Forest Pest Control
Douce, G.K., Moorhead, D.J., and Bargeron, C.T., Forest Pest Control, The University of Georgia, College of Agricultural and Environmental Sciences, Special Bulletin 16, Revised January 2002.
Weeds are unwanted vegetation that interferes with land management objectives. They are obstacles to regeneration and optimum crop growth and development. Weeds compete with crops for moisture, nutrients and light. They can be classed as weed trees, brush, vines, and herbaceous weeds.
Herbicides are chemicals that kill or suppress the growth of weeds. Plants are controlled by herbicides that act on the plant’s physiology. Different herbicides, concentration rates, application methods and equipment enable users to control targeted weeds without undue injury to desirable plants or the environment.
Herbicides are registered for the specific forest uses and application methods for which they have been tested. Uses other than those indicated on the label are unlawful and may not provide the needed control. Off-label use can cause adverse effects to non-targeted species on- and off-site by drift or movement in soil and water. Furthermore, unauthorized use may pose a hazard to human health.
Mode of Action
Translocated (systemic) Herbicides
Examples of chemicals used in forestry work are given for each category.
Factors Affecting Control
The type of application and equipment to be used for a specific job will depend on a number of things.
Before making a pesticide application you should:
The forest manager must be acutely aware of the risks and consequences of pesticide use and their application in and around forested environments. Use pesticides only when necessary to minimize pesticide impact in areas receiving direct application as well as non-target habitats and organisms which are potential recipients of pesticide drift and runoff.
Before you apply a pesticide, consider these points:
Terms commonly referred to when dealing with methods of applying pesticides in forestry include:
Application rate: The specific amount of pesticide applied to a treated acre or target system.
Broadcast: Uniform application to an entire area.
Banded: Application to a strip or band over or along each tree row.
Basal: Application to the lower portion of stems or trunks.
Cut surface: Application to a cut or incision in a tree or to a stump.
Directed: Aiming the pesticide at a specific portion of a plant.
Foliar: Application to the leaves of plants.
Over-the-top: Application over the top of the crop trees.
Soil application: Application to the soil rather than to vegetation.
Soil incorporation: Application to the soil followed by tillage to mix the herbicide with the soil.
Soil-spot treatment: Application to a small area of the soil surface.
Stem injection: Application into incisions around a tree stem.
Stump treatment: Application to the top or edges of a tree stump.
Some of the terms that describe the purpose or timing of pesticide applications in forestry include:
Cut surface: Includes trunk injection, frill, frill-girdle, girdle and cut stump treatment.
Desiccation: The “brown-out” or drying of vegetation by use of herbicides to aid in burning for site preparation.
Dormant spray: Application before buds open in the spring or after trees are dormant in the fall.
Early foliage spray: Application early in the year, but at or soon after full leaf development.
Fall foliage spray: Application in late summer to early fall, generally used with readily translocated herbicides.
Plantation weed control: Using herbicides for herbaceous weed control to ensure survival and rapid growth of planted tree seedlings.
Postemergent: Used after the crop trees or weeds begin to grow (emerged).
Preemergent: Applied before seedlings or weeds begin to grow (emerge) in the spring. This most often refers to applying a herbicide after the trees are planted, but before the weeds begin to grow.
Preplant: Applied before the crop trees are planted.
Reforestation: The process of establishing tree seedlings.
Release: The removal of woody or herbaceous weed competition from developing young stands to improve their growth.
Site preparation: Preparing an area for reforestation by clearing or other vegetation control.
Summer foliage spray: Application to mature foliage later in the season.
Timber stand improvement: Selective removal of undesirable trees to improve growing conditions for the desirable residual trees.
Since the primary pesticides used in forested environments are herbicides, the following sections will deal primarily with those materials. However, the application methodology and the calibration of equipment appropriate to apply insecticides and fungicides will involve the same basic techniques but will require different nozzle types, pressures and rates. If one of these other pesticides will be applied, use the procedures listed below and modify according to directions on the pesticide label.
Foliar and soil-active materials are often broadcast over the entire area to be treated. They can be applied to the foliage or soil by either aerial or ground mechanical equipment. Broadcast applications are common for site preparation. In some areas this method is used for herbaceous weed control and woody release.
The use of control droplet aerial (CDA) spray equipment and orienting the nozzles with the air flow causes large droplets. Boom length should be 75 percent of the total wing or blade span. Nozzles located on booms longer than this can cause excessive drift to occur. The larger the droplet, the less chance of drift to non-target sites. Drift-reducing agents and invert emulsions that change the physical composition of spray mixtures can be used to reduce chemical drift. However, when they are large, droplets may reduce the effectiveness of foliar-absorbed herbicides. Larger droplet sizes display a tendency to bounce off of leaf surfaces. Large droplets tie-up much more spray volume per drop than do smaller droplets. This may cause inadequate coverage, unless greater volumes per acre are applied.
Spray carrier volume should be adjusted to insure effective coverage of target vegetation. Water-based formulations require 5 to 20 gallons per acre (GPA) with the higher carrier volumes necessary when treating multi-story canopies and dense vegetation. Oil emulsions use carrier volumes of 5 to 10 GPA due to costs and deposition efficiency. Aerial applications for midstory and understory hardwood control requires 15 to 20 GPA to insure good coverage beneath closed pine canopies (Minogue 1996).
