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Biological Control of Southern Pine Beetle Fred M Biological Control of Southern Pine Beetle Fred M. Stephen1 and C. Wayne Berisford2 1Professor, Department of Entomology, University of Arkansas, Fayetteville, AR 72701 28 2Emeritus Professor, Department of Entomology, University of Georgia, Athens, GA 30602 Abstract Exotic invasive forest insects are frequently managed through classical biological control, which involves searching for, introducing, and establishing their exotic natural enemies. Biological control of native bark beetles, including the southern pine beetle (SPB), has been primarily attempted by conserving and manipulating their natural enemies. Knowledge of the role and biology of SPB natural enemies is increasing but is still limited, and is rarely well connected to coincident estimates of SPB host density. A rich complex of SPB native natural enemies exists, and these are discussed in greater detail in other chapters in this book. The cryptic nature of Dendroctonus species within phloem and bark, combined with the properties of many natural enemies (small size, highly aggregated distribution, lower density than their prey, and often acting late in the beetle’s life cycle), results in challenging sampling problems that are difficult to overcome. Attempts to assess impact of natural enemies have often been presented as percent Keywords of mortality, but rarely do these assessments show variation in mortality. The manner in which mortality varies with host density is important in population bark beetles regulation. Predators, parasitoids, and competitors of the SPB respond in varying parasitoid competitors degrees to SPB pheromones and tree volatiles during host selection. Variables population dynamics such as bark thickness and SPB density influence parasitoid success. In making predators oviposition choices, parasitoids tend to select the host beetle and tree species Scolytidae from which they emerged. Short SPB generation times, continuous flight, and attack by SPB adults result in infestations containing all life stages of beetles and natural enemies. Opportunities for numerical response of parasitoids to epidemic population growth should be great but have not been confirmed. Manipulation efforts indicate that providing nutrients for parasitoid adults increases their longevity, stimulates production of additional eggs, and prevents resorption of existing eggs. Parasitoids do forage in canopies of both pine and hardwood trees, possibly to acquire honeydew as adult nutrition. Predators frequently are the most abundant and visible sources of SPB mortality, and their potential role as delayed density-dependent agents may be important in the cycles exhibited by SPB populations. Simulation models that experimentally remove mortality attributable to natural enemies show how rapidly infestations grow when natural enemies are absent. Experimental research on Monochamus spp. indicates that they can cause high mortality to SPB brood as competitors and predators, and field observations suggest that they may play an important role in the collapse of SPB epidemics. 415 28.1.INTRODUCTION best definition is that of DeBach (1964), who considers biological control as a phase of natural Infestations of southern pine beetle control and defines it “as the action of parasites, (Dendroctonus frontalis Zimmermann) (SPB) predators, or pathogens in maintaining another are a veritable hive of insect activity, and organism’s population density at a lower the majority of these insects are not SPBs. average than would occur in their absence.” At Observations from a field-based perspective of its core, this definition implies that biological climbing hundreds of infested trees, sampling control is a natural process. The native complex bark that contains different SPB life stages, of predators, parasitoids, and competitors and studying the multitudes of insect species associated with the SPB, for example, reduces searching on the bark surface and burrowing and to some extent SPB population density. This developing within the inner bark and phloem, does not necessarily imply that natural enemies reveal the remarkable diversity of the SPB- will maintain SPB population density below associate complex. Many of these associated a level that is economically acceptable, but it insects parasitize, prey upon, or compete for does mean that without these natural enemies food and/or space with the SPB, and as such are the SPB would be a more significant pest. considered natural enemies of the beetle. In five other chapters of this book detailed information 28.2.2.TheFieldofBiological is presented on SPB predators (chapter 10), Control parasitoids (chapter 8), competitors (chapter 12), and fungal and mite associates (chapters Biological control of insect pests can be 9 and 11). Despite the extensive information considered an applied discipline; however, its that is known about SPB natural enemies, it is foundations are based in ecological theory. Thus safe to say that we still know much less than knowledge of insect ecology and ecosystem we should about their roles and importance interactions is usually key to successful in suppressing SPB population density. The biological control. Although much is known, purpose of this chapter is to define the field of we still do not have sufficient knowledge to biological control as it relates to the SPB and enact successful biological programs against the to briefly summarize attempts made at SPB SPB. Biological control as a discipline is often biological control. separated by the origins of pests and of their natural enemies and by the approaches used to maximize the effectiveness of the natural enemies. These approaches can be distinguished 28.2.Biological control as follows: 1. classical biological control – the Human manipulation of natural enemies to importation and establishment of exotic natural effect reduction of pest populations has existed enemies to control exotic or native pests; and for millennia; however, the term “biological 2. manipulative biological control – techniques control” was first used by H.S. Smith (1919) that manipulate natural enemies to augment to signify use of natural enemies, whether or enhance their effectiveness, or techniques introduced or not, to control insect pests. that manipulate the environment to benefit and Successful biological control of forest insects conserve existing natural enemies. Examples is well documented (Dahlsten and Mills 1999, of these approaches in relation to the SPB are Dahlsten and Whitmore 1989, Pschorn-Walcher explained and discussed below. Disruption of 1977, Turnock and others 1976, Waters and naturally occurring biological control through others 1976), and biological control is today a human intervention can and does occur. This primary tactic for management of many exotic knowledge should further stimulate our efforts forest insect pests. to gain more knowledge of forest / bark beetle / natural enemy interactions. Because 28.2.1.Definitions successful biological control must be based on Natural control is a broad concept that the fundamentals of scientific knowledge, it is encompasses the combined actions of abiotic essential to subsequently evaluate the reasons and/or biotic environmental factors to maintain for success or failure of biological control an organism’s more or less fluctuating programs. population density, within certain definable upper and lower limits, over a period of time Importation (Classical Biological (DeBach 1964). Although the term “biological Control) control” is used in many ways by different The importation of natural enemies to control authors, the most accepted and in our view the exotic (introduced) pests is by far the most 416 Stephen | Berisford common approach to biological control of forest and conserving natural enemies is for native insects (Dahlsten and Mills 1999). Invasive pests a strategy more important and more likely insects or other organisms that are accidentally to succeed. Humans can act to augment numbers or intentionally introduced from geographically of natural enemies, and they can manipulate distant areas into a new locale such as North their habitats so as to increase natural enemy America may reproduce without mortality from survival and effectiveness. Actions that natural enemies or host resistance that normally manipulate insects themselves can theoretically evolves through generations of insect-plant be considered distinct from actions that alter the interactions. The search for natural enemies of environment in which the insects live; however, the pest in its native area, coupled with their in practice it can be difficult to distinguish the importation, quarantined rearing to eliminate two (Rabb and others 1976). Conservation of secondary parasitoids or diseases, in concert natural enemies may be an approach that is with release, establishment, and control of the highly appropriate in forest ecosystems, as they pest in its new environment constitutes classical are longer lived, suffer fewer disruptions, and biological control (DeBach 1964). In forest are generally more complex than agricultural systems, classical biological control should be systems (Dahlsten and Mills 1999). considered as an essential management tool, particularly in light of the increasing numbers Study of invasive exotic pest species that are being The phrase, “know your enemy,” would seem discovered. If eradication of these exotic species to be essential to any commander of a military fails, then management of their populations campaign, and may be an appropriate
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