Study On Sublethal Effect Of λ-Cyhalothrin And Thiamethoxam On Harmonia Axyridis

Diaphorina citri is an important pest of citrus and other Rutaceae plants,which affects plant growth and causes sooty blotch by sucking buds.It is also the main transmission vector of Huanglongbing,a devastating citrus disease.Harmonia axyridis is an important predator insect in the natural agro-ecological environment.It preys on both nymphs and adults of the D.citri.At present,the control of D.citri mainly depends on chemical pesticides.During the spraying process,H.axyridis will inevitably be affected by lethal and sublethal effects of the pesticides.In order to identify the sublethal effect of insecticides on ladybeetle and coordinate the biological control and chemical control of D.citri,this study selectedλ-Cyhalothrin and Thiamethoxam,commonly used insecticides for field control of D.citri,as test pesticides,and D.citri as test prey.The effects of the sub-lethal concentration of LC₅₀ on the virulence,growth and development,predation function and detoxification enzyme of H.axyridis were studied,and the safety of the two pesticides against H.axyridis was evaluated,which provided a theoretical basis for coordinating the field application and chemical application of H.axyridis.The main research results are as follows:1.By leaf disk method,the LC₅₀ ofλ-Cyhalothrin against D.citri was 20.861 mg/L,and the regression equation of toxicity was y=-1.970+1.493x.The LC₅₀ of thiamethoxam was 1.658 mg/L,and the regression equation of toxicity was y=-0.302+1.458x.By dipping method,the LC₅₀ ofλ-Cyhalothrin against the 2nd instar larva of H.axyridis was 0.870 mg/L,and the toxicity regression equation was y=0.086+1.423x.The LC₅₀ of thiamethoxam was 2.030 mg/L,and the toxicity regression equation was y=-1.226+3.987x.Safety factor indicated that both pesticides had high risk to the 2nd instar larva of H.axyridis.2.The two pesticides significantly affected the developmental duration of H.axyridis.After the second instar larvae of H.axyridis were treated with sublethal concentrations of the two pesticides,the instars of the second instar(F_（2,54）=51.605,P<0.05)and the third instar(F_（2,52）=7.319,P<0.05)were significantly extended,and the instars of theλ-Cyhalothrin treatment group were extended by 0.96 d and 0.42 d,respectively.Thiamethoxam was prolonged by 0.37 d and 0.43 d,respectively.After treatment with thiamethoxam sublethal concentration,the 4th instar(F_（2,51）=13.550,P<0.05)and pupal stage(F_（2,45）=172.484,P<0.05 were significantly extended by 0.87d and 0.65 d,respectively.3.The predation of H.axyridis by two pesticides had a significant effect.The model of predation function response of H.axyridis to citrus Psyllid belonged to HollingⅡtype,and the parameters of predation function response were changed after two treatments.The sublethal concentration of the second instar larvae of H.axyridis had the greatest effect on predation of D.citri,followed by the third instar larvae,and had no significant effect on the fourth instar larvae and adults of H.axyridis.The instantaneous attack rate of the second instar larvae was 0.5079 and 0.4972,lower than that of the control,and the instantaneous attack rate of the third instar larvae was 0.8729and 0.9042,lower than that of the control,respectively.The predation efficiency of the second instar larvae of H.axyridis decreased to 9.41 and 7.31,respectively,which decreased by 85%and 88%compared with the control group.The predation efficiency of the third instar larvae of H.axyridis was 68.70 and 63.68,respectively,which decreased by 34%and 43.7%compared with the control group.The daily maximum predation of the second and third instar larvae of H.axyridis decreased,but the daily maximum predation of the fourth instar larvae and adults of H.axyridis had non-significant effect.4.Two pesticides affected the detoxification enzyme changes in H.axyridis.The specific activities of carboxylesterase Car E and glutathione S-transferase GST of the second instar larvae were significantly increased after treatment withλ-Cyhalothrin sublethal concentration,but the concentration of cytochrome P450 had no significant change.After thiamethoxam sublethal concentration treatment,carboxylesterase Car E and cytochrome P450 of the 2nd instar larvae of H.axyridis were significantly increased,while glutathione S-transferase GST had no significant change.