For most modern crop production systems, spraying pesticides is an essential tool in managing insects and diseases. Even with advanced integrated pest management practices and bio-controls, sprays are sometimes necessary to prevent epidemics. Whether using organic, bio-derived pesticides or conventional, chemical sprays; spray applications rarely achieve 100% control. However, increasing spray efficacy from 90% up to 95% can have a huge effect on prevent uncontrollable outbreaks. Depending on the product, sprays can be expensive and maximizing value can be important. These are a few basic rules to keep in mind that will ensure the best possible pest control, while minimizing over spray and pesticide drift.

1. Regularly Clean and Check Spray Equipment

The most common failures of sprays fall into this category. Spray nozzles may get clogged, or just worn down over time and often need cleaning or replacing. It is important to check every time a spray application is made. New spray tips will produce uniform distribution when properly set up. These sprays are so precisely manufactured that even small amounts of dirt or residue can affect the output of the spray nozzle. The tips may also just get worn, or rounded, over time and these worn spray tips typically produce a higher output of spray material, which results in uneven spraying, higher spray concentration in some rows, and errors in mixing and rate calculations. Spray tips may also be physically damaged which often results in erratic sprays, sometimes over or under-applying. See picture below:

From: TeeJet Technical Information

Using filters and adequate strainers can help prevent clogging at the spray tips, but if clogging does occur- tips can be cleaned with light brushing, or just replaced altogether. It is a good practice to run clear water through the entire system, from the tank through the spray tips. Some materials (such as surfactants) may not completely wash out with just water and will need special cleanings agents to fully clean equipment. Pesticide labels will usually include special instructions, if needed, for cleaning equipment and these should be carefully followed when present.

2. Test (and Adjust) Your Speed

Speed of your tractor ( or any other spray implementation) can have a large effect on spray coverage and efficacy. Typically, faster spray speeds will necessitate larger spray volumes (gallons per acre) in order to compensate for decrease in spray coverage associated with increased speeds. In established orchards, slower speeds may be needed in order to reach upper canopy of the trees that could be missed. Surveyors tape attached to a pvc pipe, extended to the upper portion of the orchard canopy, can be used to monitor air movement. This simple tool can be used to monitor whether tractor speed and spray fan are adequate in achieving full coverage.

3. Know Your Nozzle / Droplet Size

Selecting the correct nozzle is based on desire droplet size. Droplet size can play a large role in efficacy of a particular pesticide product, and play a huge role in determining drift. Nozzles used in agriculture fall into the following categories: fine, medium, coarse, or very coarse droplets. Different pesticides and herbicides will have different optimal droplet sizes, depending on modes of action (systemic or contact) and coverage needed for maximum efficacy.

The droplet size is determined by nozzle type and pressure. The example guide below is from TeeJeet nozzles but all nozzle manufacturers will provide this information.

How nozzle and pressure can interact to affect droplet size

Checking Coverage with Spray Cards and SnapCard App

The easiest way to check overall spray coverage is by using water sensitive spray cards which can be placed at several points through the crop.  Cards should be arranged throughout the crop, where ever the pesticides need to reach. For example:

 

phtoto: Ontario Ministry of Agriculture

After making the spray at desired speed and height, the cards are then retrieved and analyzed. The SnapCard spray app can help analyze these spray cards and determine if adjustments to tractor speed, spray pressure, etc are needed. These factors work together to determine spray coverage, and the only way to ensure they are all working together is to experiment, test and make adjustments.

4. Use Proper Pesticide Rates (Highest Label Rate is not always best!)

How much pesticide product to measure and put into the tank is determined by consulting the product label for specific crop and target pest. Sometimes the pesticide rate will be given in a range. For large pest outbreaks, usually the high end of the suggested range is the best course of action, but if plants are stressed, or there is additional risk of phytotoxic effects, the high end of the suggested range may not be the optimum rate. Unless resistance has been developed, even the low end of suggested range should be sufficient to kill target pests.

5. Optimize pH of Spray Solution

 Many pesticides/ fungicides/ herbicides perform best in an acidic solution, but ground water quality is often alkaline or neutral, depending on location of water source and time of season. Acidifying spray solutions is one step that can usually improve spray performance and is often skipped by farmers. There are several buffer solutions which are available and can be used to adjust pH of spray solution. Always consult the product label before adjusting pH, optimum pH can vary. This can be especially important with organic/ microbial-based pesticides which can drastically look efficacy if proper pH balance is not maintained.

pH scale

pH can affect the pesticide spray efficacy in multiple ways. Uptake into plant tissue is important for systemic products and  improper water pH or quality can negatively affect plant uptake on the leaf surface. Degradation is another important way that pH can negatively impact efficacy of plant protection products. Listed degradation rates for pesticides are at their optimum pH, but these rates can be increased greatly when using water with improper pH, that has not been buffered. In most cases, insecticides are more sensitive to high pH and alkaline hydrolysis, compared to  fungicides and herbicides, but there are some exceptions. There are also some pesticides that are more sensitive to low pH levels and acid hydrolysis. Always consult the label for the optimum pH.

