Sublethal Effects Of Spinosad Applied To Frankliniella Occidentalis (Thysanoptera: Thripidae)

The use of insecticides in fields may result in multiple sublethal effects on insect pests along with direct mortality.The western flower thrips(WFT),Frankliniella occidentalis(Pergande),is a key pest of vegetable,fruit and ornamental crops.Spinosad is one of the few insecticides that control wester flower thrips very well and it can be used in integrated pest management(IPM) programs.In the present study, a sublethal strain(Sub) of WFT was obtained by selecting the susceptible strain(SS) of WFT with the LC₂₅ concentration of spinosad.The sublethal effects on the toxicity,biology,biochemistry in Frankliniella occidentalis exposure to low spinosad concentrations were systematically investigated, after the second larvae treated with LC₁₀ and LC₂₅ of spinosad.This study will be helpful to reveal the possible factors for pesticide inducing F.occidentalis recurrent outbreaks and resurgences.Selected continuously with LC₂₅,we obtained a sublethal strain which named Sub strain.The susceptibility of the second instards of Sub strain is decreased with the sublethal cycles increased.In a short time,Frankliniella occidentalis has obtained the higher tolerance.After the strain was selected by sublethal spinosad for 47 generations,its sensitivity to spinosad decreased 6.7 fold compared to the SS strain(the susceptible strain),indicating that Sub strain had development low level resistance to spinosad. Sub strain showed some cross-resistance to avermectins(2.68) and emamectin benzoate(2.92).Fipronil has good toxicity to both SS strain and Sub strain and this suggests that rotation of Fipronil with spinosad may be an effective resistance management strategy.After the second instar of SS strain exposure to LC₁₀ and LC₂₅ of spinosad respectively,the development time,survival rate,longevity of adult and the fecundity were effected.Especially,the pre-adult development time was prolonged for 0.57d by the treatment of LC₂₅,the survive rate from 2nd instar to adult was decreased for 30%,and the fecundity of parent was decreased for 51.3%.These biological value was resume in the offspring,nevertheless,the egg development in offspring treated with LC₂₅ was prolonged for 0.83d,and the hatch rate was decreased for 16%.Collectively,the sublethal effect of the treatment of LC₁₀ was lower than that of the LC₂₅,and the fecundity of the treatment of LC₁₀ in its offspring was increased.The sublethal effect in the Sub strain which was treated with LC₁₀ and LC₂₅ was lower than that of the SS strain.Except the developed time of pre-adult in the parent treated with LC₂₅ was prolonged,the fecundity was decreased and the developed time of egg to adult in offspring was prolonged,there was no effect in the adult longevity of the parent and the offspring,and also no effect in the fecundity of the offspring.The population parameters of the SS and the Sub strains showed that the mean fecundity of the Sub was increased 19 eggs than that of the SS strain which indicating that the fitness and the production were enhanced with continuous sublethal treatments.the TWOSEX-MSCHart life tables of different strains were built,the population parameters were decreased.However,the fitness of over multi-gene Frankliniella occidentalis ration exposure has gradually increased.After the strain was selected by LC₂₅ of spinosad for 25 generations,its biological characteristics were restored to near-normal levels and were even greater.This enhanced population growth is viewed as a major factor in recurrent pest outbreaks and resurgences.The second instar larvae of the SS and Sub strains(RR=5.2) were treated with LC₁₀ or LC₂₅ concentrations of spinosad and then the specific activities of the detoxifying enzymes were determined after 1 h,6 h,12 h,24 h and 48 h,respectivity.In generally,the specific activity of glutathione S-transferase,the mixed-function oxidases and the carboxylesterase in Sub Strain were higher than those in SS strain.For the sublethal treatd groups,the CarE activities in the Sub strain were higher than those in the SS strain.The GST and MFO activities of different treatment strages in Sub strain were transitorily lower than those in the SS strain,but higher with prolonged treatment time.The results indicate that the activities of the detoxification enzymes in the SS strain have a increased trend when the strain were seletced with with sublethal contration of spinosad for more than 36 generations.When the strain is treated with sublethal concentration,the Sub stain has stronger ability of regulating the detoxifying enzymes than that of the SS strain,indicating that in low level resistance,the increased activity of metabolic enzymes may contribute to the resistance mechanism of Frankliniella occidentalis to spinosad.