title: Effect of a mixture of iprobenfos and malathion on the development of malathion resistance in the mosquito Culex pipiens pallens Coq.
abstract: A malathion-resistant (RM) strain of Culex pipiens pallens Coq was obtained by successively selecting a field population with malathion in the laboratory. The synergistic effect of iprobenfos on malathion toxicity and alpha-naphthyl acetate (alpha-NA) esterase assay revealed that malathion resistance in the RM strain was associated with increased alpha-NA esterase activity and the synergism was mainly due to the inhibition by iprobenfos of this activity. There was no difference in alpha-NA esterase activity between the larvae and female adults in the susceptible (S) strain, but the activity in the adults was 13-fold higher than in the larvae of the RM strain. To understand the effect of the application of a mixture of iprobenfos and malathion on the evolution of malathion resistance, an artificial strain (Syn) was generated by mixing the RM and S strains with 0.1 frequency of the malathion-resistant individuals. The offspring of the Syn strain were divided into two sub-strains, Rm and Rm+ibp, which were successively treated with, respectively, malathion alone and malathion + iprobenfos (1:2) at LC70. In the mixture, the fungicide iprobenfos acted as a synergist of malathion. After treatment for 10 generations, the resistance level to malathion was 317.4-fold for the Rm sub-strain, whereas for the Rm+ibp sub-strain it was only 38.9-fold, compared with the Syn strain. Similar results were obtained by measurement of alpha-NA esterase activity from both larvae and female adults. The alpha-NA esterase activities in larvae and female adults at F10 generation were 2.6- and 10.9-fold from the Rm+ibp sub-strain and 5.7- and 98.5-fold from the Rm sub-strain, respectively, compared with the Syn strain. The above results suggested that iprobenfos, although it cannot completely stop or prevent the onset of malathion resistance, could dramatically delay its evolution.
authors.affiliation: Shanghai Institute of Plant Physiology and Ecology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai 200032, China.
authors.forename: Li-Ming
authors.initials: LM
authors.lastname: Tao
authors.forename: Jian-Zhong
authors.initials: JZ
authors.lastname: Yang
authors.forename: Pei-Jun
authors.initials: PJ
authors.lastname: Zhuang
authors.forename: Zhen-Hua
authors.initials: ZH
authors.lastname: Tang
mesh_terms: D000818:Animals; D003465:Culex / Q000201:enzymology; D005260:Female; D005659:Fungicides, Industrial; D007305:Insecticide Resistance; D007814:Larva / Q000201:enzymology; D007928:Lethal Dose 50; D008294:Malathion; D009283:Naphthol AS D Esterase / Q000037:antagonists & inhibitors / Q000378:metabolism; D009946:Organothiophosphorus Compounds; D010574:Pesticide Synergists; D012641:Selection, Genetic
chemical_list: D005659:Fungicides, Industrial; D009946:Organothiophosphorus Compounds; D010574:Pesticide Synergists; C009897:O,O-diisopropyl-S-benzylthiophosphate; D009283:Naphthol AS D Esterase; D008294:Malathion