Quasi-synergism and penetration of insecticides.

Abstract

The toxicities of carbamates alone and combined with sesamex or thiocyanates were determined on the house fly, Musca domestica L., by topical application, oil spray, and injection methods. The results were compared with those of organochlorine (OCI) and organophosphorus (OP) insecticides. Generally, differences in toxicity were small or insignificant for OCI and OP, but large differences were observed with some carbamates between topical application and injection. There were minor differences in magnitudes of synergism between topical and spray results for OCI, and the differences for most OP also were small between methods. However, differences in the degree of synergism were found as high as > 830 times for some carbamate when tested by different methods. When kerosene was added to acetone (1:9 vol/vol) for topical application, the difference in toxicity (as LD50) for SD-8786 (2,3,5-trimethylphenyl methylcarbamate) was greatly increased from 63 to 2.3 µg/fly, whereas the difference in the degree of synergism was greatly reduced from 630 to 27 times. Apparently, penetration was an important factor in the observed high differences in synergism of some carbamates tested by different methods; the quicker the penetration, the lesser the opportunity for detoxication. In addition, the high degree of synergism for combinations of carbaryl-Thanite® (isobornyl thiocyanoacetate) (>400×) and SD-9003-(2,4,5-trimethylphenyl methylcarbamate)-Thanite (> 110) by topical application with acetone solution but no synergism by oil spray strongly indicated that the differences also were due to the differences in penetration. Thus, this type of synergism was classified as quasi-synergism. From the analysis of apparent degree of synergism with sesamex, penetration may be more important than inhibition of detoxication for carbamates tested and vice versa for higther homologues of monocrotophos. Topical application with acetone solutions may not reveal the true degree of synergism (i.e. inhibition of detoxication) for some slow-penetrating insecticides. A thorough study of topical application method for measuring toxicity, synergism, and resistance is necessary.