SYNERGISM OF DDT EFFECTS BY DMC ON LABELLAR RECEPTORS OF RESISTANT AND SUSCEPTIBLE HOUSE-FLIES

Effects of DDT or a mixture of DDT and DMC on the salt response of labellar hairs in DDT-resistant and DDT-susceptible house flies were studied electrophysiologically. The normal regular firing pattern was disrupted by either treatment, with the spikes becoming grouped into trains. The nontoxic analog DMC had no effect on the firing pattern when applied alone. In every case studied, the DDT:DMC mixture produced greater effects than DDT alone over a 30-min observation period. The differences were statistically significant in the moderately resistant Roberds strain and the highly susceptible NAIDM strain using a mixture of 100 ppm DDT:400 ppm DMC. Tests of 100 ppm DDT:100 ppm DMC in the moderately susceptible Stauffer and 100 ppm DDT:200 ppm DMC in the Roberds produced results in the expected direction but they did not reach statistical significance. DMC showed a high degree of synergism in whole-fly application in the Stauffer and Roberds strains and little, if any, in the NAIDM. The synergistic effect of DMC at the site of action may be due to its known inhibitory effect on the enzyme DDT-dehydrochlorinase, or to some as yet unknown factor. © 1979.