Guest post by Lauren Diepenbrock.

As we prepare for the upcoming field season, I decided it would be a good idea to review notes and methods from last year. Since most of my research is done collaboratively with conventional berry growers, determining the amount of insecticide that needs to be applied in my field plots is important to ensure that I put out the correct amount of insecticide to control pests and stay within label regulations. While talking with one of our grower-collaborators, he mentioned that some newer growers might benefit from this information as well, so the goal of this blog is to help all of us prepare for the upcoming season. The steps below can be used when calibrating pull-behind airblast or air cannon sprayers, such as those we use in our field trials. Information on calibrating other spray equipment can be found at the Pesticide Environmental Stewardship site.

How much insecticide do I need?

The amount of insecticide used depends on how much acreage that needs to be treated and the label rate. For this example, I am going to use Malathion 8F. I figured out how much chemical I will need per acre by reading the Specimen Label and specific registrations for my crop and state, available via www.cdms.net. I am preparing to work in blackberries in North Carolina this season, so I scrolled down until I found the appropriate information for using Malathion 8F for the control of spotted wing drosophila in caneberries. According to the registration, you can use up to 2 pints per acre in caneberries, so then the amount of chemical needed is determined by how many acres will be treated (2 pints x _____acres).

How do I figure out how much water to mix insecticide into? (AKA- “how much do I put in the tank?”)

This will depend on how many gallons per acre that the sprayer puts out. To determine this, you need to know (1) how many gallons per minute are being sprayed and (2) the speed of your tractor.

(1) To determine gallons per minute:

-Measure out a known volume of water (a bucket with a clearly labelled gallon marker will work well for this). The volume used will depend upon the size of your sprayer. You should use a water volume large enough to view clearly on your sprayer tank.

-Get the tractor ready and make sure that all of the settings (gear, rpm, etc.) are as they would be when you normally spray your field. With the tractor NOT MOVING, turn on the agitator and spray out all of the water, timing how long it takes from when spraying begins until the nozzles start sputtering and the tank pressure begins to drop.

-Do this 3 times and take an average, which will give you a good estimate of the time it takes to spray out this known volume. From this, you can determine the gallons per minute.

Example: With one tractor and sprayer rig that we use, spraying out in 2nd gear at 2500 rpm, it took an average of 206 seconds to spray out 10 gallons of water (206 seconds = approximately 3.5 minutes). So 10 gallons, sprayed out over 3.5 minutes (10/3.5) = 2.8 gallons per minute.

(2) To determine the speed of your tractor + sprayer:

-Mark off a known distance.

– Set the gear and rpm that you would normally use when applying treatments. This should be the same as those used to determine gallons per minute.

– Time how long it takes to drive the marked distance.

Example: For one of the tractors that we use, I drive in 2nd gear at 2500 rpm. To find out what this translates to in a field with the sprayer attached, we marked off 74 linear feet with 12 foot row spacing and timed how long it took to drive this distance, repeating this two additional times so that we could use the average. For this particular tractor, in 2nd gear at 2500 rpm, it took me 23 seconds to drive 74 linear feet (don’t forget the 12 foot spacing! 74 ft x 12 ft = 888 ft²), which means that I was driving at 4.3 ft²/s or about 3 mph.

Using the amount of water sprayed out and the mph, we can determine the gallons per acre.

Using our example, multiply speed by the time it takes to spray out 10 gallons:

(4.3 ft² / 1 sec) x (206 s / 10 gal) = 888 ft² / 10 gal

To make things easier, convert feet/gallon into the number of acres that you can treat with a single gallon of water:

(888 ft² / 10 gal) x (1 acre / 43,560 ft²) = 0.002 acre / 1 gal

Finally, take the inverse of this number to determine how many gallons of water are needed to treat one acre:

1 gal / 0.0002 acre = 62.5 gal / acre

Now you know the rate at which the chemical should be applied per acre and how much water needed for each acre that you want to treat. Now what?

Putting it all together

Let’s assume that I have a 4 acre field to treat. To cover 4 acres, I will need at least 250 gallons of water in the tank. It is important to prepare a little extra material, since the gallons per acre is an average and may fluctuate as during application.

We generally use a fudge factor of 1.5, so multiply the 250 gallon minimum by 1.5 and I will need to add 375 gallons of water to the tank. Assuming that I need to apply Malathion at the maximum rate of 2 pints/acre, I will need at least 8 pints (2 pints x 4 acres). Again, I am going to mix a little more just to make sure I don’t run out so multiply by 1.5 and I will now need 12 pints.

What if my sprayer tank isn’t big enough to fit it all into one tank? The easiest way to address this problem is to divide out the water and insecticide into two equal parts and mix the tank two separate times. It might take 2 or more tanks to get the whole field treated!