Biochemical and Physiologial Characteristics of Insecticide Resistance in Diamondback Moth

A pyrethroid-resistant strain of the diamondback moth, Plutella xylostella (L.), originally collected in Thailand showed cross-resistance to all the pyrethroids tested. Upon further selection with fenvalerate in the laboratory, the strain exhibited an extremely high LD50 value, 41 μg/larva, for fenvalerate. In genetic studies of the resistance, the F1 progeny showed recessive gene(s), while the B1 and F2 progenies had a more complex genetic nature. The in vivo dynamics of fenvalerate applied topically showed: (1) reduced cuticular penetration; (2) increased detoxification; and (3) insensitivity at the site of action being responsible for the resistance mechanism. A large portion of the increased detoxification was due to cytochrome P450-dependent monooxygenases, judging from a remarkable synergism of fenvalerate toxicity by piperonyl butoxide. The resistance was unusually unstable in the absence of selection pressure. The LD50 decreased about 50% in every generation. Selection pressure once with fenvalerate or even with malathion, however, increased the LD50 restoring the fenvalerate resistance. There was no significant difference in the fitness between the resistant strain and its revertant. These results may imply the presence of an unknown factor(s) necessary to maintain the insecticide resistance in diamondback moth, which cannot be explained by the conventional preadaptation theory.