Citrus Industry Magazine CEU 2013 Articles Test Series: Article #4
All about adjuvants
By Stephen H. Futch
Posted Nov. 1, 2013
(expires Oct. 31, 2014)
Adjuvants are products used with pesticides to improve the handling properties of the mixture and/or enhance the pesticide's performance. Used alone, adjuvants do not provide pesticidal activity. Adjuvants may be added to the pesticide when formulated or by the mixer/loader to the spray solution prior to application. Adjuvants may be classified as surfactants, compatibility agents, anti-foaming agents, colorants (dyes), drift control agents and more.
Applicators may use the terms adjuvant and surfactant interchangeably. However, while all surfactants are technically adjuvants, not all adjuvants are surfactants.
The selection of the correct adjuvant is important. The performance of a pesticide can vary depending on the adjuvant selected. Pesticide labels will state if a surfactant is recommended and a suggested rate to be added to the spray mix to maximize pesticide performance.
The term surfactant was originally developed to refer to surface-active agents. Surfactants are products that facilitate and/or improve the emulsifying, dispersing, spreading, wetting or other surface modifying properties of liquids. They are used to produce physical changes in the solution to enhance desired properties of the spray mixture and thus increase the spray coverage on the foliage or aid in the penetration of the solution into the plant.
Surfactants are either water-soluble (hydrophilic) or oil-soluble (lipophilic). Hydrophilic solutions have a strong affinity (natural liking or attraction) for water, whereas lipophilic surfactants have a strong affinity for lipids or oils. The most commonly used surfactants are soaps.
Surfactants are separated into four major groups: anionic, cationic, non-ionic and amphoteric. Anionic and cationic surfactants have electrical charges whereas non-ionic do not have an electrical charge. Amphoteric surfactants may have either a positive or negative charge based upon the pH of the spray solution. The most commonly recommended surfactant is a non-ionic type; however the type selected should vary depending on the pesticide or pesticides chosen.
An emulsion is a mixture of two liquids that are incompletely mixed. Emulsifying agents promote the suspension of one liquid into the other. The two common types of emulsifying agents are oil-in-water or water-in-oil mixtures. The oil-in-water emulsion agents are used to get an oil-soluble pesticide dispersed in a water mixture, thereby allowing it to be sprayed in a water-based solution. When water and oils are mixed, they will form an invert emulsion that creates a thick solution that will resist drift, improve rainfastness or delivery of the product. In some cases, an invert emulsion may be too viscous (thick) to be applied with conventional sprayers. Mayonnaise and butter are two examples of invert (water-in-oil) emulsions in which water is dispersed in vegetable oil or animal fat, respectively.
Wetting agents will decrease the surface tension of the mixture, which allows a larger portion of each spray droplet to contact the surface of the vegetation. Wetting agents increase the coverage of the spray solution, thereby increasing the effectiveness of the pesticide due to increased coverage of the target surface.
CROP OIL CONCENTRATE
Crop oil concentrates are phytobland petroleum or vegetable oils that are formulated to increase the absorption of the pesticide solutions through the cuticle or waxy layer of the leaves. Crop oils contain an emulsifier that aids them in mixing with water. Crop oil concentrates are usually 80 percent to 85 percent petroleum or vegetable oil and 15 percent to 20 percent emulsifier. Product labels will specify if a petroleum or vegetable oil formulation should be used.
Silicone-based surfactants reduce the water surface tension, which allows the water to spread outward onto the leaf surface as compared to beading up on the leaf surface like water on a recently waxed surface. Silicone surfactants can be used at very low rates ranging from as low as 0.10 percent to 0.25 percent volume/volume basis.
Stickers make the pesticide solution adhere to the plant foliage to a greater degree as compared with solutions that do not contain a sticker. By increasing the adherence to the foliage, runoff of the spray mixture, wash-off during rain or irrigation and/ or the evaporation of the pesticide from the target surface are reduced. By minimizing pesticide losses, the effectiveness of the pesticide application will increase.
