RESUMO
When eggs of various strains of the screwworm, Cochliomyia hominivorax (Coquerel), were treated with a coumophos emulsion, resistance was as follows: 1.0- to 2.0-fold (SW USA), 3.0-fold (El Salvador), 4.9 to 11.3-fold (NE Mexico), 5.6 to 10.2-fold (NW Mexico), and 78.1-fold (Jamaica). The pattern seemed to correlate with a history of insecticide use in the areas of origin. The resistance of the Jamaica population was recessive and not controlled by a single major gene, as indicated by the F1 hybrids. Further intense selection of the Jamaica strain failed to increase the shallow slopes of the dosage-mortality regression lines. Attempts at selecting insecticide-resistant strains from adults of laboratory populations (Aricruz strain) by using very high selection pressures(75 to 95 percent mortality) with insecticide-treated food proved futile. Over five to six generations the maximum tolerance seen were 1.0 to 2.1-fold to DDT, 1.5-to 6.5-fold to dieldrin, 1.7s-to 1.8-fold to propoxur, and 5.0-to 6.5-fold to coumaphos. Other selections with adults of the GDI strain (from NW Mexico) under pressure with DDT or coumaphos over 8 to 11 generations suggested great diversity in tolerance but no resistance (AU)
