Pesticide interactions

When confronted with a difficult or complex pest problem, you may be tempted to mix several pesticides together in one labor-saving, cost-efficient treatment. The reasons for this include: * Providing better control than either product would alone * Increasing the length and spectrum of control * Saving money by reducing labor and wear on equipment (due to fewer applications).

Pesticide combinations usually alter plant absorption and translocation (for herbicides), and metabolism and toxicity at the site of activity of one or more of the mixed products. While this can improve performance in many cases, not all such changes are for the better. Therefore, not all pesticide combinations are desirable. Negative effects can occur, such as reduced pest control, increased damage (phytotoxicity) to turfgrass or ornamentals and incompatibility problems between materials.

Interactions that change efficacy The responses to simultaneous or sequential application of two or more pesticides fall into three categories:

*Additive effects. When mixing two pesticides provides the same response as the combined effects of each material applied alone, the effect is said to be additive. In other words, the products neither hurt nor enhance each other. Such mixes save time, labor and equipment use.

* Synergistic responses. When two pesticides provide a greater response than the added effects of each material applied separately, the response is considered synergistic. People often mistake additive effects for synergism. However, unlike with additive effects, the chemicals in a synergistic combination are not neutral toward each other. Rather, they interact in some way that increases efficacy beyond simple addition. Thus, not only does synergism save time, labor and wear on equipment, it enhances control compared to applying each of the materials separately. With true synergism, you often can reduce the application rate of the pesticides without sacrificing control.

* Antagonism. When two pesticides applied together produce less control than if you applied each material separately, we call it antagonism. In addition to reducing control, antagonistic responses also may increase damage, or phytotoxicity, to plants. Antagonism is one kind of incompatibility, which we will discuss here in more detail.

Enhancement is yet another type of interaction, but not between two pesticides. Enhancement occurs when a pesticide is mixed with an additive to provide a greater response than if you applied the pesticide alone. A common example of enhancement is mixing an adjuvant (such as a surfactant) with a pesticide. Surfactants enhance the performance of a pesticide by increasing the efficiency with which a product enters (or sticks to or covers) a plant or pest.

Incompatibility Two or more pesticides, or a pesticide and a fertilizer, are compatible if no adverse effects occur as a result of mixing them together. Conversely, decomposition (deactivation) of an active ingredient often occurs with chemical incompatibility. This is most affected by temperature, tank pH and length of time that you hold a spray mixture in the tank before use. Physical incompatibilities usually involve the inert ingredients of a formulation--the mixture may become unstable, forming crystals, flakes, gel, oil, grease or sludge that may clog spray equipment.

For herbicides, incompatibility most often occurs when you mix an emulsifiable-concentrate formulation with wettable powders. Similarly, you should not mix emulsifiable-concentrate insecticides with fungicides or herbicides. Liquid fertilizers also can cause compatibility problems, predominately due to their high use concentration and strong ionic nature. The possible effects of mixing incompatible chemicals are many and include: * Reduced effectiveness of one or both compounds * Precipitate in the tank, clogging screens and nozzles in the sprayer * Plant phytotoxicity, stunting or reduced seed germination * Excessive residues * Excessive runoff.

Tank-mixing calls for caution You should plan and use combination treatments with great care. Not all possible combinations have been tested on all plants. As we discussed, some combinations may react to form material that clogs spray equipment and ends up costing extra time and money in equipment repair costs. Or even worse, some combinations can burn sensitive plants. Thus, a word of caution is in order. When attempting pesticide combinations that are unfamiliar to you, use a jar test (see page 16) to check for incompatibility. In addition, test the combination on a few plants or a small area of turf before larger-scale treatments. Wait at least 2 to 3 days for any problems to become apparent. Keep accurate records on compatible, safe combinations for future reference.

Be sure to follow all label instructions whenever you use pesticides. Labels generally list known antagonisms and incompatibilities, as well as any special mixing procedures and the need, if any, for adjuvants (see page 16 for mixing guidelines). Remember, you must apply these tank mixes according to label directions. In addition, you legally can apply non-registered tank mixes if all the pesticides in the mix are registered individually on the crop you're treating. However, you assume liability for injury to plants if the combination is not a labeled tank mixture.

