Stressors of Pine Forests
Southern Pine Beetle
The southern pine beetle (SPB) is one of the most serious insect pests of pine in the South. Smaller than grains of uncooked rice, these hard bodied, dark brown insects annually destroy timber trees worth millions of dollars. Additional impacts are on recreation areas, shade trees, and general aesthetics.
The beetles bore directly through the tree bark, normally producing pitch tubes that resemble kernels of popped corn. Females construct long, winding, S-shaped tunnels between the bark and wood. Lined with eggs, these tunnels eventually girdle the tree, effectively stopping the flow of water and nutrients. The beetles also introduce a fungus known as "bluestain," which furthers damages the conductive systems of the tree by plugging cells. Figure 21 shows beetle egg galleries and bluestain streaking on the sapwood. Newly emerging beetles bore directly through the tree bark, leaving innumerable holes and making the tree appear as if hit with bird shot.
As a result of the damage to the phloem, trees eventually fade to yellow, and then bright reddish brown before they lose their needles. Typically, the direction in which the infestation or spot is spreading can be determined by tree color, with more recently attacked trees (which comprise the spot head) appearing abnormally light green to yellow, and trees with older infestations appearing red or gray. Beetle spots can vary in size from just a few trees to thousands of acres.
The number of generations of beetles per year depends on several factors, including location and weather conditions. Under conditions favorable to the insects, as many as seven generations per year appear in certain parts of the South, with the life cycle being completed in as little as a month.
Typically, infestations begin in trees weakened by such stress factors as disease, lightning strikes, excessive age, or storm damage. Once the beetles become established in such trees, they are capable of attacking apparently vigorous, healthy hosts.
While SPB infestations come and go across the South, the insect is almost always epidemic somewhere in its natural range. Reliable records on southern pine beetle infestations have been available since about 1960, with certain parts of the South (e.g., eastern Texas and northwestern South Carolina) showing an inordinate predisposition toward outbreaks.
Since 1960, the USDA Forest Service has requested cooperating State agencies to report counties exhibiting more than one multiple-tree SPB spot per 1000 acres of host type, with host type determined by Forest Inventory and Analysis survey data. Additionally, Forest Service land managers reported the same data. The information was then compiled by frequency of incidence.
Figure 22 shows, by county, the number of years in which SPB has reached outbreak status since 1960.
Southern pine beetle populations have fluctuated markedly since the mid 1990's. From 1999 to date (2002), losses have been heavily focused in the eastern South, with infestations most concentrated in South Carolina and the Appalachians. In many areas of the mountains, virtually all yellow pine has been lost. White pine losses have also been heavy, with the log home industry suffering devastating consequences in East Tennessee. Impact has been exacerbated by drought, poor wood markets, and public appeals against logging on federal lands that have prevented salvage control. More specific information on the fluctuation of southern pine beetle populations in the South since 1996 can be found in archived Southern Region Forest Insect and Disease Conditions Reports: http://www.fs.fed.us/r8/foresthealth/pubs.html#conditions
Due to the cyclical nature of SPB epidemics, the South will doubtless continue to sustain periodic heavy losses from the pest. Typically, these outbreaks last two to three years, and repeat about every seven years. Long-term losses may decline slightly because of improved management methods that stress loss prevention, including increased stand and age diversity, and better detection and control practices.
The principal method for SPB surveys remains the traditional aerial sketch map survey, a procedure highly dependent upon the skill and experience of surveyors to accurately plot infestation locations. Promising new technologies, however, will doubtless find applications in SPB detection and evaluation. These technologies include global positioning systems (GPS), satellite imagery, and aerial videography.
The GPS uses a network of satellites that allows survey crews to determine location within as short a distance as 3 meters. It will allow even inexperienced sketch mappers and ground check crews to find spots quickly, easily, and more economically. Surveyors envision a time when aerial survey crews can mark SPB infestations using aircraft-based GPS, and ground crews can, in turn, use terrestrial units to travel directly to the spots.
Additionally, declassified satellite imagery is expected to improve to the point that photographs will be far superior in resolution to those of today, probably eventually allowing for real-time transmission of SPB data.
Finally, emerging technologies such as aerial videography, in combination with such advances as GPS, will enable survey crews to make meaningful progress in their ability to detect and evaluate southern pine beetle infestations.