If water pH needs to be adjusted, buffering agents, such as Buffercide, Buffer-X, Unifilm B, and LI 700 Acidiphactant, can be used to achieve desired pH level in a spray solution, and keep it there for duration of spray. There are some products that have color indicators built in to easily observe pH level of solution. Citric acid is also an inexpensive, and commonly available product which can be mixed into solution to acidify and lower pH.

6. Use Proper Adjuvants

Depending on the situation, using an adjuvant may dramatically increase spray performance, or may not be beneficial at all. Spreader, wetters, and surfactants all reduce surface tension and allow spray droplets to flatten on leaf surface. This can allow for greater leaf coverage, however, it can allow the product to be more easily washed off during rain or overhead irrigation. Stickers help with this problem and can help the product adhere to the leaf surface during rain or wind events, however, in some cases it can bind the active ingredient and decrease efficacy. Spreader-stickers combine both, but in many cases operate more like spreaders.

Silicone based spreader sticker effect on leaf surface wetting
Silicone based spreader sticker effect on leaf surface wetting

Emulsifiable oil activators may be used to enhance rate of penetration into the leaf surface ( past the waxy cuticle layer) for some systemic pesticides, or herbicides. Drift retardant products can increase the droplet size and decrease the risk for pesticide drift. The are also some other types of adjuvants that may be needed, depending on the products used, to prevent foaming in the tank. As described above, buffering agents may also be needed to achieve proper pH balance, and sometimes hard water may require conditioning or water softening agents to be added to the mix.

7. Be Careful When Combining Multiple Products

As a general rule, it best not to combine more than three different products in one tank mix. The more products you put in, the more likely that there is a problem with compatibility. This could result in decreased efficacy of one or more of the products, or physical problem with one of the products gelling or falling out of suspension. This should be considered especially when using biological or organic pesticides, due to the increased sensitivity of living organisms. Generally biologically based products should not be mixed. With chemical products, the label should clearly list all possible compatibility issues, but occasionally problems do arise that are not listed or foreseen. For this reason, a small tank or jar mix may be wise to test combinations before mixing large tanks.

The order of combining products can also be important. BASF has developed a mnemonic device ( WAMLEGS) which can be helpful in remembering the proper order of mixing. With the tank half full and agitation on, the following order should be followed:

The Mixtank App from Precision Laboratories can help make this process easier. The user simply inputs the products they’d like to mix, and the app will determine any compatibility issues as well as recommend the optimal order of adding the products to the tank mix.

8. Re-Apply Contact Pesticides to New Growth

This tip does not apply to systemic pesticides, but it’s an important consideration for most pesticides when applying to a growing crop. Even with long lasting chemicals, plants can outgrow spray coverage, leaving new growth susceptible to pests. During periods of rapid growth, more frequent applications may be needed, especially with preventative fungicides.

9. Adjust to the Weather

Spray drift can be a large problem for multiple reasons, such as under-applying target rates as well as potential health and regulatory issues. Beyond spray pressure and tractor speed considerations, weather is the most important factor to consider when controlling for drift. It is important to look at the local weather forecast when planning when to apply. In very windy locations, sometimes sprays might need to be made very early in the morning, or at night. High wind speeds are obvious to avoid, but even lower wind speeds can create drift and need to be calibrated to. Temperature has a large effect on drift as well. Typically higher temps create more drift as well. Boom height, pressure and speed will need to be adjusted based on wind speed and temperature.

Beyond drift, there are factors to consider with the weather. Temperature and humidity can play a large role in potential phytotoxicity issues. Sulfur based products and oils should avoided with very high temperatures (at the time of spray as well as forecasted). Some products may volatilize in very high temperature conditions and lose efficacy. For some products (such as copper based products) low temperatures can cause issues with phytotoxicity as well.

Rainfall immediately following pesticide sprays should be avoided when possible, however different products will have different “rainfastness.” This information is sometimes available on the product label, but not always. Dusts and wettable powders are usually more susceptible to wash off than emulsion formulas for pesticides. Adjuvants such as surfactants or oils can be helpful when there is a possibility of rainfall occurring within a day or two after applying. If some wash off does occur ( or possibly occurs) after rainfall, the product used as well as the target pest’s biology and behavior need to be considered before deciding to reapply.

10. Take Advantage of New Spraying Technologies

Beyond all these listed tips, the next step to greatly increase pesticide spray coverage and efficacy is to implement new technology to the spray rig. New nozzle designs have greatly increased droplet size and uniformity and decreased potential for drift. New air induction nozzles produce droplets with air inside that have much lower potential for drift, and these droplets disperse into smaller droplets when they hit the plant.

Electrostatic sprayers are an exciting advancement in spray technology and can greatly improve spray coverage, as well as decrease the risk of drift. Electrostatic sprayers can produce droplets 900 times smaller than those produced by conventional or hydraulic sprayers. Droplets are given a small charge and these atomized droplets are attracted the leaf surface.

With all these tips, it is always important to read the product label, calibrate and make adjustments as needed. Every individual crop will also have its own considerations.

Sources:

TeeJet technical support information

The Almond Doctor: Good Spray Coverage

Sprayers 101