Synergistic Effects And Mechanism Of Thiamethoxam On Entomopathogenic Nematodes In Controlling Bradysia Odoriphaga

The chive maggot,Bradysia odoriphaga Yang and Zhang(Diptera:Sciaridae),the specific devastating underground pest,is the major pest that restricts Chinese chive production.Currently,the predominant strategy for the management of this pest is to apply conventional insecticides.This approach has not only led to the evolution of resista nce to pesticides,but also seriously affects food quality of Chinese chive.Entomopathogenic nematodes(EPN)is one of the important parasitic natural enemy,however,EPN field efficacy is often inconsistent and unsatisfactory.The strategic combination of biological controls with reduced application rates and dosages of chemical insecticide synergists may,therefore,represent a valuable approach for the suppression of soil insect pests.In order to reduce the dosages of chemical insecticide and increase the effectiveness of EPN,the influence of various biotic,abiotic factors and application technology on the control effect of low concentration thiamethoxam mixed with EPN in controlling B.odoriphaga were discussed,and synergistic combination and the key application technology were confimed.On this basis,bioefficacy of the combined application of EPN and thiamethoxam in different processing methods and its effects on enzyme activities in B.odoriphaga larvae were discussed,and the foraging behavior and attacking ability of EPN treated with thiamethoxam was further studied,and the synergistic mechanism of thiamethoxam on EPN in controlling the pest was expounded.The result of this research may have great significance for the widespread use of EPN both theoretically and practically.The main results are as follows:1.The combination effects of EPN and thiamethoxam in the temperature,soil moisture,thiamethoxam concentration,EPN concentration and different life stages of B.odoriphaga were conducted,and best synergism effects of EPN and thiamethoxam against B.odoriphaga were screened out.There were significant differences among six EPN species(Heterorhabditis bacteriophora H06(H06),Steinernema carpocapsae NC116(NC116),S.carpocapsae All(All),S.longicaudum X-7(X-7),S.feltiae SF-SN(Sf)and H.indica LN2(LN2))in mortality.A synergistic effect was found between six EPNs and thiamethoxam.The combination effects of three EPN species(Sf,All and LN2,100 IJs/larva)and thiamethoxam(15 mg/L)was 96.94%,89.92%and 96.64%,which caused significantly higher mortality than the other three treatments(X-7,H06 and NC116),respectively.The corrected mortality of the3rd instar B.odoriphaga larvae treated with the combination Sf(60 IJs/larva)and thiamethoxam(15 mg/L)was higher at temperature 20??25?,substrate moisture10%?18%.2.The application technology of Sf mixed with thiamethoxam was innovated,and the scientific application method of reducing the dosages of chemical insecticide and increasing control effect was put forward.The combination effect of Sf(0.75 billion IJs/ha)and thiamethoxam(1.0 kg a.i./ha)applied twice(28 day intervals)was significantly higher than those of using thiamethoxam alone(6.0 kg a.i./ha),Sf alone(3.0 billion IJs/ha)and Sf(1.5billion IJs/ha)mixed with thiamethoxam(2.0 kg a.i./ha).Compared to the single thiamethoxam application,the combination effects of Sf and thiamethoxam had the highest value at over 90%of the control effects for up to 6 weeks.The survived rate and the production rate of Chinese chive after the comb ination effect of Sf and thiamethoxam applied twice were 97.75%and 61.80%,respectively.3.The combination effect of Sf(60 IJs/larva)and thiamethoxam(15 mg/L)in different processing methods(combined application of Sf and thiamethoxam simultaneous ly,thiamethoxam-treated the larvae treated with Sf and Sf-treated the larvae treated with thiamethoxam)were significantly higher than those of thiamethoxam and Sf alone.The combined treatment can significantly restrain the activities of enzyme in larvae.The corrected mortality of B.odoriphaga larvae in the combined treatment group were significantly higher than those in treatments with the respective single agent,and reached the peak(96.61%)at 72 h.The protein contents of enzyme solution in B.odoriphaga larvae in the combined treatment group at 12 h,24 h and 36 h after treatment increased by 13.88%,46.87%and 57.99%,respectively,as compared with the control group,a nd were significantly