Behaviorally Active Compounds May Not Enhance Pesticide Toxicity: the Case of Dicofol and Amitraz

A behaviorally active formamidine pesticide was used to test the hypothesis that efficacy of the chlorinated hydrocarbon acaricide, dicofol, could be increased by enhancing pest locomotion or decreasing pest avoidance of dieofol, or both, on treated leaves. Amitraz, dicofol, and mixtures of amitraz + dicofol were sprayed on plants in a manner that created discontinuous, checker-board residues. Behavior and mortality of twospotted spider mites, Tetranychus urticae Koch, were then observed. Evaluated alone, or in mixture with dicofol, amitraz resulted in significant increases in mite locomotion and, in mixtures with dicofol, eliminated mite avoidance of dicofol residues. Though both of the desired behavioral modifications were achieved, the toxicity of amitraz + dicofol was actually less than that of dicofol alone, as revealed by probit analyses of 72-h discontinuous residue bioassays. Hypotheses formulated to explain this toxicologically undesirable outcome of the mixture of dicofol and the behaviorally active compound were decreased alimentary uptake of toxicant or antagonistic chemical or physiological interactions between dicofol and amitraz. Our findings warn of the potential for unexpected and detrimental outcomes of the use of behaviorally active chemicals to enhance toxicity of conventional pesticides. Even for chemicals like formamidines, that strongly influence pest behavior, the benefits of chemical mixtures should be demonstrated empirically in the laboratory and field before being recommended for pest management.