The Preparation of Small Spray Quantities of Pesticides

Introduction

Label directions for mixing and applying pesticides come in two general scenarios: rate per volume of water (pesticide concentration) or rate per area of land (lb or qt per acre). Mixing directions for small quantities of pesticide vary with the scenario. 

Pesticides that are mixed by concentration generally have directions for application. Some insecticide application directions may state to apply until spray runs off. Some herbicide application directions may state to apply only enough spray material to wet leaves uniformly. If pesticide mixing directions state an amount of formulated material per volume of spray (such as 1 pound per 100 gallons), you should adjust the amount of pesticide to the volume of water you mix. If your label instructions state a final spray concentration, you do not have to calibrate the sprayer, but YOU MUST READ THE LABEL TO KNOW HOW MUCH SPRAY MATERIAL TO APPLY. 

If the pesticide mixing instructions state an application rate in an amount (typically lb., fl. oz., pt. or qt.) per area (usually acre, but sometimes 1,000 sq. ft.), YOU MUST CALIBRATE YOUR SPRAYER. Calibration doesn’t have to be complicated. You must determine how much spray you apply for a known area. A sprayer may be used one way to spray insecticides, and another for herbicides. Calibration should be done with a sprayer that will be used for a particular application, in the same manner that the pesticide will be applied. Calibration is a three-step process: 

1. Put a known volume (V) of water in the sprayer.  Spray the water out in the same manner the pesticide will be applied, then measure the area (A) that was sprayed. 

2. To calculate the percentage (%) of an acre sprayed by V, divide A (expressed in sq. ft.) by 43,560 (this is the number of sq. ft. in an acre). If V=1 gallon, and the spray covered 1075 sq. ft., then the area treated by 1 gallon is 1075/43560 or 0.0247 acre. 

3. Mix only enough spray to cover the area. If you need to spray 2,000 square feet and it takes 1 gallon to cover 1,075 square feet, dividing 2,000 by 1075 gives you the number of gallons of spray. In this case, 1.86 gallons are needed to cover the 2,000 square foot area. 

To calculate the amount of pesticide required for each gallon of spray, multiply the rate per acre by the percent you determined in step two above. In this problem, the answer was 0.0247. Use the conversion factors in Table 1 to convert weight or volume into a measure that you can measure with your equipment. 

Accuracy is Important

Densities of solid pesticides vary with the formulation and the amount of shaking or settling within the package during shipping and in storage. Use an electronic scale to ensure that the correct weight of the dry product is used. These scales are readily available online and reasonably priced. Many of these scales measure down to 0.1 gram. The use of an electronic scale is essential for the solid form pesticides (e.g., wettable powders, dry flowables, etc.). 

Do not use an ordinary teaspoon for measuring liquids as the common teaspoon varies from 4 to 10 ml. Use a graduated spoon. When measuring out small amounts of liquids, use a syringe marked in cubic centimeters (cc). One cc equals one ml. Graduated spoons and syringes used for a pesticide must not be used for anything other than that pesticide. Always read and follow label directions. 

Table 1a. Conversion factors for solids (Wettable powder, Dry flowable) 

U.S. customary system / imperial	Metric scale
1 lb	453.6 g
1 oz	28.4 g

Table 1b. Conversion factors for liquids

1 gallon	4 quarts
1 quart	2 pints
1 pint	4 cups
1 fl oz	2 tbsp
1 tbsp	3 tsp
1 tsp	10 ml

Reference: CSU Fact Sheet 7.615