Cyromazine toxicity to Drosophila melanogaster (Diptera: Drosophilidae) and lack of cross-resistance in natural population strains

Cyromazine is an insect growth regulator (IGR) insecticide that is effective against Drosophila melanogaster (Meigen). Both larvae and pupae of a standard laboratory strain were killed over an extremely narrow range of 0.3-0.5 ppm incorporated into the diet. Higher toxic doses killed larvae more rapidly, and lower toxic doses allowed pupariation but usually blocked pupal development. These puparia were abnormally elongated, indicating a failure of the contraction process that produces the characteristic pupal shape. When fed to adults, cyromazine had no effect on adult survival, fecundity, or fertility, even at a 10-fold larval LC50 level. In previous work, D. melanogaster strains recently derived from natural populations were found to be 25- to 100-fold resistant to at least 2 other classes of insecticides, chitin-synthesis inhibitors and carbamates. Here I tested 9 additional strains that were recently selected from widely separated geographical locations in the United States and found them to have resistance to lufenuron and propoxur, but virtually no resistance to cyromazine relative to one another or to 3 long-established laboratory strains. The results show that cross-resistance between the IGRs lufenuron and cyromazine is not present in D. melanogaster. They further suggest that cyromazine control failure in pest insect populations caused by cross-resistance may be minimal.