The Bugwood Network

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.

Vegetation Control

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.

Weed Trees
Weed trees are undesirable hardwoods and conifers. They include deformed and defective or undersized individuals of both commercial and non-commercial species. Large weed or “wolf” trees can occupy significant growing space within a stand. Weed trees reduce the economic value of otherwise healthy, desirable trees. They affect both small and commercial size trees within a stand.

Brush includes shrubs, small trees and woody perennials. These prevent light from reaching tree seedlings and deprive even taller commercial species of water and nutrients. It interferes with natural regeneration or planting and can create a habitat for rabbits and rodents that may damage newly planted stands. Over time, a build-up of brush in the understory can pose a fire hazard to the tree stand.

Vines include greenbriar, Japanese honeysuckle, wild grapes, kudzu and other plants with climbing or creeping stems. All of these grow well on good forest sites. They drag down tree branches and crowns, and compete with desirable trees for light and nutrients. Vines have vigorous sprouting habits and are some of the most difficult weeds to control.

Kudzu is a serious weed pest in tree plantations and natural stands and is a threat to mature as well as developing stands and all regeneration. Repeated herbicide applications are essential for control. This vine spreads so rapidly it can take over the site again in 2-3 years if a single root crown is left alive (Figure 58). Follow-up treatments must be made for one or more years after initial treatment. Kudzu’s ability to resprout following treatment varies with the stand age, root size and plant vigor. Old stands may resprout for several years.

Figure 58. Kudzu vine, Kerry
Britton, USDA Forest Service.

Herbaceous Weeds
Herbaceous weeds retard seedling growth in new plantations and natural stands. Tree seedlings competing with herbaceous weeds may develop poorly or die, especially in time of drought. Herbaceous weeds also create favorable cover for tree damaging animals such as mice, gophers and cotton rats. They pose the potential for loss of a new plantation by wildfire. Control herbaceous weeds with herbicides labeled for this forest use. In forest nurseries, seed orchards and Christmas tree plantings, herbaceous weed control is critical. These high-value forest crops must be free of weeds to allow for proper growth and development.


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
Herbicides may be broadly classed as contact herbicides and translocated (systemic) herbicides.

Contact Herbicides
Contact herbicides kill only the plant foliage to which they are applied. These herbicides are often non-selective, affecting most plant species whether woody or herbaceous. Their use is often referred to as “chemical mowing.” Because roots and even larger woody parts are not killed, resprouting may occur, and control is often short-lived. Due to poor control, the currently labeled contact herbicides are rarely used in forestry. However, treatments to kill and dry vegetation to increase fuel loading for site preparation burning can be used.

Translocated (systemic) Herbicides
Translocated (systemic) herbicides are those that must enter and move within the plant to be effective. They move to sites where they disrupt certain physiological functions. This enables them to severely stunt or kill the plant. Most herbicides used in forestry, whether applied to foliage, soil, bark or cut surface, are of this type. Some translocated herbicides work in more than one way. Some of these may also act as contact herbicides at higher concentrations because of the petroleum additives in the formulations.

Plant activity
Depending on the chemical molecule used in the product, herbicides affect plants in different ways. These different modes of activity are not always apparent on the outside of the plant, but they have a major influence on the success (or failure) of a particular chemical and the ability to mix different products for greater efficacy. The following eight modes of action describe the different ways in which forest herbicides can affect (and control) plants.

Examples of chemicals used in forestry work are given for each category.

  1. Cell Membrane Disrupter - Oxyfluorfen, Paraquat
  2. Respiration Inhibitor - MSMA
  3. Photosynthesis Inhibitor - Hexazinone, Simazine, Atrazine
  4. Growth Inhibitor - Pendimethalin
  5. Lipid Biosynthesis Inhibitor - Fluazifopbutyl, Sethoxydim
  6. Growth Regulator - Dicamba; 2,4-D and 2,4-DP; Picloram; Triclopyr
  7. Amino Acid Synthesis Inhibitor - Glyphosate, Imazapyr, Metsulfuron methyl, Sulfometuron methyl
  8. Pigment Inhibitor - No forestry chemicals

Factors Affecting Control
Plants vary in their susceptibility to different herbicides. They absorb various compounds differently and have different abilities to detoxify the herbicide. Herbicides start breaking down at varying rates soon after application. This breakdown is caused by microorganisms, sunlight and chemical reactions. Herbicides eventually lose all effectiveness.

Pesticide Application

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:

  • know the pest to be controlled;
  • be familiar with pesticides available for use;
  • determine if a Certified Applicator is required;
  • know the size of the area needing treatment;
  • have accessibility to the area;
  • identify the presence of sensitive areas (e.g. wetlands, streams, houses, etc.) and organisms (such as livestock, wildlife and any threatened and endangered species);
  • determine the appropriate application method;
  • properly set up and calibrate the equipment to apply materials. (see Appendix A for more information);
  • apply pesticides only under appropriate environmental conditions; and
  • always read and follow label instructions.

