Pesticide Drift Management

According to the Environmental Protection Agency (EPA), 500 million pounds of pesticides are applied annually in the United States. It is estimated that 3% of this total will drift. Drift is defined as the off-target movement of pesticides through the air at the time of application. Pesticide drift can cause several problems from pesticide exposure to nearby workers, residents and animals, to damage to susceptible crops. Drift will also reduce the amount of chemical that is available to control the intended pest and may render an application inadequate. Since the chemical and the application cost money, drift not only is environmentally wrong, but it is also a costly and inefficient way of doing business.

The Spray Drift Task Force (SDTF) was created by chemical companies to study and evaluate pesticide drift according to different methods of application in response to EPA’s growing drift concern. Several field studies were conducted, and drift amounts were quantified. According to SDTF, drift cannot be totally eliminated during pesticide application because of technology constraints, but there are ways to minimize drift to levels approaching zero.

Drift can occur in all forms of pesticide application: using agricultural airplanes, ground sprayers, airblast sprayers or irrigation systems. In general, drift can be influenced by factors in one of these four categories:

  • Spray characteristics
  • Weather
  • Application equipment and setup
  • Operator skill

Spray characteristics

Droplet Size

In this category the overwhelming factor influencing drift is droplet size. As a general rule of thumb, small droplets do not have enough mass to drop fast, so they remain airborne and exposed to air movement longer than larger droplets. Droplets are measured in microns, or micrometers (µm). One micron is equal to 39.37/1 million of an inch. The speed in which a droplet falls when released is a direct function of its size (Stokes’ law). The larger the droplet, the faster it will reach its target. The distance that a droplet will travel downwind is a function of the height of release and wind speed and is inversely proportional to its terminal velocity. In other words, larger droplets will fall faster and be less exposed to wind. The higher a droplet is released and the stronger the wind, the greater the chance that a droplet will travel downwind and drift. Droplets that are smaller than 150 µm are considered drift-prone.

Droplet size becomes an issue when choosing a particular setup that will effectively reduce drift potential but still maximize coverage and penetration. In general terms, coverage and penetration are optimized by small droplets, a direct opposition to the use of large droplets. The relationship between a droplet size and its volume is cubic; therefore, when comparing two droplets, for example a 250 µm and a 500 µm droplet, the latter carries 8 times more volume than the former. So using large droplets may be prejudicial to some contact insecticides and fungicides that depend on coverage and penetration to be effective. Systemic herbicides can be effectively managed using large droplets since coverage and penetration are not so critical.

Spray Height

As mentioned before, the height of release of spray will influence how much droplets will travel downwind. This may not be as critical for ground sprayers, but it is very important for aerial application. Agricultural pilots should try to maintain an optimum distance from the crop, generally 8-12 feet during the application. Any additional distance will give droplets an opportunity to be influenced by wind and be deposited off target.

Spray Pressure

When using hydraulic nozzles, higher pressures will produce more fine droplets and increase the drift potential of an application. Pressure has been demonstrated to not be an effective way to increase coverage and canopy penetration. Flat fan nozzles can operate from 30-50 PSI and low-drift nozzles can operate at 20 PSI.

Drift Retardants

Drift retardants are usually added to the spray mixture in order to increase the viscosity of the spray solution. Increasing the viscosity of the spray will reduce the number of small droplets (the ones smaller than 150 µm). There are several different drift retardants in the market with various degrees of efficacy. Although drift retardants are a tool to be used to decrease drift potential, their contribution is limited. You should manage nozzle type, height and operating pressure correctly to minimize drift potential. Do not rely solely on drift retardants.


Wind Speed

Wind speed is the most important factor influencing drift. High wind speeds will move droplets downwind and deposit them off target. Generally, pesticide application should be avoided if wind speed is greater than 10 mph. The most effective way to check wind speed is to use a wind meter. There are several models on the market with price ranging from $50 to $300. To be accurate, wind meters should be used in places with no obstructions, such as buildings or large trees that may mask wind speed. There are also portable weather stations that can be mounted outside the sprayer and constantly monitor weather conditions, sounding an alarm if wind speed surpasses a chosen threshold.

Wind Direction

Wind direction will influence where off-target spray droplets will be deposited. A careful operator will try to apply pesticides whenever the wind is blowing away from sensitive areas. The use of a spray buffer downwind to protect sensitive crops is an effective way to minimize drift.

