When choosing nozzles/droplet sizes for spray applications, applicators must consider both coverage needed and drift potential. As a rule, smaller droplets provide better coverage, but larger droplets are less likely to drift.

Nozzle type and application pressure govern droplet size (see Nozzles: Selection and Sizing, Virginia Cooperative Extension publication 442-032), which in turn affects system output (application rate), target deposition, uniformity, efficacy, and the risk of drift. Off-target movement in the form of spray drift is a major concern because it diverts the chemical from the intended target, reduces efficacy, and deposits pesticide where it is not needed or wanted. When a pesticide drifts, it may cause both environmental and economic damage, including exposure to people and property, injury to susceptible vegetation, harm to wildlife, deposition of illegal residues on crops, and contamination of water supplies.

In some cases, increased droplet size may reduce efficacy. Although fine or very fine droplets (if applied uniformly) can, theoretically, provide the best cover­age, small, lightweight droplets may not penetrate plant canopies. Droplets produced by air-induction nozzles may break vs. bounce when they hit the target, providing better coverage than droplet size rating alone would indicate. In many cases, choosing a nozzle/droplet size for a job is a tradeoff between good coverage and drift potential.

So … what’s an applicator to do?! Many labels pro­vide recommendations and/or requirements regarding droplet size, nozzle selection, and sprayer configuration. Research in application technology supports recommendations in nozzle-selection guides and directions on product labels. However, in the absence of specific guidance, Charts 1 and 2 may serve as a starting point.

Droplets, Coverage, and Drift:

Very fine droplets (VMD less than 145 microns) are collected efficiently by insects or needles on coniferous plants, but they tend to remain in the air stream, which carries them around the stems and leaves of weeds.

Fine and medium-size droplets (VMD between 145 and 325 microns) will deposit efficiently on stems and narrow vertical leaves such as grasses if applied when there is some air movement.

Coarse and very coarse droplets (VMD more than 325 microns) will deposit most efficiently on large, flat surfaces such as the leaves of broad-leaved weeds.

Insecticides and fungicides generally require smaller droplets (Chart 1) than herbicides to obtain adequate coverage of the target. For foliar herbicides, however, experimental results suggest that applications using droplet sizes in the range of 100 microns to 400 microns do not significantly differ in weed control unless application volumes are extremely high or very low. (Exceptions may exist for specific herbicides.)

Summary:

1. When choosing a nozzle, consider both flow rate and droplet size.

2. Base decision on target and properties of active ingredient.

3. Avoid using nozzles and pressures that will produce a volume median diameter (VMD) of less than 200 microns (fine–very fine).

4. Always read the label. Pesticide product labels may specify what droplet size to use and how much liquid to apply to a given area. This will direct nozzle selection and, in turn, affect spraying equipment configuration and calibration.

References and Additional Information:

Strategies to Reduce Spray Drift, Kansas State University Agricultural Experiment Station and Cooperative Extension Service publication MF-2444; www.oznet.ksu.edu/library/ageng2/mf2444.pdf. (accessed: October 23, 2008)

Fine Tuning a Sprayer with “Ounce” Calibration Method, VCE publication 442-453; http://pubs.ext.vt.edu/442-453/.

Protect Virginia’s Sensitive Areas: Control Your Drops-and-Control Your Drift! Virginia Tech Pesticide Programs and Virginia Department of Agriculture and Consumer Services, Office of Pesticide Services; www.vtpp.ext.vt.edu/index.php/html/main/CYD_1005.html. (accessed: October 23, 2008)

Nozzles: Selection and Sizing, VCE publication 442-032; http://pubs.ext.vt.edu/442-032/

Spray Nozzle Classification by Droplet Spectra, ASAE Standard S572 FEB04; American Society of Agricultural and Biological Engineers, St. Joseph, MI; http://asae.frymulti.com/standards.asp (accessed: January 12, 2009)

Acknowledgments:

The authors would like to express their appreciation for the review and comments made by Mike Parrish, Extension agent, agriculture and natural resources, VCE Dinwiddie County Office; Mike Weaver, Extension specialist, Virginia Tech Pesticide Programs, Virginia Tech; and Paul Sumner, Extension specialist, University of Georgia.

Chart 1. Droplet range for application/pest control¹.

Chart 2. Droplet size classification chart

Other resources in:

• Agricultural Insects / Pests
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• Pesticide Education

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• Patricia A. Hipkins
• Robert Grisso

Other resources from:

• Entomology
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