Resistance And Mechanism Of Spodoptera Exigua (H(?)bner) To Pyrethroids

An artificial diet consisted of soybean flour, barley, brewers yeast, vitamin C, cholesterol, choline chloride, agar, sodium benzoate and acetic acid, can support the development of Spodoptera exigua. No significant differences were found in development duration, eggs hatching percentage, pupation rate, emergence rate, female adults life or sex ratio compared with those feeding on natural diet ( water spinach), but pupal weight and fecundity of the female adults of 5. exigua reared on artificial diet were higher. Life parameters including the eggs number per female laying, eggs hatching percentage, larvae survival rate, larvae duration, pupation rate, emergence rate, pupal weight and female adults life kept stable during 14 consecutive generations of S. exigua reared on the artificial diet.The relative susceptible toxicity baselines of S. exigua to major insecticides in the laboratory condition were established with topical application and leaf dipping method. Resistance monitoring of S. exigua collected from Jianxin of Fuzhou, Shangjie of Minhou, Jianou of Nanping, Tong'an of Xiamen, Huangshi of Putian and Longhai of zhangzhou respectively, was carried out by the topicial application of the 4th instar larvae. The resistance monitoring results indicated that 5. exigua had developed extremely high resistance to pyrethroids (alpha-cypermethrin, fenvalerate, fenpropathrin, deltamethrin, cyhalothrin) ; the medium to high level resistance to chlorpyrifos, profenofos, phoxim, methomyl and endosulfan had occurred; the low to medium level resistance to abamectin, tebufenozide and chlorfenapyr was found; But Spodoptera exigua was still susceptible to chlorfluazuron. Insecticides resistance developed very quickly with the larvae instars increasing.Cyhalothrin, fenvalerate and alpha-cypermethrin resistant strains (Cyh-R, Fen-R, Cyp-R) of S. exigua were respectively obtained by laboratory selection of a strain collected from Fuzhou, Fujian Province in 2000 for 12, 10 and 10 generations. After selection, their resistance increased by a 18.1,27.6 and 45.4 times respectively. When theselection pressure was moved, the resistance level decreased. The resistance ratio ofCyh-R declined from 522.9 to 60.8C after 10 generations). Fen-R from 669.7 to 90.8(after 8 generations ) and Cyp-R from 2737.2 to 180.5 ( after 9 generations ). This was found in field-collected resistant strain, too. Without contacting with pyrethriods, the resistance declined rapidly and then stayed at a 6-10 fold level. It seemed very difficult to recover susceptibility completely to the 3 pyrethriods.Cross-resistance was found in Cyh-R strain, which was 6.5 fold to fenvalerate, 5.6 fold to alpha-cypermethrin, 8.5 fold to deltamethrin and 12 fold to methomyl, but no cross-resistance to chlorpyrifos. phoxim, endosulfan. tebufenozide. Fen-R and Cyp-R strains showed significant cross-resistance to deltamethrin, but were still sensitive to cyhalothrin, methomyl, chlorfluazuron, tebufenozide and chlorfenapyr.The mode of inheritance of resistance to cyhalothrin, fenvalerate and alpha-cypermethrin in S. exigua was evaluated by log dose-probit mortality curves. The genetic analysis results indicated that cyhalothrin, fenvalerate and alpha- cypermethrin resistance appeared to be controlled by multiple autosomal genes and the major genes involved were incompletely dominant.The effects of cyhalothrin, fenvalerate and alpha-cypermethrin resistance on fitness in S. exigua were evaluated in terms of developmental and reproductive characteristics. Life tables of Cyh-R, Fen-R, Cyp-R and susceptible strain (S) were also constructed to determine relative fitness by intrinsic rate (R0) . The results showed that the 3 resistant strains possessed significantly reproductive and developmental disadvantages, including prolonged larval duration, lighter pupa weight, lower mean eggs number per female laying and decreased eggs hatching percentage when compared with S strain. Cyh-R, Fen-R and Cyp-R strains were calculated to have a fitness value of 0.39, 0.53, and 0.47 rel...