Organochlorine-resistant mosquitofish from an insecticide contaminated environment in the Mississippi Delta showed a 3.4-fold tolerance to pyrethrum when compared to a susceptible strain. The 24-hr LC50 values for pyrethrum in susceptible and resistant mosquitofish were 27 and 93 ppb, respectively. Results of studies with sesamex, an inhibitor of mixed-function oxidase (mf0) enzymes, indicated that the tolerance to pyrethrum is partially, but not solely the result of increased levels of mfo enzymes. Additional studies with allethrin show that increased carboxylesterase enzyme levels cannot explain the pyrethrum tolerance which was not accounted for by mfo enzymes; thus yet another mechanism must be involved. Resistant mosquitofish that had been maintained in an insecticide- free environment for several years also possessed a tolerance to pyrethrum but to a lesser degree than the original parents from the insecticide- contaminated area. Data suggest that the observed tolerance to pyrethrum is apparently in part a genetically based resistance involving two or more resistance mechanisms.
