Insecticide Resistance Study in Plutella xylostella (L.) VI. An Experimental Analysis of Organophosphorus and Synthetic Pyrethroid Resistances

Abstract

The organophosphorus and synthetic pyrethroid resistances in diamondback moth, Plutella xylostells (L.), were compared in two groups of resistant strains. The first group consisted of seven field strains which carried mixed origins of resistance; the second group were six laboratory-bred strains specially selected by designated insecticides so that the insects with known origins of resistance. The resistance and cross resistance characters of laboratory-bred strains are used in analyzing and explaining the test results of field strains. The resistance inducing ability and the cress resistance relationship of organophos-phorus and synthetic pyrethroids were the main target of this investigation. Variable responses in diamondback moth were obtained for different insecticides. There are fast inducers for resistance as profenofos and fenvalerate; compounds induced resistance slowly as mevinphos, prothiofos, permethrin and cypermethrin. Stability of the resistance was also noticed as both mevinphos and prothiofos induced unstable resistance. Generally, the results of laboratory simulation in regard of organophosphorus were similar to what were actually happened in field strains; on the other hand, the laboratory-bred synthetic pyrethroid resistant strains failed to attain the high resistant level of those field strains. In cross resistanco tests, the organophosphorus insecticides caused significant cross effect on many other organophosphorus compounds with few exceptions. The cross resistance among synthetic pyrethroids were rather common except the diamondback moth responded much stronger to fenvalerate and decamethrin than permethrin and cypermethrin. The resistance crossed over from organophosphorus to synthetic pyrethroids was evident in our laboratory-bred strains while the synthetic pyrethroid resistance crossed to organophosphorus was rather miner and selective. From the evidence of cross resistance, the high synthetic pyrethroid resistance of field diamondback moth may result from the combined effects of many insecticides. In a cross examination of permethrin-and cypermethrin-resistant strains for the possible role of extra alpha-cyano group on some synthetic pyrethroids, the result indicated the diamondback moth responded similarly to both compounds in resistance as the presence of cyano group will not discriminate the resistance mechanism.