Weather predictions for the next several days suggest excellent conditions for pesticide spraying. So make use of this opportunity!

High pressure is building and this means, statewide, relatively high temperatures, 90 to near 100 degrees F toward the end of the week. Winds are expected to be moderate, in the 5 to 10 mph range. So, not too windy, but not too stable. (Thus inversions will likely not be an application hazard).

The main concern will be heat. Some pesticides, growth regulator herbicides in particular, may be more prone to vapor drift when applied at higher temperatures. Additionally, the risk of crop injury from a pesticide may increase with higher temperatures. So, check the label to avoid unpleasant surprises. If high temperature is a problem, you will likely see precaution statements like: (cont. on next page)

  • “Do not apply XXXXX adjacent to sensitive crops when the temperature on the day of application is expected to exceed 85 degrees F as drift is more likely to occur.”
  • “Do not apply XXXXX when the temperature on the day of application is expected to exceed 85 degrees F as significant crop injury may result.”

On the flip side, this heat spell might provide near perfect conditions to for certain pesticides. Interestingly, glufosinate, a contact herbicide used on Liberty Link Soybeans, may actual work best at higher temperatures with bright sunny days. So consult the label for advantages as well as precautions.

For more information on high temperature issues and herbicide use, consult NDSU Extension Weed Specialist Rich Zollinger’s summary.

Another concern with applications in high temperatures is evaporation of spray droplets. This is a serious problem with small (fine) droplets below 200 microns. The table from NDSU Extension Circular AE-1210, Reducing Spray Drift, illustrates the potential for small droplets to evaporate at 90 degrees F and 36% relative humidity.

Evaporation is a two-fold problem. Obviously, drops that do not hit the target because they evaporate results in less pesticide available to control the pest. But of more importance, is increased drift potential.

The small droplets illustrated in the table drift easily in even light winds because they fall very slowly. Thus, applicators should use label directed water volumes, pressures, and nozzles to create larger droplets. The appropriate droplet size is usually stated on the label and will often range from Medium (226-325 microns) < Coarse < Very Coarse < Extremely Coarse < Ultra Coarse (650 microns or more).

Bottom line:

  • Look for and act upon high temperature statements found on the label.
  • When making an application, create a droplet spectrum that minimizes fine drops which are sensitive to evaporation and drift, especially those that are 200 microns or less.
  • If the label directs you to use a MINIMUM of XX gallons of water by ground or XX gallons by air, don’t short the water. If you do, you will likely be creating a significant number of small droplets which are prone to evaporate and drift.