Reducing Pesticide Drift

This information comes from "Reduce Pesticide Drift", PIP-35, Prepared by Robert G. Bellinger, Pesticide Coordinator, P. Mac Horton, & Clyde S. Gorsuch, Extension Entomologists (Rev. January 1996)

Drift is a side effect of pesticide use associated with ground and aerial application and is an important environmental concern. Drift is the uncontrolled airborne movement of spray droplets, vapors, or dust particles, away from the intended point of application. While drift in agricultural and forest situations is more commonly thought of, drift may also occur indoors in air currents caused by ventilation systems and forced-air heating and cooling systems. All drift is illegal and can cause potential injury to non-target plants and animals, and has the potential for producing illegal residues on non-target sites.

Virtually every pesticide application produces some amount of drift. How much drift occurs depends on such factors as the formulation of the material applied, how the material is applied, the volume used, prevailing weather conditions at the time of application, and the size of the application job.

Pesticide applications which are directed upwards or made by aircraft are the most likely to be subject to drift. Pesticide application by aircraft can result in residue problems on sites that are distant from the actual application site. Pesticides released close to the ground or substrate are not as likely to be suspended in the air as those released from a greater height or distance from the target.

Lightweight particles, especially dusts and low volatility vegetable oils, are very easily carried by air currents. Heavier formulations such as granules and pellets settle out of the air very quickly. High pressures and small nozzle openings produce very fine spray droplets with accompanying high drift potential. Lower pressures and larger nozzle openings produce coarser sprays with larger droplet sizes having less drift potential.

Fumigants and nonfumigant pesticide formulations having a high vapor pressure (are volatile) may produce vapors. Vaporization (volatilization) increases as: air and surface temperatures increase; relative humidity decreases; particle or droplet size decreases; and air movement increases.

Drift control is your responsibility! In fact, many labels specify that drift control is the responsibility of the applicator. Drift cannot be completely eliminated, but it can be greatly reduced.

General rules

  • Use as coarse a spray as possible and still obtain good coverage and control. For sprays, use formulations which give large diameter (150 - 200 microns or larger) spray droplets. Droplet size is one of the most important factors affecting drift, however, addressing droplet size alone is not sufficient to reduce the probability of drift and potential damage.
  • Don't apply pesticides under windy or gusty conditions; don't apply at windspeeds over 15 mph, ideally not over 5 mph. Read the label for specific instructions.
  • Maintain adequate buffer zones to insure that drift does not occur off the target area. Read the label.
  • Be careful with all pesticides. Insecticides and fungicides usually require smaller droplet sizes for good coverage and control than herbicides, however, herbicides have a greater potential for non-target crop damage.
  • Choose an application method and a formulation that is less likely to cause drift. After considering the drift potential of a product/formulation/ application method, it may become necessary to use a different product to reduce the chance of drift.
  • Use drift control/drift reduction agents. These materials are basically thickeners and are designed to minimize the formation of droplets smaller than 150 microns. They help produce a more consistent spray pattern and aid in deposition. Some of these are: Chem-Trol, Intac, Lo-Drift, Nalco-Trol, Nalco-Trol II, StaPut, Wind-Fall, Arborchem 38-F.
  • Choose the formulation carefully. Water-based sprays will volatilize more quickly than oil-based sprays. However, oil- based sprays can drift farther because they are lighter, especially above 95° F.
  • Apply pesticides early in the morning or late evening; the air is often more still than during the day.
  • Don't spray during thermal inversions, when air closest to the ground is warmer than the air above it. When possible, avoid spraying at temperatures above 90°-95° F, ideally not over 85° F.
  • Know your surroundings! You must determine the location of sensitive areas near the application site. Some crops are particularly sensitive to herbicides which move off-site. Homes, schools, hospitals and other care facilities, surface waters, water treatment facilities, etc. are considered sensitive areas. Be aware of the location of seasonally flowering crops and the location of honey bee colonies. You should know the location of sensitive areas within a one-half mile radius of sites on which you would make, or have someone else make, pesticide applications, and one mile downwind. Make pesticide application decisions with these locations in mind.
  • Don't apply pesticides near lakes, reservoirs, ponds, rivers, streams, marshes, etc. Add a drift retardant on fields near any water source! Read the label for specific instructions.
  • Use a solid cone or fan spray nozzle. These produce larger droplet sizes than hollow cone nozzles.
  • Be sure you are getting the spray deposition pattern you think you are; service and calibrate your equipment regularly.
  • Check your system for leaks. Small leaks under pressure can produce very fine droplets.
  • Determine wind direction and take this into account in determining application timing, equipment and whether or not to make an application. The wrong wind direction can cancel out everything else you have done to reduce drift.
  • When chemigating, turn off the end gun. Use drop booms instead of upwardly directed nozzles.
Rules for ground applications
  • For applications of liquid and dry formulations, commercially available or homemade shrouds or skirts attached over or behind the application equipment can help prevent spray droplets and pesticide particles from becoming airborne.
  • Using air blast sprayers in orchards is a highly visible application method with significant potential for drift problems. Be sure the machine is properly adjusted to direct the spray into the tree canopy. On most sprayers, one or more of the upper nozzles will not deliver spray to the tree and should be shut off. Use only the nozzles that actually deliver spray to the tree.
  • In orchards or in shade tree plantings, whenever possible, cut off the spray for missing trees in the row. Spray that does not enter the tree canopy is wasted and contributes significantly to drift problems. Reducing drift saves you money by reducing the amount of spray material needed.
  • For ground rigs and hand sprayers, use low pressures and don't spray too close to the target surface to reduce spray-back aerosol.
  • For applications by PCOs, lawn care technicians, homeowners, etc. be sure windows, doors, and foundation vents of nearby buildings are closed. Know where ventilation system intakes are relative to your spray application. Be sure windows of nearby parked vehicles are closed.
  • Be careful of applications near painted surfaces or other non-target surfaces.
  • As an added precaution, remove articles within and adjacent to the application area which may become contaminated, such as: lawn furniture, recreational equipment, pet or livestock feeding items, and removable housing and bedding materials. Remove animals.
Rules for aerial applications
  • Try to get good field-end coverage on initial spray runs; crossing the ends of fields which are bordered by trees or other obstacles usually means flying higher and increasing the chance of drift.
  • Fly slow. Fly low. Slow speeds are combined with lower pump pressures to produce larger droplets. Herbicides should be applied at a lower height than other pesticides.
  • For fixed wing aircraft, don't use a whirl-plate, rather, use a 1/16 to 1/18 inch diameter orifice plate directed straight back.
  • Be sure the positive shut-off is working properly, and use it!
  • Nozzle orientation affects wind shear across the nozzle face, and subsequently droplet size. Use a nozzle orientation that will give the desired droplet size.
  • Boom length should be no more than 75% of the wingspan of fixed wing craft, or of the rotor diameter on helicopters to reduce drift caused by wingtip and rotor vortices.
  • Use Microfoil boom, Tru-Value boom or equivalent drift control system. See the pesticide label.
  • When there is any possibility or concern of drift, use a drift retardant as a standard part of your spraying service. Using drift retardants can promote a positive environmental concern and help eliminate legal problem.
Remember, ALWAYS read and follow label directions.