Every exterior pesticide application is affected in some way by the weather. The weather conditions that may cause drift are wind, precipitation and temperature inversions. Applications involving small particles of spray, mist or ULV applications are subject to being blown off site to non-target sites by wind or a combination of weather conditions. Most pesticides labeled for exterior applications contain label warnings about application during the rain. Some products have instructions for application during wind or other weather conditions. Most states have regulations that require documented steps to prevent off-target drift and response to drift when it occurs.

As a fundamental practice, all applicators making exterior applications should assess the conditions and the possibility of drift to an area with unwanted consequences. Then they should make a decision to proceed, postpone or proceed with modifications.

Every discussion of drift must include the observation that the higher the wind speed, the more drift it will cause. Similarly, the smaller the particle, the easier it is for drift to occur. Applicators must be trained in the use of both the application equipment and the product being applied.

Measuring Wind Speed.

Applicators cannot make sound decisions regarding wind speed if they cannot measure wind speed. There are a number of weather-related apps for smartphones that can provide current and predicted wind conditions from nearby weather stations. There are some handheld wind gauges (anemometers) that can be very helpful and prices are now reasonable. Of course, there are many methods for estimating wind speed using flags, portable wind socks or other low-tech devices.

General recommendations from various sources suggest that, from 0-10 miles an hour, applications can be made in coordination with common sense decision-making to ensure the product goes where you want it. When wind speed is between 10-15 mph, careful steps may need to be followed to manage drift; some products prohibit application in wind more than 10 mph. If it’s more than 15 mph, applications are not recommended.

Each different type of application may have a number of different methods for reducing the incidence and/or severity of drift, but there are some methods that are common to all applications. Some of the methods for reducing drift include:

  • Increase the size of the spray droplets
  • Decrease the pressure being used
  • Decrease the distance of the spray nozzle to the target
  • Reduce the amount of active ingredient by using the lowest effective concentration
  • Reduce the amount of material applied by using less diluent
  • Consider selecting a different formulation, such as a granular

Even when conditions favor drift, if the area to which product may drift is not an area of concern, a certain amount of drift may be tolerable. Of course, this depends heavily on the material being applied and the nature of the area to which drift may occur. This simply reinforces the value of well-trained applicators who are familiar with both the product and the equipment they are using.

There are a number of common applications that may require drift management. Certainly, ULV treatments such as mosquito adulticiding require special training and are particularly susceptible to drift. Other applications that can be affected are barrier treatments using a mist blower, exterior treatments using a power rig and herbicide applications among others.

Special Situations.

Two situations that warrant special attention are applications to tall building exteriors and applications during the condition known as a temperature inversion. When making applications to the higher floors of tall buildings, it is important to remember that any overspray or tiny droplets will drift much farther in even the lightest wind due to the long height of the fall. When addressing these concerns, take the same precautions as other drift management situations and apply to the special conditions represented by tall buildings and their surroundings.

Temperature inversions can be complicated, but the simple explanation is that two layers of air form a “surface” between them that small droplets cannot penetrate. This usually occurs in still conditions and early mornings when a cool air layer is trapped under a layer of warm air above. Normally the air gets cooler as you go up, but when this gets “inverted,” the resulting layer can prevent pesticide droplets from falling back to the surface. Temperature inversion is a complicated situation, but if it occurs regularly in your area, applicators should be aware of it and be trained to recognize the conditions.

If you think drift might occur, you should contact the owners of the affected area and let them know what you are doing and what, if any, precautions they should take. If they are adamant and insist no application be made, you are far better off letting neighbors come to an agreement than to complete the application and “let the chips fall where they may.”

Most states have regulations that must be followed in such situations. Some of the regulations are more of a burden than others, so applicators should know the regulations that exist where they are working. If a written drift management plan is required in your state, all of your applicators need to be familiar with it and trained to use it. There are many resources through the Cooperative Extension Service in your state and online to help you develop a plan and implement it.

Rose Pest Solutions’ Mark Sheperdigian is a 1982 graduate of Michigan State University with a bachelor’s degree in entomology. He has held a variety of positions in service, sales, management and technical support. He is vice president of technical services for Rose Pest Solutions in Troy, Mich.

Copesan is an alliance of pest management companies with locations throughout North America. To learn more, visit www.copesan.com.