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Host Defenses
An important component of epidemiology of bark beetles centers on the interaction of the insect with its host.
Considerable effort has been directed to research on host defense mechanisms, because of their critical role in regulating
colonization by bark beetles. In pines (Pinus spp.) both preformed and induced defense systems have been identified.
During colonization, bark beetles directly confront the defense mechanisms of the host. The colonization process is greatly
influenced by variation in defense capacity associated with different pines, (e.g., Iongleaf pine is more resistant to D. frontalis
colonization than loblolly or shortleaf), and with different individuals of the same host species. Within a forest landscape
there is also a seasonal variation in host defense capability. Lorio (I988) suggested that normal physiological changes
associated with tree phenological development and response to variable environmental conditions cause regular and predictable
fluctuations in resistance/susceptibility to D. frontalis. Lovelady (1994) provides a comprehensive review and interpretation
of seasonal variation in host defense in relation to population dynamics ofD. frontalis.
Lightning-struck hosts, which have greatly reduced capacity for defense against colonization, serve as refuges for
bark beetles. For many years foresters have recognized that infestations olD. frontalis are often associated with a lightningstruck
host. The initial conceptual model of epiderniology of D. frontalis included lightning-struck hosts as a prominent
component, although only circumstantial evidence supported the contention (Coulson et al. 1983). Subsequent research has
suggested that lightning-struck hosts are an essential feature of the natural history of the insect. This research considered the
interaction of D. frontalis with disturbed hosts (Coulson et al. 1986a, Flamm and Coulson 1988, Flamm et al. 1992) as well
as an analysis of lightning as a disturbance regime (Lovelady et al. 1991, Lovelady 1994).
Of particular concern in the study of epidemiology is how host defenses are distributed at the landscape scale.
Herbivory by D. frontalis produces disturbance patches (infestations) in the forest matrix that can be observed relative to
other features of the landscape, i.e., the crowns of infested trees become discolored and are visually detectable. The disturbance
patches are typically associated with old-growth forest stands occurring on poor sites. Hazard rating systems have
been developed to grade forest stands with respect to vulnerability to D. frontalis infestation (Lorio 1980, Branham and
Thatcher 1985, Mason et al. 1985). Several different methods have been devised. Each rating system has novel features, but
all involve integration of a subset of variables: tree species, radial growth, height, and DBH; stand basal area (pine, hardwood,
total), species composition, site index, and degree of crown closure; and landform classification. When applied to a
forest landscape, the hazard rating systems provide a general view of the distribution and abundance of host defenses against
the insect. It is noteworthy that the hazard rating systems integrate a substantial knowledge base on the interaction of D.
frontalis, host plants, and site conditions.
To summarize, we have described the nature of the interaction of D. frontalis with host defenses and have identified
several sources of variation important to population dynamics of the insect. Although the hazard rating systems have proven
useful in defining vulnerability of forest landscapes to herbivory by D. frontalis, they ignore the seasonal dynamics of host
defenses associated with different tree species, with individuals of the species, and with lightning-struck hosts. Below we
will consider how the spatial and temporal sources of variation in host defense can be integrated in the context of the natural
history of the insect. In effect, we view host defense capacity as a variable in landscape structure that can be characterized
and defined. The distribution of this variable across the forest landscape influences epidemiology of D. frontalis.
Outbreaks and Forest Landscapes
Herbivory at the landscape scale is significant because it is at this level of integration where the ecological and forest
management consequences of insect outbreaks are interpreted (Pickett and White 1985, Forman and Godron 1986, Barbosa
and Schultz 1987, Turner 1987, Platt and Strong 1989, Turner and Gardner 1991, Holling 1992a, Wiens 1992, Coulson et al.
1993). Advances in the geographic information system (GIS) technologies and the development of statistics for analysis and
description of spatially referenced data have greatly expedited studies of insect outbreaks in forest landscapes (Liebold and
Barrett 1993).
Historically, interest in epidemiology of D. frontalis has been primarily associated with the economic impact of the
insect on forest resources. Emphasis has focused on investigations of herbivory in the context of impact on forest stand
structure, i.e., the research has considered the effect of process (herbivory) on pattern (the configuration of elements that
constitute the landscape mosaic). For example, Fitzgerald et al. (1994) examined the relation of various suppression tactics,
employed to modify D. frontalis populations, on the subsequent development of new infestations within the surrounding
forest landscape. 273