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Drifting in the Wind (06/18/20)

Each year applicators are placed in a near impossible task of meeting the expectations of the pesticide label and being able to control pests in a timely way.

Each year applicators are placed in a near impossible task of meeting the expectations of the pesticide label and being able to control pests in a timely way. 2020 has been another extraordinary year from a planting perspective, but in recent weeks wind conditions have been far from ideal.

I have had several calls from anxious applicators over the past week asking, what if I spray when winds are gusting well above 15 mph? It depends on a whole range of factors. But ultimately it comes down to droplet size and the length of time it takes to reach the target. Fine spray drops fall very slowly as compared to coarse drops. Fine drops have very little mass and so they cannot overcome the friction of the air as well as coarse ones. That means the fine drops are subjected to the wind for a relatively long period of time. The following illustration compares different droplets sizes for lateral movement at various wind speeds:

wdsci.4

The chart was derived from EPA’s National Aerial Applicator’s Manual published in 2009. Generally, 150 micron or less drops are considered highly driftable. They fall slowly and therefore move much farther down range in the wind. In theory, a 20-micron drop will move 1100 feet before it hits the target. But in most situations, they never make it that far, especially on a warm day because they simply evaporate. What follows is the evaporation rate for various-sized spray drops on a relatively hot and dry day.

 

wdsci.5

In sum, making an application in high wind conditions will result in significant down range movement of fine spray drops. Applications higher than 24 inches above the target will exacerbate fine droplet movement. Sprayers operating at pressures well above manuafacturer recommendations will result in substantial generation of fine spray drops. Flat Fan nozzles produce a high percentage of fine spray drops and should be avoided in high wind conditions. Even nozzles that generate a high percentage of coarse or greater droplets will still produce a sufficient number of fine drops that can move down range. North Dakota Administrative Rule makes it clear:

 

“Applications must not occur when the atmospheric conditions favor the off-target drift of

pesticides or prevent the proper deposition of pesticides to the target area.”

 

If you apply in high wind conditions, you are likely risking violation of the law and angering a neighbor. Plus, it is a public relation black eye. We are living in an environment that tolerance for off-target movement is moving towards zero. As a pesticide specialist, and as a veteran of applying pesticides commercially, I fully realize the pressures you are all under. I want you to know that I wish you the best on the very important and difficult work you all do.

 

Andrew A. Thostenson

Pesticide Program Specialist

This site is supported in part by the Crop Protection and Pest Management Program [grant no. 2017-70006-27144/accession 1013592] from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed are those of the website author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

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