Adjuvants and the Power of the Spray Droplet

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Why does a pesticide work well one time and not so well the next when everything seems the same? Variable weather could play a role. Or the pest could be less susceptible to the active ingredient. At other times, an inconsistent pesticide performance seems inexplicable.
In many ways, pesticide performance is similar to the way a seed corn hybrid’s yield varies from one year to the next or within a given field in the same year. The genetic pedigree of that hybrid determines its yield potential, but just because a hybrid has built-in potential doesn’t necessarily mean it will produce the highest possible yields.
Performance potential is useful when discussing pesticides, too. Every pesticide molecule in a jug or bag has the built-in potential to control the pests listed on its label. You can maximize performance consistency when you appropriately match and time the right pesticide to the targeted pests, mixed at the labeled rate, and applied with calibrated equipment under favorable environmental conditions.
However, even if you make every attempt to account for these factors, pesticides can still fail to produce expected or consistent outcomes. For example, a pesticide may not perform as planned because the pest developed resistance to it.
But there are other factors that could have affected the pesticide’s performance. An important factor is the water in the spray solution. This will be our focus: conditioning the water that will carry the pesticide and modify the droplet characteristics can often improve spray quality and potency, which increases pesticide performance and consistency.
How Water Affects Pesticide Performance
Water is the most common carrier of pesticide spray applications. A pesticide application to plant foliage relies on pesticide molecules in water droplets to transfer to the intended target pest. The process of transferring an effective pesticide dose includes:

  • Selecting the right product
  • Calculating the rate
  • Measuring the product accurately
  • Uniformly applying the spray mixture with an appropriately set up and calibrated sprayer under favorable environmental conditions

Pesticide applicators often ignore (or underappreciate) water quality. Poor water can lessen the effectiveness of the pesticide in the spray mixture, influence spray quality, and affect how the pesticide interacts with the atmosphere and the leaf surface.
Here are some examples of water quality’s effect on pesticide performance:

  • Certain pesticide molecules can be deactivated when they bind with cations (positively charged molecules) in the water such as calcium (Ca+), magnesium (Mg+), and iron (Fe+). Sodium (Na+), manganese (Mn+), and potassium (K+) are significant in some regions.
  • Spray water that is too acidic or too alkaline may degrade some pesticides or make them less soluble in the mixture.
  • Smaller spray droplets may evaporate after exiting the sprayer nozzle before they even reach the target.
  • The wind can blow smaller spray droplets away from the intended target (this is called spray drift).
  • Spray droplets can bounce, shatter, or roll off the leaf surface (a lack of spray retention).
  • Spray droplets that remain on the leaf surface may form a bead and fail to spread.
  • The deposit formed after the spray evaporates may not have the right chemistry or enough active ingredient present.
  • The active ingredient contained in the spray residue on the leaf surface may fail to penetrate the leaf.

The common denominator among these multiple fail factors is that they are all linked to the molecular, chemical, and physical properties of water in the final spray mixture. Spray deposit retention on the leaf and the chemical composition of the spray droplet will ultimately determine the effectiveness of the pesticide application.
To make more effective applications, it is important to understand what makes up a spray droplet and how they work.

Fred Whitford
Gregory Lindner
Bryan Young
Donald Penner
Jason Deveau
Douglas Linscott
Heping Zhu
Richard Zollinger
Eric Spandl
Bill Johnson
Kiersten Wise
Aaron Patton
Chuck Champion
Nick Harre
Norman Wagoner
Kevin Leigh Smith
Purdue University