The Truth Behind Tank Mixes By Ron Oetting

Tank mixes were common years ago, but are they worth it now?

“Can I use tank mixes?” This is a question I haveheard from growers for more than 25 years, and I still do not have an answerthat is backed up by facts and figures. Tank mixes were a common practiceduring my early experience with greenhouse growers, but mixes are not as commontoday. When I first started working with pest management in greenhouse production,it was common for chrysanthemum growers to use weekly sprays of tank mixes.These tank mixes usually included a pesticide for worms, one for mites and atleast one for diseases. It was also common to apply a systemic pesticide,usually aldicarb, as a general preventative against several pests. Integratedpest management was being researched and practiced, especially for field andorchard crops, but not much emphasis had been placed on ornamental crops andresearch in this area was in its infancy.

When Liriomyza leafminer became a management problem in themid 70s, there was a change in traditional management strategy. Pesticideresistance became a new topic of concern, and recommendations for pestmanagement took on a new picture. There was a rush to find a management toolfor leafminers, and pest biology became an important part of this strategy.Prior to this time, the only concern was pure efficacy of pesticide against thetarget pest. Researchers across the country diligently searched new and oldchemistry for a solution to the problem of leafminer resistance. When asolution was found, it was used extensively to reduce leafminer populations; asa result, resistance was developed against some of these compounds. The primeexample was the rapid development of leafminer resistance against permethrin inabout 1980.

Resistance management really became an issue when westernflower thrips became resistant to available pesticides and spread across theUnited States and around the world. Thrips were already hard to control in themid 70s, but there were a few insecticides that were effective. Methomyl wasthe standby, and acephate was new on the market at that time — both wereeffective. By the early 80s, western flower thrips and the accompanying tospovirus(tomato spotted wilt virus) had become a dominant concern in pest managementfor greenhouse growers. This was a result of western flower thrips’ resistanceto available insecticides and the lack of any means of controlling thetospovirus vectored by the thrips. A management approach was to develop arotation of insecticides to control western flower thrips. The theory was thatthis rotation would slow down the development of resistance when newinsecticides were found. At that time, the recommendation of rotating chemicalswas the standard, and tank mixes were discouraged in extension recommendations.

What is the role of tank mixes, or the lack thereof, inresistance management today? This is a good question with answers supported byvarious theories, but the most common theory is that mixing compounds willresult in resistance developing faster than if each pesticide is usedseparately in a rotation. The purpose of this article is to address some of thereasoning behind the use of tank mixes and rotations in pest management.

Tank Mix Problems

The first major problem with tankmixes is the increased potential for phytotoxicity. I have seen numerousexamples of increased phytotoxicity as a result of mixing two or more chemicalstogether. Two serious examples come to mind. One grower lost his newly rootedpoinsettias because of a foliar spray of a tank mix of three chemicals alltargeting silverleaf whitefly, and a second situation resulted in complete lossof a poinsettia crop because of the mixing of an insecticide and anoff-the-shelf spreader sticker formulated for homeowners. In these situations,all chemicals were sprayed at label rates. None of the selected pesticides wereillegal. Data does not exist for all possible mix combinations, let alone for allplants. Growers who use tank mixes must take the responsibility of checking outa tank mix on a few plants to make sure that phytotoxicity will not occur.

Another problem sometimes encountered with tank mixes is theincompatibility of components in the mix. This incompatibility results becausethe chemistries of the two compounds are not compatible, and they react witheach other, often reducing the activity of mix against the target pests. Itcould also cause the new tank mix to be more phytotoxic to the host plant asdiscussed above. Often, if the chemicals are incompatible, there is a chemicalreaction between the two compounds, which can be seen in the mixed solution.The common test of physical compatibility is to mix the correct proportions ofeach pesticide component with water in a quart jar and shake the mixture.Observe the mixture to see if they mix together uniformly or if there isseparation of layers, precipitation to the bottom of the jar or other abnormalmixing characteristics. The problem of incompatibility is not limited to mixingtwo or more pesticides but could also be the result of mixing a pesticide withfertilizer or other horticultural chemicals.

Resistance Management Programs

The most common tank mix is the use of a pesticide with anadditive to enhance efficacy of the pesticide. Additives such as buffers areadded to adjust the pH, without which the pesticide will break down too fast ornot be as active as it should be; surfactants are often used to increasecoverage on plants, especially waxy plants; pyrethrin has been used as aflushing agent to irritate a pest and make it move, allowing better contact andcontrol; attractants have been used to attract pests to feed on the pesticidecomponent of the mix; and sugar has been mentioned as an additive for increasedthrips activity. Many of these additives are not only enhancing the activity ofthe primary pesticide, but some of them also have pesticidal activitythemselves. The pesticide label often contains guidelines concerning the use ofadditives in the tank mix.

