Methods

The data shown in Table 1 and Maps 1960-1996 was collected to provide a regional record of longterm patterns of southern pine beetle outbreak. Such broad scale datasets are crucial to proper understanding of factors that control episodically varying pests, yet are rare for forest insects. However, it is important to recognize the limitations of the data presented here. This data was collected by state and federal pest control specialists to assist their own pest control objectives as well as to fulfill federal cost-sharing reporting requirements. Fundamental differences in methodology are inevitable particularly in light of the regional coverage and lengthy period described. Such difference necessarily limits the comparability of the data.

The two types of data presented in this publication, county-level outbreak intensities (Map Figures 1960-1996) and state-level damage estimates (Table 1), are derived from three sources of information: aerial spot detection surveys, ground checks of detected spots, and surveys of host forest extent. This section defines the data presented and describes how it was assembled.

Aerial surveys:

Because host damage and reproduction by southern pine beetles occur primarily in well-defined patches called spots, locating and enumerating spots are fundamental to estimating their population and impact. Active spots are principally identified through detection flights (Hain, 1980). Flights are conducted periodically through the active season, with flight timing dependent on expected level of beetle activity, season, objectives, and operational capabilities (Billing, 1979). States do not record very small spots, less than five or ten trees in size, because of their limited potential for damage [For example, Texas increased its detection threshold to ten active trees in 1974 (Billings, 1979)], and for programmatic reasons some states do not survey or report spots on federal lands. Survey efforts may historically have been less intensive during years of limited beetle activity or in counties thought to be at low risk, leading to under-reporting of spot numbers. Dull (1980) discusses some of the sources of error associated with aerial spot detection.

Ground checks:

Pockets of mortality observed in the air may be caused by agents other than southern pine beetle. Ground checks allow confirmation of the beetle’s role and permit improved estimation of spot size for subsequent damage estimates (Mayyasi et al., 1975). States may prioritize detected spots by their damage potential, restricting ground checks to those spots most likely to benefit from control (Billing, 1979) [Because beetle activity was light in 1989 and 1990, SC performed no ground checks].

Host surveys:

Because southern pine beetle only attacks certain species of pine, measures of spot frequency are typically expressed relative to the amount of potential host available. For the maps in this publication, spot numbers in each county have been divided by that county’s acres of susceptible forest, producing “spots per thousand acres of susceptible host type.”

“Susceptible host type” refers to forests dominated by suitable host species. Loblolly and shortleaf pines are the most common host species of southern pine beetle, although pitch and Virginia pines are also susceptible. White, slash and longleaf pines are rarely attacked by southern pine beetle and thus are not treated as susceptible. Pines within mixed forests can be attacked, although less frequently than in stands with high pine basal area (Lorio, 1980).

All the states in the survey obtain their county-level estimates of acreage by forest type from the U.S. Forest Service’s Forest Inventory and Analysis (FIA) survey. States combine the acres in the FIA forest type categories “loblolly-shortleaf” and “oak-pine” as their estimate of susceptible acres [For GA, prior to 1972, only one-half of all mixed oak-pine acres were included as susceptible.]. The FIA survey is conducted approximately every ten years, with states apparently using the most recent survey data available for their calculations [For 1972-1990, GA estimates were based on linear interpolations between survey years.]. Thus, estimates of susceptible acres can be up to ten years out of date.

Levels of infestation:

The above descriptions suggest that the states forwarded to us estimates of spots per thousand acres. This is generally not the case. Rather, most state infestation levels have been reported by broad categories. For the years prior to 1978, data is only available on whether a county was in outbreak status or not, where one spot per thousand acres or greater serves as the definition of outbreak. Starting in 1978, infestation levels have been divided into three ranges:

Category	Spots Per Thousand Acres Susceptible Host Type
Low	0.1 to just under 1
Middle	1 to just under 3
High	3 and greater

The new “low” category captures less intense infestations than were reported in earlier periods – only the middle and high categories fit the previous definition of “outbreaks.”

Damage estimates:

State-level damage estimates (Table 1) are divided into pulpwood and sawlog volumes killed and estimated amounts salvaged, with volumes valued using that year’s statewide prices. Estimates of amounts killed and salvaged are derived from spot counts, ground checks, and other available information. Estimates of that year’s statewide stumpage values are then simply applied to the volumes killed to produce estimates of total value of loss.

Update:

This publication updates data found in an earlier publication (Price et al., 1990) that presented similarly collected data on outbreaks from 1960 through 1990. These older data are reproduced here for the convenience of the reader. Only data for Georgia from 1972 to 1990 were revised, based on improved estimates of susceptible host acres [The previous version have used more dated estimates of host acres and had included one-half of the mixed oak-pine acres]. Data for all states after 1990 was solicited for this publication from the state and federal pest control specialists listed in the Contributors section and maps and was proofed by the responsible contributors for accuracy. Their assistance has been essential to this effort and is gratefully acknowledged.

[ Back ]