Carrier Volume and Coverage
When the pesticide label does not prescribe a carrier volume or concentration, the sprayer operator must decide the appropriate volume. Use sufficient carrier to disperse or dissolve the product and create enough spray to contact all target surface(s) with minimal runoff. The degree of contact is called coverage, which is a combination of the percent surface area covered and the droplet density on that surface. The operator must consider the following factors when choosing a volume:
- The level of coverage required reflects the product’s mode-of-action. For example, a contact product generally requires a higher droplet density than a locally systemic product (which has limited translocation in plant tissues). A miticide intended to saturate bark is a dilute application that often incurs runoff. Plant growth regulators have very specific coverage requirements and should not be generalized.
- The location and nature of the target. For example, if the target is a mobile insect found predominately on the upper-side of the leaf, it may be controlled with less carrier than a disease found deep in the plant canopy. Further, the orientation and surface texture of the target will affect how spray is retained and how it spreads.
- The impact of environmental conditions, sprayer design and the crop size, density and developmental stage. For example, the more plant canopy to be protected per hectare, the more carrier volume will be required. More volume is required when sprayer air is poorly adjusted, the weather is dry and/or windy and the distance-to-target is long or convoluted (such as tree-tops or deep in unpruned canopies).
To understand the relationship between carrier volume and coverage, the sprayer operator requires a feedback mechanism. Visual inspection of foliar “wetness” or spray residue is subjective and transient, and therefore insufficient. Water-sensitive papers distributed within the target canopy provide a fast, repeatable and quantifiable means for evaluating coverage. Most conventional foliar products require minimal coverage of 10–15% with a droplet density of 85 droplets/cm2.
Smartphone apps such as the GRDC’s SnapCard (https://www.agric.wa.gov. au/grains/snapcard-spray-app) quickly calculate and record spray coverage for future consideration in light of the level of protection achieved. For more information on quantifying coverage, see the Sprayers101 website at www.sprayers101.com and use the keyword “coverage” in the search engine. Download a copy of Airblast 101, A Handbook of Best Practices in Airblast Spraying (https://sprayers101.com/airblast101/)
General Mixing Steps
- Read all product labels — Know the product formulation (which affects mixing method and order). Look for information about the influence of carrier pH, hardness and any requirement for adjuvants. Defer to label instructions should they differ from these mixing steps.
- Shake any liquid products — This ensures the active ingredient and inert ingredients are thoroughly mixed.
- Add carrier to the tank — For water, fill the tank 50% with the required volume. For oil, fill the tank 75%.
- Agitate — Agitation should continue through the mixing process. Excessive agitation may create foaming. If possible, reduce the level of agitation or use a defoamer adjuvant (50% of which should be added during step 3, and the remainder during step 7.)
- Add products in order — The formulation type dictates the order in which tank-mix partners should be added. Refer to Product Order by Formulation below. If using an inductor, flush with water between additions.
- Wait and check — Dry products and water-soluble packets must fully disperse and/or dissolve before adding the next product. Several factors affect the duration, but 3–5 minutes is typical.
- Add remaining carrier.
- Measure pH — This is best done after all products are added to account for their impact on pH and buffering capacity. If required, pH adjusters can be added at the end of mixing to ensure the solution is in the range required by the label.
Product Order by Formulation
Pesticide labels usually provide directions for mixing different materials, which is the sequence for mixing. The order in which you add each product to the tank is critical.
- Dry Formulations — This includes water dispersible granules (WDG or WG), wettable powders (WP) and soluble granules (SG). Allow more time for these products to dissolve and/or disperse completely. Best practice is to premix these products with water in a slurry before adding to the tank.
- Anti-drift adjuvants, compatibility agents or anti-foamers — Consult labels as these products may require multiple additions or a different order than indicated here.
- Liquid Formulations — Liquid pesticide formulations mix in water to form a solution. Some pesticides may be oil-based, such as emulsifiable concentrates (EC), and form an opaque (milky) emulsion that requires moderate agitation and may be prone to foaming.