higher than those in the control group,Sf treatment group and thiamethoxam treatment group.The activities of SOD,CAT,AChE and GTSs in larvae in the combined treatment group decreased by 47.48%,28.73%,71.04%and 29.97%at 24 h after treatment,and by 46.34%,42.22%,58.37%and 11.87%at 36 h,respectively,as compared with the control.The combined treatment can significantly restrain the activities of SOD,CAT,AChE and GTSs in larvae.The combination effect of thiamethoxam-treated larvae 36 h treated with Sf had the highest value at 76.35%and was increased by 4.40 times as compared with the untreated larvae treated with Sf.Compared to untreated larvae treated with Sf,the activities of SOD,CAT,AChE and GTSs in larvae in thiamethoxam-treated larvae treated with Sf treatment group decreased by 27.66%,14.61%,19.42%and 19.79%at 48 h,and by 33.71%,36.78%,24.13%and 3.26%at 60 h,respectively.The thiamethoxam-treated larvae treated with Sf treatment group can significantly restrain the activities of SOD,CAT,AChE and GTSs in larvae.The combination effect of Sf-treated larvae 24 h treated with thiamethoxam had the highest value at 87.16%and was increased by 1.65 times as compared with the untreated larvae treated with thiamethoxam.Compared to untreated larvae treated with thiamethoxam,the activities of SOD,CAT,AChE and GTSs in larvae in Sf-treated larvae treated with thiamethoxam treatment group decreased by 38.50%,38.30%,25.80%and 51.33%at 60 h,respectively.The Sf-treated larvae treated with thiamethoxam treatment group can significantly restrain the activities of SOD,CAT,AChE and GTSs in larvae.4.The foraging ability of thiamethoxam-treated Sf was significantly higher than that of untreated Sf in the processes of dispersal,orientation and recruiting.Compared to untreated Sf,the distance of thiamethoxam-treated Sf to water,larvae,pseudo stems and larvae+pseudo stems was increased by 22.52%,16.17%,8.88%and21.06%at 12 h,respectively.The dispersal of thiamethoxam-treated Sf to larvae+pseudo stems was higher at temperature 20??25?and pH=7.Compared to untreated Sf,the orientation effect of thiamethoxam-treated Sf to larvae+pseudo stems,larvae,pseudo stems and water was 1.46,1.21,1.57 and 1.80 times higher than that of untreated Sf.At the same time,the chemotaxis index(CI)values of thiamethoxam-treated Sf to larvae+pseudo stems was 0.23,showing high attractant,while untreated Sf was0.13,showing weak attractant.The recruiting effect of thiamethoxam-treated Sf to larvae,pseudo stem and pseudo stems juice-treated larvae were significantly higher than that of untreated Sf from 4 h to 12 h,respectively.The recruiting effect of thiamethoxam-treated Sf to pseudo stems juice-treated larvae had the highest value at 49.83%and was increased by 1.25?1.98 times highe r than that of thiamethoxam-treated Sf to larvae and pseudo stem5.The corrected mortality of B.odoriphaga larvae and the attack rate,consumption rate,daily maximum lethal amount and searching effect of thiamethoxam-treated Sf on B.odoriphaga larvae were higher than those of the untreated Sf,while the handling time was shorter than that of the untreated Sf.The corrected mortality of B.odoriphaga larvae exposed to thiamethoxam-treated Sf was significantly higher than that exposed to the untreated Sf,and was increased by 2.13 times as compared with that exposed to the untreated Sf at 6 h post treatment.When the concentration of Sf was 6 400 IJs/petri dish,the functional responses for killing action of untreated and thiamethoxam-treated Sf on the 3rd instar larvae of B.odoriphaga could be described by Holling's type II and?disc equation.The attack rate(a'=0.5592)of thiamethoxam-treated Sf increased by 42.76%,the handling time(T_h=0.0081 d)decreased by 44.90%,the consumption rate(a'/T_h)increased by 2.59 times,and the daily maximum lethal amount(N_amax)increased by 1.81(Holling's type II)and 1.41(Holling's type III)times,respectively,as compared with those of the untreated Sf.The searching effect of both untreated and thiamethoxam-treated Sf decreased with increasing larval density of B.odoriphaga.When the larval density of B.odoriphaga was 40 per petri dish,the lethal effect of thiamethoxam-treated and untreated Sf increased with increasing Sf concentrations,while the searching effect first increased and then decreased.The searching parameter and mutual interference parameter of thiamethoxam-treated Sf were higher than those of the untreated Sf.