhibitor, or deterrent, produced primarily by males, This inhibitor pheromone signals that the host tree under attack is fully colonized and causes newly arriving beetles to land on and infest nearby trees; hence, the infestation (spot) enlarges in area. Although pheromones may be similar among closely related beetle species, they are generally species specific. For example, a primary inhibitory chemical produced by SPB, verbenone, is being experimentally used in place of felling a buffer strip of uninfested trees and has been shown to effectively disrupt spot growth under certain conditions (12). Similarly, of the bark beetles that attack conifers in the western United States, an inhibitory pheromone produced by the spruce beetle, methylcyclohexenone (MCH), has been successfully used experimentally as a biopesticide (15). Journal of Arboriculture 22(2): March 1996 6 9 New Advances in Beetle Management for Single Trees Another form of biorational technology is the area of host resistance, the use of host attributes to reduce susceptibility, including a multitude of characteristics from morphological (such as bark features) to chemical (such as host-produced chemicals as deterrents and antifeedants). A semiochemical produced by certain plants, with repellent properties to many species of coniferfeeding bark beetles (8,9,10) is 4-allylanisole (4AA). The oleoresin (pitch or sap) of pines is predominantly composed of compounds known as monoterpenes. 4-AA belongs to a class of compounds known as phenylpropanoids, produced in a metabolic pathway unique from the monterpene pathway. It is typically found in small quantities in the oleoresin of many pines, including those utilized by SPB, such as loblolly (Pinus taeda), shortleaf (R e&in&) longleaf (I? palustris) and slash (R elliottii) pines. In laboratory studies, >80% of SPB, regardless of sex, were repelled by the presence of this chemical. In field studies the addition of 4-AA significantly reduced SPB collections relative to collections in traps with attractant only or attractant plus verbenone, SPB’s own inhibitory compound. At the same time, natural enemies of SPB, such as the predatory clerid beetle, did not seem to be affected. SPB in Urban Areas In 1992 two lightning-struck pines in residential settings in Pineville, Louisiana, were treated with 4-&I. Treatment consisted of placing nine 20mm polyethylene vials equipped with cotton wicks at 1 m (1.1 yd) intervals on the damage side of the tree trunk from level up to 8 m (8.7 yd). In both cases, the trees were not attacked by SPB for the 30 days of 4-AA treatment. These results encouraged us to consider the use of 4-AA in urban settings to protect high-value trees from SPB attack. Pines in urban areas are as vulnerable to SPB as they are to lightning strikes. In fact, the two often follow one another; a general rule of thumb is that if the lightning doesn’t kill the tree, the beetles attracted to it probably will. Although generally no data are kept on pine loss in urban areas, a call to local private tree contractors suggests that 30 to 40 pine trees are removed from residences annually because of lightning strikes and beetle infestations, at an average cost of approximately $250 per tree. Under any of these circumstances, 4-AA and other biorationals may provide an environmentally neutral means of protection from infestation. Once the threat of infestation has subsided, treatment can be immediately ceased. While no nonlethal material can be expected to provide 100% protection, 4-AA has the potential to significantly lower the risk of bark beetle infestation in landscapes in urban habitats where pines are often highly prized features. SPB Outbreak in Gainesville, Florida An unprecedented and severe SPB outbreak occurred throughout a 155 km2 area of greater Gainesville, Florida (Alachua County) during 1994, killing nearly 17,000 pines and impacting more than 320 public and private landowners. In 1995 the outbreak spread throughout the county, with beetles killing another 22,000 pines on about 300 ownerships in just the first 6 months of the year. The beetles infested not only loblolly pine, the preferred host of SPB, but also attacked slash, longleaf, spruce (I? g/&a), and, in 1995, included the Florida endemic sand pine (R clausa), a rarely reported host of SPB. While Ips and BTB have been common colonizers of dead and dying pines, there is no record of SPB for at least the past 50 years in the Gainesville area. The nearest known infestation occurred in old-growth loblolly pines in 1947 near Silver Springs, about 65 km (40 miles) south of Gainesville (5,6). Since 1960, most SPB in Florida have been in the central and western sections of the Panhandle (13), and only recently have there been scattered infestations in northeastern Florida close to the Georgia border. The pine mix, stand structure and condition, and history of land use in the Gainesville area undoubtedly contributed directly or indirectly to the intensity and duration of the outbreak. The majority of the pines in the urban Gainesville area at the time of the outbreak were mature, sawtimber size (>25 cm or 10 inch diameter at breast height, dbh) and somewhat patchily distributed, and included