The Dilemma of Tank-Mixing By Dr. Raymond A. Cloyd Assistant Professor Department of Natural Resources and Environmental Sciences University of Illinois Tank mixing, which involves combining two or more pest control materials into a single spray solution, is gaining in popularity because of the potential for improved pest control. In addition to the benefits of tank mixing there are several problems that growers need to be aware of before tank mixing pest control materials. This article will address the advantages and concerns regarding tank mixing. First, why do greenhouse growers tank-mix? One reason is convenience. It is less time consuming, costly, and labor intensive to combine two or more pest control materials into a single spray solution and spray one application as opposed to making two or more applications. Another reason growers tank-mix is because of improved pest control or enhanced effectiveness. This is known as synergism. Synergism occurs when two pest control materials mixed together perform better (kill more insect and mite pests) than when the two pest control materials are applied separately. Research has demonstrated that pest control materials containing azadirachtin (Azatin/Ornazin) or the entomopathogenic fungus Beauveria bassiana (Botanigard/Naturalis) are more effective when tank-mixed with another pest control material compared to either if used by itself. Some compounds are strictly referred to as synergists. For example, piperonyl butoxide or PBO, which is not an insecticide, is normally mixed with pyrethroid-based pest control materials (i.e. Talstar, Decathlon, Tame, Mavrik, and Astro) to block enzymes present in insects that are capable of breaking down pyrethroids. Another method that growers may use to enhance performance is to mix a pyrethroid based pest control material with a different type of pest control material. The pyrethroid will irritate insects or mites and cause them to become more active, thus increasing their exposure to any spray residues. Just as synergism improves the performance of two or more pest control materials, the opposite referred to as antagonism may result. Antagonism occurs when mixing two or more pest control materials reduces the effectiveness of one or all the components of the pest control materials. In addition to reduced effectiveness, there is the potential for plant injury (phytotoxicity). Be sure to read the label prior to tank mixing because pest control labels generally state (although this may not always be the case) which materials can and cannot be tank-mixed. If in doubt, contact the manufacturer of the pest control materials for information. One caution against tank mixing is to avoid the potential for insects or mites to develop resistance to two pest control materials. Resistance may develop, whether to two materials with similar modes of activity (cross-resistance) or two with different modes of activity (multiple resistance). Resistance occurs faster to those pest control materials with site-specific modes of action in the pest (i.e. nerve poisons). More research and data are needed to address this issue. Another reason to avoid tank mixing is incompatibility, which is a physical condition that prevents pest control materials from mixing properly in a spray solution. This may result in reduced effectiveness or phytotoxicity. Incompatibility may be due to the chemical or physical nature of the pest control materials, impurities in the water, water temperature, or the types of formulations that are mixed. In order to determine compatibility between two or more pest control materials, growers should conduct a "jar test." This involves collecting a sample of the spray solution (i.e. one-pint) into an empty glass jar or other container, and letting the solution sit for approximately 15 minutes. If the materials are not compatible there may be noticeable separation or layering, or precipitates such as flakes or crystals may form. However, if they are compatible then the solution may appear homogeneous or look like milk. The continued loss or changes in registration (i.e. extended restricted entry intervals) of older, broad-spectrum pest control materials due to federal rules and regulations has led to an increased development and availability of pest control materials (often referred to as "reduced risk") that are more selective. That is, they control a narrow-spectrum of pests. In addition, these materials have low impact on natural enemies such as predators and parasitoids. Although the presence of pest control materials that demonstrate specificity is desirable, it does create a dilemma when growers are dealing with multiple pests. For example, a grower that has thrips, whiteflies, and spider mites as major pests may have to tank-mix two (possibly more) pest control materials in order to obtain the same spectrum of control that a single broad-spectrum pest control material might provide. Will this result in an enhanced development of resistance and will this situation expedite the reduction in effectiveness of these pest control materials? In all cases, it will probably depend on the pest control materials being tank-mixed. Another concern is that as new plant varieties become increasingly available there may be differences in tolerance to tank mixes based on variety sensitivity. In order to avoid the problem of phytotoxicity it is recommended to test a spray mixture on a sample of plants prior to using on an entire crop. In summary, the issue of tank mixing can be argued from both a positive and negative perspective. Although, there are substantial advantages to tank mixing, growers should still be cautious about tank mixing in order to avoid potential problems including resistance, reduced effectiveness, and phytotoxicity. Dr. Raymond A. Cloyd is Assistant Professor, Extension Specialist in Ornamental Entomology/Integrated Pest Management, University of Illinois, Department of Natural Resources and Environmental Sciences. 228 Edward R. Madigan Laboratory. 1201 West Gregory Drive. Urbana, IL 61801. Phone: (217) 244-7218 Fax: (217) 333-4777. Email: rcloyd@uiuc.edu