Certain Pesticide Mixtures
Cause Label ConfusionAnytime a field is treated with more than one pesticide there is a risk for an interaction among the
pesticides. With most products the interaction does not significantly influence product performance, and thus is of little concern to farmers. However, with some products the interaction may result in an economically significant response. The interaction may be reduced performance (e.g. reduced grass control when tank mixing a broadleaf herbicide with an ACC-ase herbicide such as Assure® or Poast Plus®) or reduced crop safety (e.g. tank mix of Accent® and Clarity®). In most situations these interactions are inconsistent, thus making it difficult to access the risk associated with the combination of products.
However, the potential for organophosphate insecticides to increase herbicide injury on corn is well documented. Organophosphate (OP) insecticides can reduce the margin of crop safety to any herbicide that is metabolized by the cytochrome P-450 enzyme system. This enzyme system is responsible for degrading several important herbicides, including the sulfonylureas (Accent®, Beacon®, etc.) and Callisto® (mesotrione). The OP insecticide can reduce the speed at which the cytochrome P-450 enzyme metabolizes the herbicide, allowing the herbicide to remain at a toxic concentration in the plant for a longer time than if the insecticide wasn't present. Several factors influence the potential for crop injury when one of these herbicides is used on a crop treated with an OP insecticide, including the relative margin of crop safety to the herbicide, the insecticide used, and any factor that influences crop vigor. The risk of a herbicide/insecticide interaction varies among OP insecticides largely due to the mobility of the insecticide within the crop. For example, in corn, terbufos (Counter®) generally has a greater risk for herbicide interactions than chlorpyrifos (Lorsban®) because it is absorbed by corn roots and translocated within the plant more readily than chlorpyrifos.
Although the physiological cause of the OP/herbicide interaction is well understood, end users (i.e., farmers, ag chemical dealers) of these products may be confused by the conflicting statements found on the product labels. For example, the current Callisto® label states “severe crop injury may occur if Callisto® is applied postemergence to corn crops that were treated with Counter® or Lorsban®.” The original label went even further, stating not to use Callisto® postemergence on corn treated with Counter® or Lorsban®. Dow AgroSciences, the manufacturer of Lorsban®, has stated that the risk of significant injury from postemergence applications of Callisto® on corn treated with Lorsban® is equivocal, and thus feel the warning should not be on the Callisto® label. To support this position, Dow AgroSciences released a supplemental label (2ee) recommending postemergence applications of Callisto® on corn treated with a soil application of Lorsban®. Syngenta has maintained that the label warning is warranted in order to minimize the risk of crop injury since they are likely to be held responsible for any injury complaints, and recommends not using Lorsban® or Counter® if Callisto® is to be applied to the corn postemergence. Since Callisto is a pigment inhibitor, symptoms of injury are easily noticed; however, under most situations any crop response will be short-lived and should not threaten yields (in corn). (Iowa State University, 1/3/03 via AgNet, 1/7/03).