Air Temperature and Relative Humidity

High air temperature and low relative humidity go hand-in-hand in creating a worst-case scenario for pesticide drift. Under these conditions, spray droplets can evaporate very fast and become more susceptible to wind forces. Air temperature can also influence atmospheric stability and off-target movement of spray droplets.


Evaporation will reduce the size of the droplets released in the spray. For typical applications with ground applicators, droplets of 50 microns and less will completely evaporate to a residual core of pesticide before reaching the target. Droplets greater than 200 microns will have no significant reduction in size before deposition on the target. Evaporation of droplets between 50 and 200 µm is significantly affected by temperature, humidity and other weather conditions. Some pesticide formulations are more volatile than others. For example, 2,4 D or MCPA esters are susceptible to vapor drift, while 2,4-D or MCPA amines are practically non-volatile.

Thermal Inversions

Thermal inversions occur naturally. Usually warmer air tends to rise in the atmosphere and cooler air tends to sink closer to the ground. This upward and downward movement generates turbulence and aids in mixing particles in suspension. During stable conditions, a layer of warm air can stay overhead and not promote mixing with colder air that stays below, closer to the ground. If small droplets are released into the atmosphere in this situation, there will be great chances of off-target movement.

Inversions are part of a daily atmospheric cycle, occurring in the early morning hours when the ground cools the air layer immediately above it. Inversions tend to dissipate during the middle of the day when wind currents mix the air layers. It is very important that applicators recognize thermal inversions and do not spray under those conditions.

Application equipment and setup

Low-drift nozzles

Almost all major agricultural nozzle manufacturers have recently introduced their version of low-drift nozzles. These nozzles are designed to create larger droplets at the same flow rate and operating pressure than comparable standard flat-fan nozzles. This has been accomplished by adding a pre-orifice to the nozzle tip assembly just ahead of the conventional discharge orifice, creating a venturi effect. The pre-orifice reduces pressure at the exit orifice, creating larger droplets to reduce drift significantly.

Shielded Sprayer Boom

Partially covering a sprayer boom with a shield has been shown helpful in reducing spray drift. Researchers have conducted wind tunnel tests to determine the effect of having a shield near a nozzle on the path droplets follow after they are released from the nozzle. Results from laboratory tests conducted at the University of Missouri indicated that a mechanical shield could reduce spray drift deposit by up to 70 percent.

Air-Assisted Spraying

One of the most effective ways to prevent drift of small droplets is using high-velocity air flow to aid the process of transporting droplets from the nozzle to the target. Several studies have shown that air assistance reduces drift deposits from boom sprayers. One common air-assist system uses a stream of air to direct the spray to the target by entraining droplets in a downward-moving air curtain. With these types of air-assisted sprayers the air assistance system is an add-on to the existing conventional boom sprayer. Droplets are generated by conventional nozzles using hydraulic pressure. Nozzles direct the spray droplets into the air jets. These air jets offer some protection against drift and improve the penetration of spray into target canopies. Factors that must be considered when operating air-assisted sprayers include the wind velocity and direction, travel speed and direction with respect to the wind direction, and ground cover (plant canopy) being sprayed. For example, some studies indicate that more drift can be created by using this type of air-assisted system if high air flow and air velocity are used when spraying ground with no vegetative cover.

Operator skill and care

Under a given spray situation, any one of the previously mentioned factors may be the most critical in reducing drift hazards. Ultimately, it is the applicator’s job to determine the critical factor and to take precautions against it. By exercising good judgment regarding both equipment and weather factors relative to each application, applicators can minimize drift potential in nearly every case.

Drift Mitigation Strategies

Conscientious and experienced operators rarely get into serious trouble with drift damage because they understand drift and take steps to avoid it. Here are some management strategies to reduce spray drift for different spray application situations.

Ground Boom Sprayers

Use nozzles that produce large droplets whenever possible, if biological effectiveness can be maintained. 

