Pesticide drift: just how controllable is it?

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

Drift is one of the key arguments associated with efforts to prevent pesticide exposure to Island residents. 

But it turns out 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.

(Image: A Hawai`i farm’s spray hoods, yellow boxy units which are affixed over spray nozzles during use to prevent drift.)

On a recent windy day, I drove by a subdivision neighbor who was using a spray rig 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.

To learn how it’s done in agricultural industry, I toured a West Kauai seed company, attended a class on proper pesticide use, and conducted some online research.

Among the things I learned was that my neighbor was doing just about everything wrong that day. Spraying during high winds. Using too fine a spray droplet size. Spraying from too high. Using too wide a spray pattern for the need. And not mechanically controlling the spray.

For the farmer, the drift discussion starts with this point: The farming industry has 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. (Yeah, the big seed companies use a fair amount of organic pest control products.)

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

So, how do you control drift? There is a LOT of literature in this area. Here and here and here are a few resources.

Some of the key messages regarding drift control are these, all of which are employed by modern farms. 

Don’t spray on windy days. That’s an absolute rule. One of the reasons farmers sometimes spray at night is that wind speeds may be lower then. A standard for a lot of products 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 on spray days, identify additives that may be required, and so forth.

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 Hawai`i seed companies that surround the finer spray with a cone of bigger droplets to prevent their drifting.

Droplet size is also 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, herbicide 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 Hawai`i pesticide application study paper.

You don’t want them to shatter into smaller and more driftable droplets, 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, including their tendency to “stick” to the target plant.

“An adjuvant is any substance added to a spray tank to modify a pesticide’s performance, the physical properties of the spray mixture, or both,” says this University of Hawai`i publication.

Sometimes an adjuvant will be added to a mixture before spraying, to accomplish one or more of several tasks. 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 others might make them less likely to foam up, make the formulation thicker, or increase its ability to get into the plant.

From an effectiveness standpoint, sticking means it’s getting where it’s meant to be. But that’s also important from a drift perspective. If it’s sticking, it’s clearly not drifting.

Some pesticide labels require an adjuvant be added.

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. The one shown with this article is boxy, but I also saw cone-shaped hoods that follow the pattern of spray developed by the particular nozzle being used.

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 into a gas.

It is all complicated stuff, and while it makes you worry about the neighbor who hasn’t read any instructions or taken any training, it gives a lot more confidence about the professional farming community and its approaches.

And if you’re the neighbor planning to begin spraying stuff, here’s a resource to help do it more safely—protecting both yourself and your neighborhood. 

© Jan TenBruggencate 2015