Spray drift is one of the most important issues facing pesticide applicators. Movement to off-site locations can be caused by wind, poor calibration/adjustment, and operator error.
1) Select a nozzle that produces coarser droplets
Use droplets that are as coarse as practical to provide necessary coverage.
2) Use the lower end of the pressure range
Higher pressures generate many more small droplets (less than 100 microns). Under most conditions, do not exceed 40 to 45 psi.
3) Maintain a constant travel speed and pressure during spraying
If an automatic regulator is fitted, remember, small increases in speed result in large increases in pressure. The delivered air and spray must be given time to penetrate the canopy.
4) Lower boom height
Wind speed increases with height. If boom height is a few inches lower, off-target drift is reduced.
5) Increase nozzle size
Larger capacity nozzles reduce drift. If you use nozzles that put out 10 to 15 gallons per acre (GPA), increase to nozzles that put out 15 to 20 GPA.
6) Spray when wind speeds are less than 10 m.p.h.
More spray will move off-target as wind increases.
7) Spray when wind is moving away from sensitive areas
Leave a buffer zone if sensitive areas are downwind. Spray the buffer zone when the wind changes direction.
8) Do not spray when the air is very calm
Calm air, or an inversion, reduces air mixing, which means spray can move slowly downwind. Inversions generally occur in early morning or near bodies of water.
9) Use a drift control additive when needed
Drift control additives increase the average droplet size produced by nozzles.
The USDA-NRCS designs windbreaks which can trap drift. Contact your NRCS Service Center, which you can find at: http://www.wa.nrcs.usda.gov
The following information is taken from the National Pesticide Applicator Certification Core Manual:
Spray drift refers to the off-target movement of a pesticide during a liquid application. This is the result of small spray droplets being carried off-site by air movement. Spray drift occurs more frequently than the other two types of drift because almost all spray applications result in some off-target movement. Avoid most problems associated with spray drift by paying close attention to spray droplet size and the wind direction and speed. Larger spray droplets are less likely to drift than smaller ones.
Air movement is the most important environmental factor influencing the drift of pesticides from target areas. The movement of air is influenced by the temperature at ground level and the temperature of the air above it. The best time to spray is when the spray droplets move slowly upwards in the absence of windy or inversion conditions.
Low relative humidity and/or high temperatures also can increase the potential for spray drift. Under these conditions, the evaporation rate of water increases, resulting in smaller spray droplets that drift more easily. Avoid spraying during these times.
Reduce outdoor drift problems by spraying when the wind speed is low, by leaving an untreated border or buffer area in the downwind target area, and by spraying downwind from sensitive areas such as residential properties, schools, crops, waterways, or beehives.
Vapor drift refers to the movement of pesticides as gaseous vapors from the target area. Some pesticides are volatile and can change readily from a solid or liquid form into a gas under the right conditions. This most often occurs with high air temperatures. Pesticides that have volatilized into a vapor or gas may drift farther and for a longer time than they would have as spray droplets. Whenever possible, choose a pesticide formulated as a low-volatility product. Avoid applying volatile pesticides on hot days.
Particle drift refers to the movement of solid particles from the target area by air during or just after an application. These solid particles may include pesticides formulated as dust or soil particles to which pesticides are attached. Some pesticides can remain active on soil particles for long periods after they are applied.
The applicator is ultimately responsible for managing drift. Applicators must assess the vulnerability of neighboring properties and those areas downwind of the application site. Evaluate weather conditions for temperature inversions, wind direction, and wind speed before making the all-important decision about whether to spray. The applicator may have to make adjustments to the application equipment to reduce spray drift. Consider using low-volatile formulations or adding a drift-control additive or thickener to help minimize drift.