You'll find information specific to insecticides on page 16 and herbicides on page 17. Fungicides, of course, also can interact antagonistically or additively. However, the issue of fungicide synergy is somewhat controversial. Many turfgrass experts feel fungicide synergy is non-existent or simply insignificant for any practical purpose. However, the principles and methods that this article illustrates for dealing with other pesticide interactions apply to all pesticides.

Dr. L. Bert McCarty is associate professor of turfgrass science at Clemson University (Clemson, S.C.). Dr. Beverly Sparks is professor of entomology and extension entomologist at the University of Georgia (Athens, Ga.).

* Read the label. This, of course, is the first step you should take whenever you apply any chemical. Labels typically inform the user of frequently used mixes, known incompatibilities and the need for any adjuvants.

* Perform a jar test. Anytime you try a new mix, it is a good idea to do this (see heading, below right).

* Test pH. Many incompatibilities result from excessively alkaline (but occasionally extremely acidic) pH in the tank. If necessary, you can add buffers to counteract this problem.

* Make a test application. It is always a good idea to make a test application anytime you use a product or mix you haven't used before. Such a test will expose any phytotoxicity or antagonism before you make a wide-scale application. If you overlap a few strips, this also can show you how much of a margin of safety you have. Wait a few days for any symptoms to become visible.

* Take care with fertilizers. If you add fertilizers, be aware that they can have substantial effects on the chemistry of a tank mix, especially on pH. Fertilizers also increase the salt load of a mix. It's best not to exceed 1 ounce of dissolved solids per gallon of spray solution.

* Don't mix iron sulfate with phenoxy herbicides. Iron sulfate is incompatible with amine formulations of some phenoxy herbicides and can cause a precipitate to form, clogging spray equipment.

* Mix no more than one soluble or emulsifiable chemical with any insoluble products such as wettable powders or flowables.

* Avoid mixing strongly acid materials with strongly alkaline materials.

* Apply sprays soon after mixing. Mixes that sit for several hours or longer are prone to degrade, especially if pH is alkaline.

Proper mixing procedures * Mixing order. Pesticide labels usually provide directions for mixing different materials, often describing the sequence of mixing. Whenever a label provides such directions, you should follow them. In general, the proper order in which to add herbicides to a tank mix is: * Wettable powders (WP) * Dry flowables (DF) * Flowables (F) * Emulsifiable concentrates (EC) * Solubles (S).

The tank agitator should be operating continuously while you're mixing a tank. After you make an addition, thoroughly agitate the mix before adding another product. You also may need to add a compatibility agent to keep the components of the mixture suspended and prevent them from reacting. Compatibility agents are adjuvants that reduce the risk of incompatibility in pesticide or pesticide/fertilizer combinations. Sometimes, labels suggest their use, but even when they don't, many applicators still use them whenever they work with mixes. If you use a compatibility agent, it should be the first thing you put in the tank.

* Pre-mixing. Pre-mixing in a smaller, separate container or tank is necessary for many pesticide formulations.

* Wettable powders. Make a slurry in a separate container by adding small increments of water until it forms a gravy-like consistency. Slowly add this slurry to the tank with the spray-tank agitator running.

* Dry flowables and water-dispersible granules. Pre-mix with 1 part flowable to 1 part water (start with the water and add the flowable to it) and then pour the mix slowly into the tank.

* Liquid flowables. Pre-mix liquid flowables by adding 1 part liquid chemical to 2 parts water (or liquid fertilizer) before blending in the tank. Many labels for liquid-flowable products describe the proper mixing procedure. Remember, when you pre-mix chemicals as we describe here, you still need to follow the mixing order listed previously.

Testing for compatibility: The Jar Test You can test the compatibility of a new mix with the "jar test," which consists of the following:

* In the proper sequence (see "Mixing order," at left), place 1 teaspoon of each chemical that will go into the mix in a quart jar containing about 1 pint of clean water. Close the lid tightly and shake it vigorously between each addition.

* Observe the mixture for settling out, layering, formation of gels, flakes or other precipitates, or a change in temperature of the mixture (rapid heating).

* If you see any of these effects, the chemicals are incompatible and you should not use them together. If nothing happens, let the jar stand for about 30 minutes and then check it again to be sure. If you still see no problem, the mix is probably okay.

Again, applicators should make a test application to a small area before wide-scale treatment with any new mix. This will reveal any antagonism or phytotoxicity that develops as a result of the mix. Be sure to wait a few days for any problems to become visible.

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