Water Soluble Packaging
Water-soluble packaging (WSP) is often used for dry formulations. The PVA (polyvinyl alcohol) packaging should dissolve completely when added directly to the tank water (not the basket filter). Protect them from moisture by leaving them in outer packing until just before use and do not handle them with wet gloves. Reseal them to protect remainder.
Do not mix WSP with any product incompatible with the PVA packaging. which includes residues from prior applications of:
- Oils (e.g., Superior Oil)
- EC formulations containing mineral or vegetable oil
- Chelated micronutrients
- Water-soluble fertilizers
Compatibility of Spray Materials
Tank-mixing is adding more than one formulated product in the tank at the same time for efficiency, resistance management and improved performance. However, the odds of incompatibility increase with the number of tank-mix partners
Physical incompatibility can result in the solution thickening, foaming, separating or falling out of suspension, which in turn leads to poor coverage uniformity or plugged / damaged spray equipment. Chemical incompatibility (i.e. antagonism or synergy) can result in reduced pesticide efficacy or cause plant injury when sprayed on the crop.
For information on compatibility, check the product label, product manufacturer or distributor. Do not decide on tank-mixes during loading; instead, do so off-season. Before tank-mixing pest control products, ensure the following:
- each product is registered for use in Canada on the crop.
- each product is used according to the label.
- the tank-mix only includes an adjuvant when specifically required by one of the product labels.
- the application timing of each product is compatible with crop and pest staging.
- no product is specifically excluded on any other of the tank-mix product labels.
Registered product labels can be downloaded through Health Canada’s label search webpage at http://pr-rp.hc-sc.gc.ca/ls-re/index-eng.php. Search for the following keywords:
- Do not mix
- The trade name of any intended tank-mix partner
Jar Test for Pesticide Compatibility
If labels do not include compatibility, or you are considering a new tank-mix, use a Jar Test to test physical incompatibility. Note, this will not reveal a chemical incompatibility. When performing a jar test, do so in a safe and ventilated area, away from sources of ignition, and always wear personal protective equipment (PPE).
- Measure 500 mL of carrier into a 1 L glass jar. Be sure to use the same carrier at the same temperature used in the sprayer.
- Add ingredients according to Table 2–3. Tank-Mix Order for Pesticide Compatibility Test, stirring after each addition.
- Let the solution stand in a ventilated area for 15 minutes and observe the results. If the mixture is giving off heat, these ingredients are not compatible. If gel or scum forms or if solids settle to the bottom (except for the wettable powders) then the mixture is likely incompatible.
- Keep records and retain the jars for the season. They may indicate products prone to settling or separating after prolonged rest (e.g., parking the sprayer overnight). They may also indicate potential problems during re-suspension or cleanout.
If you experience a physical incompatibility issue in the sprayer, do not immediately add water, ammonia, non-ionic surfactants or detergents to the tank. This may create further problems. First, contact the manufacturer or dealer for more information. Then, perform a Reverse Jar Test by sampling the solution and attempting to break down a small volume before doing so in the sprayer. If you succeed in re-suspending the solution, it may no longer be viable and must be safely discarded.
For more information on pesticide handling and operator safety, consult the Ontario Pesticide Education Program (OPEP) Grower Pesticide Safety course (www.opep.ca/resources).
Quantity for 500 mL or 500 g of Product Labeled for1,000 L of Final Spray Volume
|1||Compatibility agents||5 mL (1 teaspoon)|
|2||Water-soluble packets, wettable powders and dry flowables. Include a ~1cm2 cutting of the PVA packaging.||15 g (1 tablespoon)|
|3||Liquid drift retardants||5 mL (1 teaspoon)|
|4||Liquid concentrates, micro-emulsions and suspension concentrates||5 mL (1 teaspoon)|
|5||Emulsifiable concentrates||5 mL (1 teaspoon)|
|6||Water-soluble concentrates or solutions||5 mL (1 teaspoon)|
|7||Remaining adjuvants and surfactants||5 mL (1 teaspoon)|