Those of us who can remember cropdusters and the pesticide fog trucks driving through neighborhoods have reason to be concerned about pesticide drift.

It’s still an issue. On a recent windy day, I drove by a subdivision neighbor who was using a personal sprayer to kill weeds in the cracks in her driveway. She was directing a fine spray from hip height, and it was apparent that most of the herbicide was blowing down the road in a big cloud.

Drift is, of course, one of the big issues in the industrial farming debate, where some folks assume that vast quantities of agricultural pesticides must surely be blowing through our communities.

Syngenta field on Kauai, bird watcher

At the seed company Syngenta’s field on Kauai, a worker keeps an eye out for birds.

Nick Grube/Civil Beat

To learn more about drift, I toured a West Kauai seed company and talked with two of their workers, attended a class on proper pesticide use and conducted some independent research.

It turns out my neighbor was doing just about everything wrong. Spraying during high winds. Using too fine a spray droplet size. Spraying from too high above the target. Using too wide a spray pattern for the need. And not mechanically controlling the spray.

Modern drift management is a mature science, and safety measures are well established. There’s an impressive array of equipment, chemical formulations and best management practices to prevent unwanted pesticide movement. Those clouds of pesticides may be coming from home users who don’t pay attention to safety protocols, but the professionals have lots of tools to prevent chemical drift.

Farmers have no interest in letting their pesticides drift. Pesticides are expensive — they are one of the big costs of farming, whether you’re using organic or non-organic compounds. (Yes, organic farmers use pesticides, and the big seed companies use organic pest control products as well as conventional ones.)

Farmers clearly get the political climate as well—nobody wants drift—not their bosses, not their customers, not their neighbors, not the larger community and not the regulators.

So, how do you control drift? There is a LOT of readily accessible literature in this area, much of it from the nation’s universities, including the University of Hawaii.

Some of the key messages regarding drift control are these.

Don’t spray on windy days. A standard is that if it’s blowing more than 10 miles an hour, the spray rigs stay in the barn. Pesticide labels establish permissible wind speeds, can require buffer zones, set air temperatures allowable, identify additives that may be required and so forth. These label requirements are law and are enforced in Hawaii by the State Department of Agriculture. One of the reasons farmers sometimes spray at night is that wind speeds may be lower then or weather conditions in general may be more favorable at night.

Control droplet size. Tinier droplets are more likely to get caught on the breeze and travel. So spray rigs are outfitted with nozzles that set droplet size to reduce drift potential. There are nozzles used by Hawaii seed companies that surround the finer spray with a cone of bigger droplets to prevent their drifting.

Droplet size can be controlled through the pressure applied. You might get a finer spray at high pressure of 20 pounds per square inch, but a satisfactory droplet size at lower pressure of 15.

Droplet size can also be controlled by how fast the spray rig is moving, and whether the spray nozzle is facing with the direction of travel, or straight down, or backwards. Going slow and aiming backwards results in bigger droplet size.

Control spray height. The higher the spray nozzle, the less control you have in where the product goes. Thus the industry’s fertilizer and pesticide applicators keep the spray nozzles as low as they can be to best accomplish the task.

Sticking, bouncing, shattering. Droplet size and velocity can also impact the effectiveness of the spray. “Droplets that strike the target’s surface will do one or more of three things: shatter, bounce back, or stick to the surface,” says a University of Hawaii pesticide application study paper.

You don’t want them to shatter, and don’t want them to bounce back. But it’s a fine dance: “Generally, small droplets make drift riskier but they have the potential for more thoroughly covering the target’s surface. On the other hand, large droplets may not cover the target’s surface so thoroughly but they do lessen the risk from drift.”

Control the characteristics of the liquid spray. Spray professionals may add products, called adjuvants, to change the characteristics of the spray. A key task of an adjuvant might be to make the product stick better to its target crop, perhaps by reducing the surface tension and increasing the product’s “wettability”. But other adjuvants might make them less likely to foam up, make the formulation thicker, or increase its ability to get into the plant.

Mechanically control drift. Farming companies in the Islands use a range of hood designs to control drift. The hood fits over the spray nozzle, ensuring that no (or very little) spray can escape. There are cone-shaped hoods, box-shaped hoods and others, designed for the crop and conditions.

Pick the right product. Modern agricultural chemicals are formulated to reduce both drift and volatilization. Volatilization is the term for another part of the drift discussion: when instead of drifting particles of spray, the chemical converts directly from a solid or liquid into a gas or vapor.

It is all complicated stuff, and while it makes you worry about the neighbor who hasn’t ready any instructions or taken any training, after my discussions, I was left with a lot of confidence about the professional farming community and how it handles this complex issue.

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About the Author

  • Jan TenBruggencate
    Jan TenBruggencate was the science and environment writer and Kauai Bureau Chief for the Honolulu Advertiser. He left to start a communications consulting firm, Island Strategy LLC. His science writing has generated awards from the Hawaii Audubon Society, Hawaiian Academy of Science, The Nature Conservancy, the Conservation Council for Hawaii and others.