Most agricultural pesticide labels list use rates on a per acre basis, but this can pose an issue for many producers of specialty crops. Often only a small fraction of an acre needs to be treated and it is important to apply these pesticides at the correct dose. Over-application is illegal as it may result in excessive residues on the harvested produce and injury to plants; it would also be a waste of money. Under application can result in ineffective control, foster the development of resistance, and also be a waste of money. So, determining the correct amount for the proper dosage is required. While there are many different methods to do this, this article outlines the simple process I use to determine amounts of products needed for small acreages.
1. Select the rate per acre
While this sounds simple and straight forward, select the rate listed on the label for the crop to be treated; many labels list a range of rates allowing the user to select a rate within the range. Generally, the low rate is used for small plants and light insect or disease pressure. As the plants get larger or insect and disease pressure increases, higher rates within the range should be selected. However, do not apply more than what is listed on the pesticide labelling.
2. Determine the area to be treated
The simplest method is to measure the width and length of the area to be treated and multiply those two numbers together. For example, if the area to be treated is 24 feet by 150 feet, then the area would be 3,600 square feet (24 x 150 = 3,600). There are 43,560 square feet in an acre, so in this example, the acreage to be treated would be 3,600 divided by 43,560, or 0.0826 acre (3,600 / 43,560 = 0.0826).
While this method works well for rectangular fields, it can be difficult to determine the area of some irregular-shaped fields. On-line mapping programs, like Google Maps, can calculate the area of irregularly shaped objects. Bring up the field with the mapping program and right click on the edge of the area to be treated and select ‘measure distance.’ Add more points around the perimeter by right clicking on those points until the object is encircled. The area of the object will be provided in square feet. Convert square feet to acres by dividing the number by 43,560 square feet.
3. Determine the amount of product needed for the area to be treated
Take the rate per acre you selected in #1 and multiply that by the acreage you calculated in #2. For example, you want to apply 5.2 fluid ounces per acre and the area to be treated is 0.0826 acres, then the amount of product needed would be 0.43 fluid ounces (5.2 x 0.0826 = 0.43). You would need to put the 0.43 fluid ounces in the correct amount of water and apply it evenly to the area to be treated. But it can be difficult to measure 0.43 fluid ounces, so I would convert this to milliliters (ml) so I can use a plastic syringe for accuracy. There are 29.6 ml in a fluid ounce, so in our example, you would multiply 0.43 by 29.6 to get 12.7 ml. This is the amount of pesticide needed to treat the field and this can be measured easily and accurately with a syringe.
Another way to measure small amounts of fluids is to use teaspoons or tablespoons. A teaspoon is approximately 5 ml and a tablespoon is 15 ml. So, 12.7 ml would be about 2 1/2 teaspoons. But keep in mind that these spoons must to be dedicated to only measure pesticides and not used for other purposes.
4. Determine the amount of water needed
The method I use is to completely fill the sprayer I plan to use with plain water, and then spray a known area. Then, I measure the amount of water needed to refill the sprayer. For example, I measure out an area 3 feet wide by 100 feet in length (300 square feet). I fill my sprayer and apply water to that area in the same way I plan to apply the pesticide (same speed, pressure, nozzles, etc.). Suppose it took a quart of water to refill the sprayer. To determine the water per acre, take the water needed and multiply by 43,560, and then divide by the area we treated. So, in this example, 1 quart x 43,560 / 300. The result is 145.2 quarts per acre which equals 36.3 gallons per acre. Multiply this by the area to be treated from #2 to determine the amount of water you need to mix with the pesticide. Here we would need 0.0826 acres x 36.3 gallons. The total amount of water needed would be 3 gallons of water (0.0826 x 36.3 = 3.0).
So to apply the pesticide in this example, put 12.7 ml in 3 gallons of water to treat an area 24 by 150 feet in size.
The same process can be used with dry pesticides, but the conversion from ounces to grams would be 1 ounce equals 28.35 grams.
By Ric Bessin, Entomology Extension Specialist