Environmental Concerns

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:

  1. Do not apply a pesticide in windy or rainy conditions when the chances of drift and wash off/runoff are high.
  2. Choose an application method and a pesticide formulation that will minimize the potential for movement of the material to off-site locations.
  3. Restrict or minimize the use of volatile pesticides on areas in or around sensitive on-target plants or animals, especially during hot weather.
  4. Generally, liquid pesticides applied by broadcast methods are more subject to drift than are granular formulations and their application methods.
  5. During liquid application, spray droplet size should be maintained within the recommended range for the proposed target and the application method to be used. In general, large spray droplet sizes (> 300 microns) reduce the potential for pesticide drift. Large droplets do not evaporate as quickly as smaller droplets, so more material will potentially be available to hit the target site, especially during hot, dry weather. However, target spray coverage is usually improved as droplet size decreases (up to a point) since there are many more small droplets than large droplets per given volume of spray material. Another drawback with large droplets is that they may bounce off of and not adhere to leaf surfaces, resulting in poor coverage and increased off-site contamination.
  6. Use additives to minimize drift and enhance efficacy as appropriate.
  7. Materials applied to the soil surface can be moved off-site through runoff.
  8. Individual stem application of pesticides can reduce the possibility of non-target impacts of the pesticide.

Application Terminology

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.

Application Methods

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.

Many forestry herbicides enter the plant through the green foliage and young stems. Plants that are shielded from foliar sprays by taller or adjacent plants will not be controlled as well as those fully exposed. Adjuvants are added to the spray mixture to aid in the coverage effectiveness or safety of these herbicides. However, some formulations already include adjuvants. Always follow label directions. Adjuvants may be particularly useful for late-season use as foliage becomes waxy and difficult to penetrate.

Soil-active herbicides may be applied to the soil as liquid or granular formulations. Control will not occur until there is adequate rainfall or sufficient soil moisture. After rainfall dissolves and moves the herbicide into the soil, it is taken up by the roots of established plants. Pre-emergence herbicides applied to the soil kill vegetation as seeds germinate or new plants grow through the treated ground. Season of the year, soil moisture, texture and pH, as well as organic matter and rainfall, greatly affect soil-active materials.

Aerial Application
Aerial application is commonly used to apply pesticides in forestry. This is because tract size is often large, access is difficult; and the vegetation is often tall and dense. Large acreage can be treated more economically and in less time by air. Untreated buffers are established around the perimeter of the treatment area. Firebreaks, flagging, or Global Positioning Systems (GPS) are used to mark treatment boundaries and flight lines. Both fixed- wing aircraft and helicopters are used for forestry applications on insecticides and biological control agents. Forest herbicides are labeled for helicopter application and not by fixed-wing aircraft. Since most states require a separate training and testing for aerial certification, only a brief discussion will follow here.

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
Ground application equipment can be more versatile than aircraft. They can treat small or large areas, do banded or broadcast application, and are not so limited by weather.

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.

For boom type sprayers, flat fan-type nozzles should be used to apply broadcast herbicides. Flat fan nozzles produce an elliptical pattern, where the edges are light and the center is heavy. These should be spaced on the boom for 30 - 40 percent overlap. When it becomes necessary to apply herbicides in bands, use an even fan or flood nozzle. These nozzles produce a uniform pattern across the area sprayed. The fan nozzles should be operated at pressures of 20 - 40 pounds per square inch (psi). Flood nozzles are designed to operate at lower pressures 5 - 15 psi. The capacity of both type nozzles should be 15 - 20 gallons per acre (GPA) when operated at 2.2 - 4 miles per hour.

Typical boom sprayer.

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
Manual applications are usually applied using backpack sprayers, mist blowers, hand-cranked broadcast spreaders, spotguns or one of various injection devices. The commonly applied manual treatments used in forestry are:

  • Directed foliar sprays
  • Basal sprays and stump treatments
  • Tree injections
  • Soil spots and granule/pellet applications

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.

The hack-and-squirt is an effective and economical means of selectively controlling undesirable hardwood stems. A lightweight hatchet is used to cut into the tree stem through the cambium and a herbicide is sprayed on the cut from a trigger squeeze bottle.

The hypo-hatchet is a hatchet with an internal herbicide delivery system connected by a hose to an external herbicide container. When the hatchet strikes a tree, the blade must penetrate into the sapwood. The impact of the striking action drives a piston forward that delivers 1 ml of herbicide into the cut. The rate cannot be adjusted. Daily cleaning and lubrication of the impact piston is required maintenance, along with periodic replacement of rubber O-rings and seals. Always wear safety glasses when using the hypo-hatchet because of frequent herbicide splashes. CAUTION: All hoses and fittings should be checked daily for leaks and appropriate repairs made to prevent applicator exposure.

Tubular Tree Injectors
Tubular tree injectors have a long metal tube fitted with a chisel-type blade that is used to cut through the tree bark into the sapwood near the base of the tree. The unit is equipped with a lever, handle or wire, which is pulled to deliver the herbicide (usually 1 ml) from the cylinder into the cut. The delivery rate can be adjusted for accurate calibration.

Waist-high injections by the hypo-hatchet and hack-and-squirt methods are just as effective and as fast to perform as basal injections. With larger stems, apply more herbicide by basal injections because of the larger groundline diameter compared to diameter at breast height.

Tree Injector Equiptment

Treating Stumps
Treating stumps with herbicide can prevent resprouting of many species. This can be an effective, lowcost treatment following harvest for site preparation and after partial cuts for timber stand improvement. Hand clearing treatments using saws or axes for pine release can be enhanced by treating the stumps with herbicide to prevent regrowth.

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
Granules and pellets can be applied by hand-cranked spreaders, air-blown backpack spreaders, and hand-broadcast.

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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