Compatibility agents are utilized when two or more products are incompatible with each other and are added to the same spray mixture. This could be a physical or chemical incompatibility which causes uneven distribution in the spray tank. The compatibility agents will aid in suspension of the pesticide into the spray solution.
ACIDIFIERS AND BUFFERS
The pH of the spray solution will vary greatly depending on the water source as well as products being mixed into the spray tank. Well water pH from underground sources will usually range between 7.0 (neutral) to 9.0 (alkaline). The pH can vary significantly with season and source.
Most pesticide applications are recommended to be applied at a pH of 5.5 to 7.0. As the pH increases above 7.0, some pesticides are subject to degradation. In most cases, the buffers should be added to the solution to reduce the pH to the desired range before adding the pesticide. As each pesticide is added to the tank, the pH should be rechecked to ensure the modified solution is still within the recommended pH range.
Acidifiers are added to the pesticide spray solution when the mixture is alkaline (pH > 7.0) and the need exists to neutralize the solution to a lower pH. Buffers will change the pH of the water solution, which will be maintained even if the pH of the solution changes.
In the case of Aliette, the label states that buffering to pH 6 or higher is recommended to avoid phytotoxicity when copper has been used prior to, with or following the application of Aliette. Other products may also have information on the label as to the target pH of the spray solution.
ANTI-FOAMING AGENTS AND SPRAY COLORANTS
Anti-foaming agents are used to reduce air entrapment in the spray mixture that may occur during agitation or spraying. Foaming of the spray solution can create problems in mixing or application. Colorants are used to cause the treated area to be colored to ensure complete coverage of the spray solution over the desired target area. In citrus, colorants are frequently added to the spray mixture when treating stumps of trees that have been removed due to various reasons. The colorant will remain on the cut surface to indicate that the stump has been properly treated with herbicides to inhibit regrowth.
DRIFT CONTROL AGENTS
Drift control agents will increase the average droplet size when the solution is sprayed. Larger droplets are less likely to drift (move away from the target site) as compared with smaller droplet sizes. The use of drift control agents can be very important if applying products that may impact sensitive crops or plants which may be located near the application site. There are no drift control agents available to completely eliminate drift.
When mixing any two or more products together to create a tank mix, it is always wise to make sure this new pesticide solution will not damage the crop or create a combination that will not mix evenly in the tank. The pesticide mixture should not create clumps or separations in the spray tank, causing an uneven application of the spray mixture.
Common causes of incompatibility could include impurities in the spray tank or water, order in which the pesticides were mixed into the tank, types of formulations being used, water pH and/or amount of time the mixture has been in the tank.
When mixing pesticides together into the spray tank, follow the W-A-L-E mixing order:
- add enough water to fill the tank about one-half full or one-half of the desired spray volume. Any water-conditioning agents should be added to the tank at this time;
- add the wettable (W) powders;
- agitate (A) the spray solution;
- add the liquids (L); and,
- add the emulsifiable concentrates (E) to the spray solution last.
Adding the emulsifiers to the tank first makes it very difficult for the wettable powders to evenly mix into the spray solution. During this entire process, the agitation systems should be operating to keep the solution mixed during the filling process. After adding each material to the tank, the pH of the spray solution should be checked to make sure it is not too high or too low. Any necessary pH adjustments should be done as needed prior to the addition of pesticides. Once all materials are added and mixed, the tank should be filled to the desired level based upon the desired mixture concentration. If possible, the agitation system should be running until the entire tank mixture is applied. If the entire spray mixed volume is not able to be applied, the tank agitation system should be run for a sufficient time to ensure all materials that may have settled to the bottom of the tank are evenly mixed back into the spray solution.
Source of information: Applying Pesticides Correctly, Fred Fishel, SM1, University of Florida.
Stephen H. Futch is an Extension agent at the Citrus Research & Education Center, Lake Alfred, FL.