Drift is the uncontrolled movement of a pesticide away from its target area. Drift can damage susceptible off-target sites (plants, animals, etc.), reduces pest control (which wastes the pesticide and money), and contaminates the environment (water pollution and illegal pesticide residues). This can result in lawsuits, administrative action, or criminal fines. Be able to recognize situations that increase potential drift problems and know how to deal with them.
The main types of drift are particle drift and vapor drift.
Particle drift occurs when spray droplets physically move away from the target site during application. The main causes of particle drift are:
- Environmental factors: wind speed above 10 mph, low humidity, or high temperature;
- Improper spray practices
- Equipment problems.
Droplet size is one of the most important factors that affects particle drift. Ideal spray nozzle droplet size depends upon the type of pesticide being applied. Contact pesticides usually require smaller droplet size to be most effective. Larger droplet size may be more effective for systemic pesticides.
Spray nozzles produce droplets of many sizes. The average droplet size from a specific nozzle is its volume mean diameter (VMD). The unit is a micron (1/25,400 inch). If a nozzle output has a VMD of 300 (droplets of very fine rain) microns, 50% of the spray volume is in droplets that are less than 300 microns and 50% are above 300 microns.
To obtain a uniform spray pattern and to minimize drift, keep the operating pressure within the recommended range for each nozzle tip. To maintain a proper spray pattern, adjust nozzles according to the manufacturer’s recommendations on nozzle spacing and spray angle.
High sprayer pressure or nozzles that produce fine droplets cover the target uniformly and completely. However, fine droplets can drift long distances.
Lower pressure and coarse nozzles produce large droplets. The drift potential is lower but coverage is not as good.
Changes in operating sprayer pressure cannot be used safely to make major changes in sprayer output. Doubling the spray pressure (for example, from 20 psi to 40 psi) does not double the flow rate. You must increase the pressure four times (20 psi to 80 psi) to double the flow rate.
Exceeding the recommended pressure range often results in more drift potential. This increase in pressure would produce more fine spray particles, increasing the potential for spray drift.
Temperature and humidity affect evaporation of spray droplets. High temperature and low humidity results in faster evaporation, droplets get smaller. The smaller droplets are moved farther by winds.
Vapor drift occurs when a pesticide evaporates and moves in the air as a gas. This is most likely to occur when the temperature is above 85° F. Vapor drift can happen for several days after application. The best way to prevent vapor drift is to read, understand, and follow pesticide label directions. Avoid using volatile herbicides near or upwind of sensitive crops.
Studies have shown that a sizable percentage of pesticides may never reach the intended target site because of drift. Significant drift can damage or contaminate sensitive crops, poison bees, pose health risks to humans and animals, and/or contaminate nearby soil and water. It is impossible to eliminate drift, but it is possible to reduce it to a tolerable level.
Follow drift reduction instructions on the product label. Read the label and reference guide to learn proper nozzle and pressure combinations.
Drift Reduction Technique
|Follow drift reduction instructions on the product label||Read the label and reference guide to learn proper nozzle and pressure combinations.|
|Select a nozzle to increase droplet size||Use the largest droplets that provide necessary coverage.|
|Look at new technologies||Air-induction and venturi nozzles may help reduce drift.|
|Lower sprayer height if possible||The higher the nozzle above the target, the greater the potential for drift.|
|Use appropriate travel speed||High travel speeds may result in a bouncing boom.|
|Be aware of wind speed and direction, as well as nearby sensitive areas||Spray when the wind speed is between 3 and 10 miles per hour. More spray volume moves off-target as wind increases or direction changes. Wind currents can drastically affect spray droplet deposition. Spray downwind from sensitive areas, such as houses, schools, sensitive crops, waterways, or beehives.|
|Use buffer zones/no-spray zones near sensitive areas||Leave a buffer/no-spray zone if sensitive areas are downwind.|
|Do not spray during a temperature inversion||Temperature inversions prevent the dissipation of spray particles.|
|Use a drift-control additive when needed||Drift-control additives increase the average droplet size produced by the nozzles but they do not make up for poor spraying practices.|
By Lee Townsend, Extension Entomologist