Determining Proper Use Rates


As we begin the process of determining proper use rates for turf and ornamental pest control, understanding and documenting the goal of the pest application is a good starting point. In many landscapes, a strong history of a certain pest or disease problems, such as bipolaris leaf spot or prostrate spurge, suggests preventive pest control applications. In these situations, reduced application rates that deliver lower amounts of active ingredient to the pest are often required. Conversely, in scenarios where existing pests are currently causing significant damage, additional formulation product is generally required.

Checking nozzle discharge uniformity is an important step in determining proper use rates.
Photo by Frederick Fishel, University of Florida.

The severity of the problem may also influence the use rate. Using the example of white grub control, if close inspection reveals that only a few small .25-inch larvae are present per square foot in early fall, less insecticide would be necessary to control them than if eight to 10 fully grown .75-inch larvae were detected in late summer. In addition, where the insect is in its life cycle makes a difference. If large numbers of annual white grubs are found in midspring, just before they would normally change to the non-feeding pupal stage, they are of much less concern than if the same number of grubs were found in late summer, with at least three months of feeding remaining prior to the onset of cooler fall conditions.

Application schedule

Another consideration when determining proper use rate involves the reduced rate of pesticides applied sequentially over the growing season in an effort to achieve higher levels of pest control. These are often referred to as “split applications,” and are common with preemergence herbicides used to control annual grasses and broadleaf weeds. For optimum control of these weeds, various research studies have determined that treatments consisting of a half-rate in midspring followed by a half-rate in early summer (or similar sequence) are equally or more effective than a single full-rate application made at the traditional time.

Read the label

Don’t use the mantra of “when all else fails, read the label.” Label reading can be a great rainy day or winter project. Pick out one or two labels of the products that you know the least information about, and sit down and read through them. This often works better if you read out loud to another person (or persons) on your staff or crew. When you read aloud, you get the benefit of three readings. First, the reading in your own mind; second, hearing your own words; and third, the benefit your listener(s) derive from hearing you speak.

The label can tell you a lot of helpful information:

  • whether to add a surfactant to the tank mix;
  • restrictions on use at certain high or low ambient temperatures;
  • plants that do not tolerate the application;
  • suggestions on tank-mixing with other products;
  • the amount of water to add to the tank mix;
  • and much more.

Determine application rates

After you’ve decided how much product needs to be applied—the dosage or use rate—then you need to make sure the equipment being used is accurately delivering the product. Perform a simple calibration test by assessing the performance of your equipment in three ways: power unit ground speed, nozzle discharge and nozzle pressure.

Measure ground speed over a known distance. With a tape measure, mark off and flag a known distance, say 100 feet. Drive the spray rig down the designated stretch of turf with the power unit operating and using normal application procedures. Using a stopwatch, document the time required to travel the measured distance. To increase the accuracy of the test, make three passes and average the time.

Nozzle discharge tests can be an eye-opening experience. To determine the output of sprayer nozzles, place a stationary bottle under each nozzle and begin spraying. Collect spray output at various time intervals such as 15 seconds, 45 seconds and 1 minute. After each interval, compare the output from adjacent nozzles. It’s common for volume disparities to exist; if they are greater than 5 percent, replace those nozzles and retest.

To measure pressure, attach a pressure gauge to nozzle caps and observe the reading for system pressure at the nozzle. This procedure measures differences between the observed pressure at the normal pressure gauge location and actual nozzle pressure. Again, repairs may need to be made.

Cost of misapplication

The costs of misapplication are very real in terms of pest control and actual application cost. Pest control failures due to the inaccurate delivery of active ingredients increase the likelihood of callbacks and the need for follow-up treatments, which can lead to both immediate and long-term customer dissatisfaction. In addition, actual application costs may be affected, especially in cases where spray volumes exceeding label or intended rates were delivered.

John C. Fech is a horticulturist, certified arborist and frequent contributor located in Omaha, Neb. Frederick P. Baxendale is a professor and extension entomologist with the University of Nebraska-Lincoln.