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4.5 Control Methods

4.5.1 Silviculture

Silvicultural methods for controlling pests include practices that favor the appropriate species for the site or increase the vigor of the plants left on the site. During site preparation, thinning, or any other stand improvement activities, opportunities exist to favor the healthiest and most natural components of an ecosystem. Normally, vigorous, mixed-age and mixed-species forests are more resistant to devastation by native pests than are single-species plantations.

4.5.2 Genetics

Often, a portion of a population is less affected by a pest than is the remainder of the population. This ability to tolerate attack by a pest may result from inherent resistance in the population. When resistance is genetically based, favoring and propagating resistant individuals will add a measure of protection to the next generation. Breeding to enhance genetic resistance takes advantage of a natural process, augmenting it but not significantly altering it. However, as managers breed genetically resistant plants, pest populations adapt to attack the newly developed resistant host material. The process of genetic manipulation is, therefore, an ongoing process, not a permanent solution.

In recent years a new technology, genetic engineering (which involves altering the genetic structure of living organisms at the molecular level), has emerged. Pests can be engineered, altering their genes to make them less successful in reproducing or less aggressive in attacking potential host material. Alternatively, hosts can be genetically engineered to make them more resistant, or even toxic, to invading pests. Currently, little genetic engineering is being done with southern forest trees. The potential of this method is unclear because use of this method is currently very controversial. Genetic engineering is perceived by some as having the potential to accidentally kill beneficial organisms or even to create new pests.

4.5.3 Quarantine

State and Federal agencies often restrict the movement of live plants or animals across State or National boundaries unless they are declared free of pests. These quarantines have been fairly effective in reducing the spread of known pest organisms but have failed to stop many organisms that are not pests in their native environment but become pests when moved. As discussed elsewhere, quarantine restrictions have been ineffective in preventing the introduction of ornamental plants, which subsequently are shown to have no natural enemies in their new ecosystems. Plant quarantine to ensure the health of incoming vegetative materials and prevent the dissemination of infested or infected materials is a critical process for protecting the future health of the southern forests.

4.5.4 Sanitation

Sanitation involves removing infected or pest-infested materials from an ecosystem in an attempt to reduce or eliminate pest impact in response to pest outbreaks, or as a part of regularly scheduled stand maintenance activities. Affected trees are selectively removed, leaving the healthy vigorous ones. Sanitation can be highly effective if symptoms are readily visible. However, where symptoms are masked, large numbers of infected or infested tees may be left.

Prescribed fire is often used to suppress pests either by killing them or destroying or modifying their habitats.

4.5.5 Chemical Control

When properly applied, pesticides are very useful in suppressing or eradicating pest organisms. Pesticides used in the southern forests include insecticides, herbicides, rodenticides, and fungicides (USDA Forest Service 1992).

Pesticides can suppress pest populations by killing the pests outright or by moderating their activity. They may be applied from the air or the ground. New pesticides have been developed that kill only the intended pest or affect a very limited number of target organisms. In southern forests a limited number of treatments (2-4) in the 40- to 120-year rotation will occur.

Despite an impressive record of success in controlling pests, and progress to improve their selectivity, pesticide use in the South has declined steadily in numbers of acres treated as well as in rates of pesticide applied per acre. Data are not available on pesticide use on industrial and private land in the South. It is believed that the downward trend in pesticide use is not as marked on these lands as it is on National forests.

4.5.6 Biological Control

Biological control involves the use of one organism to moderate or control the behavior of another organism. In biological control, the manager attempts to locate a natural enemy of a pest and augment its population to control unacceptable population levels of the pest. Viruses, bacteria, fungi, and insects have all been used in biological control (Stairs 1971). Apparent biocontrol of an epidemic population of gypsy moth by the fungus Entomophaga miamiaga, the use of a virus against sawflies, and the use of another fungus against the introduced pine sawfly are examples of successful biocontrol. Biological control, however, suffers from a problem very similar to genetic control. Often, this process has only provided short-term solutions. Natural enemies of a pest organism may fail to colonize the same niche as the pest, and either totally fail as biocontrol agents, or themselves become pests in a niche, they do colonize.