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FUNCTIONAL HETEROGENEITY OF FOREST LANDSCAPES:
HOW HOST DEFENSES INFLUENCE EPIDEMIOLOGY
OF THE SOUTHERN PINE BEETLE
ROBERT N. COULSON, JEFFREY W. FITZGERALD, BRYAN A. MCFADDEN,
PAUL E. PULLEY, CLARK N. LOVELADY, and JOHN R. GIARDINO
Knowledge Engineering Laboratory, Department of Entomology and Department of Geography, Texas A&M University
College Station, Texas 77843, USA
INTRODUCTION
Insect outbreaks are autogenic disturbances that are normally observed at the landscape scale as levels of herbivory
above an average or expected amount. When favorable environmental conditions coincide with optimal resource availability,
populations increase in size and outbreaks occur (Rykiel et al. 1988). Epidemiology is the study of causes for change in the
distribution and abundance of an insect species at tile landscape level of ecological integration. Typically, studies of epidemiology
have considered herbivore populations in the context of their impact on food and habitat resources. In this paper we
focus on the association of forest landscape structure and the process of herbivory, i.e., the effect of landscape pattern on the
process of herbivory (Turner 1989). Our goal is to examine epidemiology of the southern pine beetle, Dendroctonusfi'ontalis
Zimmermann (Coleoptera: Scolytidae), in the context of functional heterogeneity of forest landscapes. Our objectives are:
(i) to review briefly the current state of knowledge on epidemiology of D. frontalis, (ii) to consider the concept of functional
heterogeneity of forest landscapes and how it relates to epidemiology of D. j?ontalis, (iii) to consider ways and means of
measuring functional heterogeneity of forest landscapes, and (iv) to illustrate the relation of functional heterogeneity and
herbivory by the insect.
Although emphasis is directed to D. frontalis, it is noteworthy that this insect is a member of a guild consisting of
four other bark beetle species: D. terebrans (Olivier), Ips calligraphus (Germ_), L grandicollis (Eichhofl), and I. avulsus
(Eichhoff) (Coleoptera: Scolytidae). The population systems of these insects are interrelated. How the members contribute
to epidemiology of the guild is unknown.
Background
Epidemioiogy of the Southern Pine Beetle
[=lammet al. (1988) provide a review of literature on the natural history of D. frontalis. A general conceptual model
of epidemiology of the insect was proposed by Coulson et aL (1983, 1985a, b, 1986a) and Rykiel et al. (1988). This model
considers the process of herbivory in the context of variables thought to influence epidemiology, i.e., meteorological conditions,
the lighming disturbance regime, forest stand conditions, background levels of the insect, and landscape structure.
Studies by Flamm and Coulson (1988), Flamm et al. (1992), Lovelady et al. (199 I), and Lovelady (1994) have employed
empirical methods to address specific aspects of the conceptual model.
Outbreaks of D. frontalis are the result of population fluctuations observed at the landscape scale. There are three
separate processes associated with D. fi'omalis epidemiology. The first, which operates at the population and community
levels of integration, involves factors associated with initiation of infestations. The second, which operates at the ecosystem
level of integration, deals with factors that influence growth of established infestations. The third, which operates at the
landscape level of integration, deals with the network of habitat units and population centers occurring in a heterogeneous
land area consisting of multiple interacting ecosystems. An insect outbreak is an epidemiological condition that represents a
composite of the three processes operating across different scales of time and space. The actual process of herbivory by D.
fromalis occurs at the population and community levels of ecological integration and results in mortality to host pines.
However, the effects are propagated through component ecosystems to the landscape level (Coulson et al. 1986b, Rykiel et
al. 1988, Lovelady et al. 1991).
Mattson, W.J., Niemel_, R, and Rousi, M., eds. 1996. Dynamics of forest herbivory: quest for pattern and principle. USDA
For. Serv. Gen. Tech. Rep. NC-183, N.C. For. Exp. Sta., St. Paul, MN 55108.
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