Housefly collected from the 3rd Yumenoshima dumping-island (YS strain) in Tokyo Bay of Japan developed 4,900-fold resistance from only 3.8-fold in the 3rd instar larvae to a juvenile hormone analog, pyriproxyfen, after laboratory selection with this chemical. The selected strain was thus named YPPF strain. Linkage group analysis of pyriproxyfen resistance revealed that resistance to this chemical in YPPF strain was multifactorial, with the most important factor being located on the 2nd chromosome. Toxicity bioassays of pyriproxyfen with synergists showed that the resistance of houseflies was depressed to 29-fold from 400-fold in white pupae by a ytochrome P450 inhibitor, 2-propynyl 2,3,6-trichlorophenyl ehter (PTPE). It is obvious that cytochrome P450 monooxygenase plays an important role in housefly resistance to pyriproxyfen.
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