The big question is whether tank mixes are bad forresistance management programs? Resistance is the result of a pest becomingmore tolerant of a pesticide, which was once effective against it. This beginswhen a few individuals are more tolerant to a chemical than the remainder ofthe population. These individuals survive and pass this genetic trait oftolerance on to the next generation. As a result, more individuals in the nextgeneration are less susceptible to being killed by the pesticide. If thiscontinues for several generations, the tolerance can be much greater than whatwas present in their ancestors. Most new pesticides now have resistancemanagement instructions on their labeling, and most recommend limiting thenumber of applications on a crop and/or using a rotation of a different mode ofaction. The purpose of restricting the frequency of use and limiting exposureto that chemistry is to decrease the chance of having a population that hasdeveloped resistance against that chemistry. How does the use of tank mixes fitinto resistance management programs? The key concern in tank mixes andresistance management is whether the mix is targeting one pest or differentpests.

One-Pest Mixes

The use of tank mixes, which targets only one pest, goesagainst the general philosophy of many researchers and is not good resistancemanagement. My general perspective is that we do not use tank mixes targetingone pest unless it is absolutely necessary. One philosophy is that by mixingtwo effective insecticides from two different classes, you should be able tokill all of the individuals of the species, and there will not be anyindividuals to carry on resistant genes. This is good in theory, but I don’tthink it is true that we will kill all individuals in the greenhouse. It ismore probable that a few individuals that do survive will demonstrate sometolerance to both pesticides and will pass that on to the next generation.

You probably would not be thinking about mixing compounds ifyou were not already having problems controlling this pest with one or both ofthe components of the mix. We also know that if you can remove the populationof a particular pest from exposure to an insecticide, in some cases, they willactually recover and become more susceptible to that insecticide again.Therefore, the rotation of compounds is a better management practice against aparticular pest species. There are many different types of pesticides anddifferent pests that enter this picture, and each must be considered separatelyto make a sound decision on tank mixing two or more pesticides to manage asingle pest species.

There are exceptions where mixes are recommended, but these usuallyfollow the loss of efficacy of the primary component. For example, when weencountered problems with abamectin for the management of western flowerthrips, we used a tank mix of abamectin and horticultural oil, and it improvedefficacy. The use of oil is acceptable in tank mixes by many researchers.Another example was when acephate and a pyrethroid were used to managesilverleaf whiteflies when no other control was available. If possible, it isstill a good practice to rotate these tank mixes with other alternative controlpractices.

A question is often raised about mixing two insecticidestogether to kill different developmental stages of a pest. A common example isan insecticide spray for adult fungus gnats and an insecticide drench forimmatures in the medium or a miticide to kill active mites and an ovacide tostop reproduction. This is a little more valid than the situation discussedabove, but I still feel that we need to be careful with these mixes. We need tofully evaluate the situation. Do we really need to attack both developmentalstages? If the crop is going to be in the house for a few weeks, then aneffective material will reduce the population, and you can still rotatepesticides appropriately. If you are ready to ship the crop this week, thenthere may be more justification for looking at controlling all stages.

Mixes for Different Pests

What about mixing together pesticides that target differentpests? A mix that targets different pests might include an insecticide for aninsect, a miticide for a mite or a fungicide for a disease. There are varyingopinions on the validity of this mix, but the truth is that growers are goingto use them. The biggest problem is the already-mentioned phytotoxicity.

A commonly asked question is what is the difference in atank mix and spraying the pesticides separately within a very short time ofeach other? The answer is probably nothing as far as resistance management goesbut everything when it comes to phytotoxicity.

Mixes targeting different pests make better sense inresistance management, but care should be taken that the components do notoverlap in their activity. For example, using abamectin for mites and spinosadfor thrips does not make sense. Both are used for thrips control, so they overlapin that use and should be rotated in a thrips management program. In addition,abamectin is used at a lower rate for mites than for thrips. Applying aninsecticide at a sublethel dose is a sure fire way to increase the probabilityfor resistance. If you are going to mix different pesticides for differentpests, you need to really study the characteristics of the components. Youdefinitely do not want to mix a fungicide with a fungal bioinsecticide that hasinsecticidal activity, such as Beauvaria bassiana. Care should be taken to testany tank mix before using it on the entire range. Spray a few plants, andobserve for plant damage and pest control to see if they are safe andcompatible.

Tank mixes are much easier for growers to use than trying tospray all pesticides at different times. The use of mixes will save time, laborand applicator exposure to pesticides. However, if the end result is that youloose the effectiveness of those pesticides, this could be a temporary saving.More information is needed to assist growers with compatibility and otherconcerns of tank mixes. Pesticide labels do contain more information aboutmixing and resistance management than in the past, but growers still need totake care to test pesticides and mixes on a few plants before using over alarge area.

Ron Oetting

Ron Oetting is professor of entomology at The University of Georgia. He may be reached by phone at (770) 412-4714 or E-mail at [email protected]

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