Solid formulations of soil active materials are also applied aerially. Uniform distribution of solid materials is more difficult than with liquid formulations. Fine particles and dust from the granules can increase the risk of off-site drift. To minimize streaks or skips in the treatment area and off-site movement, apply solid formulations only when wind speeds are less than 5 miles per hour.
Mechanical Ground Application
Crawlers, skidders, 4-wheel drive farm tractors and the sturdier ATV’s (all-terrain-vehicles) can apply herbicides. The selection depends on the job to be done and the site conditions. Ground machine application has definite limits of terrain and stand conditions.
The pesticide application equipment mounted on the machine must be suitable to do the job. Broadcast type sprayers are most commonly used. The application equipment must be able to cover a sizable area efficiently and must be durable. Each component of a properly working sprayer is important for efficient and effective application. The main limit of ground equipment is usually the presence of brush tall enough to mask a major portion of the spray pattern. Spray coverage of plants must be nearly complete, not just on one side, for effective kill.
An alternative to broadcast foliar application would be to broadcast a soil-active herbicide. This may be in a granular form that can be applied before full leaf growth masks the distribution. Several liquid formulations also have soil activity.
Banded applications are made with herbicides labeled for herbaceous weed control. Some are labeled for application over-the-top of newly planted trees. These foliar or soil-active materials are applied in four- to six -foot wide bands. For resistant perennial species, make a late summer herbicide application at higher rates before seedlings are planted or select a different herbicide.
Manually Applied Ground Application
Directed foliar sprays are best used to release 1- and 2-year-old pine stands when brush competition is less than 6 feet tall. Apply the pesticide spray on the target foliage. Direct the spray away from pine foliage and growing tips. The benefits of release can be lost when herbicides are misapplied to needles and shoots, and pines are damaged. Directed foliar sprays are usually applied with a backpack sprayer and a spray wand equipped with a full cone, flat fan, or adjustable cone spray tip.
Full basal sprays require that the lower 12 to 20 inches of target hardwood stems be completely wet on all sides with the spray mixture. Full basal sprays are effective on target stems. A backpack sprayer is used with a wand or spray gun fitted with a narrow-angle flat fan, cone or adjustable tip.
Streamline basal sprays can control many woody plants including hardwoods up to 2 inches in diameter at breast height (dbh). Trees of susceptible species up to 6 inches in diameter can be controlled. However, treatment of small hardwoods less than 2 inches dbh results in the most control.
For stems less than 2 inches dbh, apply the stream of spray up and down single stems for about 6 to 8 inches, or spray across multiple stems creating a 2 to 3-inch-wide band. Direct the spray stream to smooth juvenile bark at a point about 6 to 24 inches from the ground. Stems that are beyond the juvenile stage, thick barked, or near 3 inches in diameter require treatment on both sides, unless they are susceptible species. Back-and-forth bands can also be sprayed on larger stems. Apply in late winter and early spring when leaves do not hinder spraying the stem. The best application time will depend on the herbicide, species and location. Avoid applications in young pine plantations on hot days if an ester herbicide formulation is used because pine injury may occur from vapor drift.
Tree injection can be used alone or in combination with other individual stem treatments for site preparation, pine and hardwood release, timber stand improvement, stand conversion and creating cavity trees for nesting. This physically-demanding method requires workers who can repeatedly and precisely chop into tree trunks deep enough to properly deliver herbicide for uptake in the sap flow. Frequent sharpening and maintenance of injection tools is needed for best results.
Commonly used tree injection methods are: the hack-and-squirt, hypo-hatchets, and tubular tree injectors.
A backpack sprayer can be used that has a wand or spray gun equipped with a straight stream, fan or hollow-cone nozzle. Alternatively, a sawyer can carry herbicide in a utility spray bottle for treating stumps after cutting; or use a wick applicator for small-diameter stumps.
Treat freshly cut stumps as soon as possible after cutting. For stumps over 3 inches in diameter, completely wet the outer edge, or cambial area, with the herbicide. Smaller stumps are usually completely wetted. To be successful, treat all small stumps. The sawyer or companion applicator should treat soon after felling so no stumps are skipped. Treat older, cut stumps with the stream-line mixture. The mixture is applied to the outer 1-inch edge of the stump until runoff and to the base of any sprouts. Stump treatments within four hours of cutting have been shown effective—the sooner the better. Spots of soil-active herbicide are applied to the soil surface in grid patterns or around target stems for site preparation and pine release. This method is effective in controlling stems up to 10 inches dbh. Apply exact amounts of herbicide, specified in milliliters (ml), to the soil surface at prescribed spacings. The effectiveness of the treatment depends on the applicator’s accuracy and consistency in amount applied and spacing.
Spots are applied to the soil by using a spot-gun or a spray-gun equipped with a straight-stream spray tip. The spotgun delivers a set amount while the spray-gun method requires training to judge the amount applied. A spotgun is an adjustable graduated cylinder or syringe operated by squeezing the handle. A forceful squeeze can project spots up to 15 feet. A spray gun uses pressure from the backpack sprayer to project spots to over 20 feet, requiring less exertion. Both can be connected to a backpack sprayer, and the spotgun can also be connected to a side-pack container.
Granules and Pellets
Hand-cranked broadcast spreaders can distribute granular or pelletized herbicides on small tracts and areas with steep slopes or rough terrain. They can be used where machine spreaders are not suitable. Advantages of hand-operated spreaders are that they are small, simple, inexpensive and generally reliable hand tools. Unfortunately, uniform application is often difficult to obtain, and treatment is slow and laborious.
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