Effects Of Surface Soil Environment In Tobacco Fields On Control Of Spodoptera Litura By Isaria Cataniannulata

Isaria cateniannulata is widely existing in nature.It has a good control effect on many kinds of mites,Lepidoptera and other pests and has a broad applying prospect in biological control.In this paper,Spodoptera litura,an important economic pest harmful to flue-cured tobacco production,was selected as the target pest.Based on the identification and indoor bioassay of 8Isaria spp.strains,GZUIFR04XS8 was obtained,which had high virulence to the prepupa and the second instar larvae of S.litura.And the pathogenicity of the strain to S.litura larvae under indoor conditions and field conditions was further defined.The conditions of killing S.litura pupae by I.cateniannulata strain GZUIFR04XS8 in soil were studied.The dynamic changes of population of the strain GZUIFR04XS8 in different soil environments were detected by selective medium containing Cu2+.So that the ideas and practical basis for controlling S.litura by I.cateniannulata can be provided.The main conclusions of this article are as follows:?1?Evaluation of the superior strain,I.cateniannulata GZUIFR04XS8Eight Isaria spp.strains were identified by morphology and molecular systematics.The lethal rates of each strain to prepupae and 2nd instar larvae were tested by soil mixing method and immersion method.The pathogenicity and pathogenic conditions of high virulent strains to2nd-5th instar larvae were tested,and the control effect was evaluated in the field.The results showed that strains GZUIFR08XS1,GZUIFR08XS8 and GZUIFR08XS13 were I.cateinannulata,strains GZUIFR08LYS4,GZUIFR04XS5,GZUIFR04XS5,GZUIFR04XS12 and GZUIFR04XS16 were I.farinosa,and 8 strains had pathogenicity to pupa and 2nd instar larvae of S.litura.When inoculated with the concentration of 2×10⁸ spore/mL by mixing soil method,the lethal rates of GZUIFR08XS1 and GZUIFR04XS8 to prepupae were all 100%.When the concentration was 1×108 spore/mL,the lethal rate of GZUIFR04XS8 to the second instar larvae was 78.21%,which was significantly different from other strains?P<0.05?and became the target strain.The lethal rate of GZUIFR04XS8 to S.litura larvae increased with the increase of spore concentration,and the corrected mortality rate was close to 100%at the concentration of 1×10⁹spore/mL.The LC₅₀0 of the 2ⁿdd to 5^thh instar larvae were respectively 9.64×10⁵ spore/mL,3.61×10⁶spore/mL,1.20×107 spore/mL and 6.47×108 spore/mL.The best infection temperature of GZUIFR04XS8 to the 2nd instar larvae of S.litura was 23?.In the field environment?near saturated humidity,temperature 21.3?25.4??,the applying amount of I.cateniannulata was5×10¹44 spore/mL,and the control effect of I.cateniannulata was 69.74%against S.litura spontaneously generated,and 80.40%against the artificially fed young larvae in the experimental area.?2?Control effect of I.cateniannulata strain GZUIFR04XS8 against S.litura prepupae in soilThe lethal rates of prepupae by mixing soil method,spraying method and soaking method were evaluated.The results showed that when inoculated with the concentration of 2×10⁷ spore/g by mixing soil method,the emergence rate of S.litura was the lowest and the stiff pupa rate was the highest which were respectively 23.33%and 70.00%.The test that GZUIFR04XS8 killing pupa under different soil conditions and different concentrations of mixed fungi found that when the temperature was 20?,the infection rate became the highest which was 88.33%.At the same temperature,the rate of stiff pupae in soil with 40%water content was the highest,which was90%.In the range of 104?108 spore/g soil mixed with fungi,the lethal effect of GZUIFR04XS8to S.litura increased with the increase of the amount of soil mixed fungi,and LC₅₀0 was 8.89×10⁵spore/g.When the temperature was 20?and soil moisture content was 40%,the lethal rate of strain GZUIFR04XS8 to S.litura pupae was 100%under the condition of 1×108 spore/g soil mixed with bacteria.?3?The discovery of Cu2+selective medium and the persistence dynamics of GZUIFR04XS8 in soilIt was found that the selective medium with 1.0 g/L CuSO₄ could better isolate GZUIFR04XS8 from soil and the formula was glucose 40 g,peptone 10 g,streptomycin sulfate0.1 g,penicillin potassium 0.1 g,copper sulfate 1.0 g,water 1000 mL.This medium had a good inhibition effect on bacteria and 4 kinds of common contaminated fungi,such as Penicillium,Aspergillus variegatus,Rhizopus and Mucor.The detection rate was 60.48?86.29%.The change of population of GZUIFR04XS8 in soil was greatly affected by temperature,water content,soil microorganism,soil texture and field inoculation.The population of GZUIFR04XS8 in soil showed a rapid decline in the early stage and a slow decline in the later stage.The high temperature environment was not conducive to the persistence of I.cateniannulata,while-5?was the most favorable for its persistence.The highest number of biomass was 546830 cfu/g which was kept for 180 days,and was 2.80 times,4.59 times and 14.32 times of that at 4?,15?and 25?during the same period.Sandy loam soil is the best soil for I.cateniannulata to persist.On the 180th day,the number of I.cateniannulata was 129366 cfu/g,which was 1.79 times as many as that in sandy soil and 2.99 times as many as that in clay soil during the same period.Soil microorganisms showed inhibition on the number of I.cateniannulata in the early stage,but promoted it in the later stage.The flora in sterilized soil was greater than that in non-sterilized soil in each persistent period.After 180 days of persistence,the flora in stertillized soil was 391029cfu/g which was 1.16 times as many as that in non-stertillized soil.In addition,in the field environment,the decline rate of flora of I.cateniannulata was accelerated,and the higher the initial amount of inoculation was,the more the flora was to be lost.It was found that the selective medium with 1.0 g/L CuSO₄ could better isolate GZUIFR04XS8 from soil.On the 90th day,when the initial inoculation amount was 1 g/kg,the flora could not be detected.However,when the initial inoculation amount was 5 g/kg,the flora of I.cateniannulata was 218775 cfu/g,which was respectively 1.23 times and 1.76 times of that when the initial inoculation amount was respectively10 g/kg and 15 g/kg.