  1. Keep the boom close to the spray target.
  2. Use greater spray volume and use nozzles with larger orifices.
  3. Use lower system pressure and check pressure gauge accuracy.
  4. Use drift retardants if droplet size cannot be controlled with nozzle selection.
  5. Follow label recommendations to avoid drift with highly volatile pesticides.
  6. Avoid spraying on extremely hot, dry days, especially if sensitive vegetation is nearby.
  7. Do not spray when conditions are favorable for an atmospheric inversion.
  8. Although the distance droplets will drift is a function of many other factors such as droplet size, RH, air temperature and boom height, it is best not to spray when wind speeds are greater than 10 miles per hour.
  9. Avoid spraying near sensitive areas located downwind. Leave a buffer strip 50 to 100 feet wide. Spray this strip later when the wind shifts.
  10. Be sure to keep good records (wind speed and direction, temperature, RH). Evaluate spray results.
  11. New label requirements may use a drift model to determine the required buffer width. Travel slower (and lower boom height) and use nozzles that produce large droplets near sensitive areas. This will reduce the required buffer zone width. Then, as distance from the sensitive area increases, nozzles can be changed to produce smaller droplets, if desired, and boom height raised so travel speed can be increased.
  12. Select the time when drift is likely to be low for spraying the parts of the field near sensitive areas.
  13. In the future, label requirements may specify nozzles that produce a certain droplet size spectrum, such as fine, medium, coarse, very coarse, etc. Carefully check the label to determine the optimum droplet size and proper nozzle size for a spray application.

Orchard and Vineyard Sprayers

Many of the same principles for reducing drift for boom sprayers apply to orchard (air blast) spraying as well. With air blast sprayers, we have to remember that the droplet release height is where the air jet no longer controls the droplet trajectory and the wind begins to direct droplet paths. In some cases, this may be several feet above the tree canopy. Some of the practices for reducing drift are:

  1. Turn off the sprayer when there are no trees present.
  2. Keep the spray as close to the target as possible.
  3. Minimize the small droplet fraction; however, remember that the purpose of spraying is to control pests. Large droplets may not provide the control desired without increasing the application rate.
  4. Match the sprayer air jet (volume and direction) and nozzle system to the trees being sprayed.
  5. Be prepared to use special techniques near sensitive areas, especially for the last few downwind rows. These may include:
    1. use large droplets;
    2. use techniques to direct spray at targets better, such as a handgun;
    3. spray the last few rows upwind only;
    4. wait and spray when the wind shifts;
    5. use special measures or equipment to create barriers or direct spray at the target (tunnel sprayer, etc.).

Aerial application

Experiments have shown that spraying with fixed-wing airplanes or helicopters may have a higher potential to produce drift when compared with other methods of spraying. This is caused by the high travel speed, the wing-tip vortices that tend to trap droplets and the release height. There are only a few options available for reducing drift when spraying with aircraft. The singlemost effective strategy to reduce drift from aerial spraying is having aircraft operators who are carefully trained to make good decisions on when to spray and when to stop spraying. Here are some other recommendations:

  1. Spray height should be 8-12 feet above canopy cover.
  2. Boom length should be less than 70% of wingspan.
  3. Use a spray offset carefully; calculate how far the wind will carry the main spray swath and plan your spray paths to allow for this spray adjustment.
  4. Use global positioning instruments in aircraft to assist you in making sure you are spraying the correct field and in selecting the proper spray paths.
  5. Choose the largest droplet size that will produce the desired biological effect.
  6. Point nozzles backwards (toward the tail of the aircraft) to reduce formation of the small droplets.
  7. Participate in calibration clinics offered throughout the country.


Drift is undesirable for economic, environmental and safety reasons. Efficient applicators don't spend money for pesticides to watch them drift away from their target fields. Today's chemicals are more potent and require more precise application. Unsatisfactory pest control could result if a significant portion of the chemical is lost in drift. This could require re-spraying the same field. You may even find yourself in court if spray drift damages sensitive crops in a neighbor's field.

The environmental effects of spray drift are equally costly and unacceptable. By reducing drift to a minimum, you can reduce the potential for pollution of streams, lakes and other water supplies that could endanger fish and wildlife.

Regardless of how accurately an application is made, the possibility of drift is always present. You can minimize this possibility by selecting the right equipment and using sound judgment when applying pesticides. Your judgment can mean the difference between an efficient, economical application or one that results in drift, damaging non-target crops and creating environmental pollution.

Reducing spray drift not only improves application efficiency but also reduces the risk of safety- and health-related problems caused by drift. Because it is impossible to eliminate drift altogether, always wear protective clothing when applying pesticides. A respirator is a must, especially if your tractor does not have a cab.

If you have any doubts about a spraying job that might result in drift, wait until you no longer have that element of doubt. Your goal should be to eliminate off-target movement of pesticides, no matter how small it may be.

8/28/2009 12:19:01 AM
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