Reducing Spray Drift

Bulletin 816-00

Droplet Size

Spray droplet size is by far the most important factor affecting drift. Spray droplet diameters are measured in micrometers. A micrometer is 1/25,000 of an inch and is usually referred to as a micron (Fig. 3). For reference, the thickness of a human hair or a sheet of paper is roughly 75 microns.

Fig.3. Spray-droplet diameters are measured in microns.

In general the longer the droplets remain airborne, the greater the chances they are going to be carried by wind away from the application site. Small spray droplets are more susceptible to drift than the larger droplets because they tend to remain airborne much longer than the larger droplets. For example, as shown in Figure 4, while it takes 4 minutes for a 20 micron droplet to travel a vertical distance of 10 feet, it takes only 2 seconds for a 400 micron droplet to travel the same distance.

Fig.4. Comparison of droplet fall rates.

Research shows there is a rapid decrease in the drift potential of droplets greater than about 150 or 200 microns. Droplet size where drift potential becomes insignificant depends on wind speeds, but lies in the range of 150 to 200 microns for wind speeds of 1 to 9 miles per hour (Bode, 1984). Small droplets can drift long distances because of their light weight. For instance, as shown in Fig. 5, the theoretical distances that water droplets would be carried while falling 10 feet in air having a uniform horizontal velocity of 3 miles per hour would be only about 8 feet for 400-micron droplets, but about 1,000 feet for 20-micron droplets.

Fig.5. Smaller droplets drift longer distances.

Spray particles under 50 microns in diameter remain suspended in the air indefinitely or until they evaporate. They should be avoided because there is no way to control deposition of these small droplets. For instance, there is no need to use small droplets when applying soil-incorporated or systemic herbicides. However, small droplets are desirable when applying insecticides and fungicides because they provide better penetration into the canopy and better coverage. Good coverage is essential with insecticides and fungicides because of the small size of the target organism.

Air-blast sprayers (Fig. 6), which are commonly used to apply insecticides and fungicides in orchards, produce a relatively high number of small droplets. However, some of the droplets never reach their target because they are directed upward where there is no canopy. Therefore, spray drift is a more serious problem with air-blast sprayers. An option to decrease drift with these sprayers is to install a baffled extension manifold that directs the spray toward the tree canopy.

Most pesticide applications should attain a balance between drift reduction provided by large droplets and good coverage provided by small droplets. Generally, spray-droplet size should be no finer than necessary to do an effective job. Recommended droplet sizes for application of fungicides, insecticides and herbicides are 150-250, 200-300 and 250-400 microns, respectively.

Fig.6. Air-blast sprayers are more prone to spray drift.