Screening And Application Of High Virulence Beauveria Strains Against Agrotis Ipsilon

Agrotis ipsilon（Lepidoptera:Noctuidae）is an important underground pest,causing great yield losses in crops including tobacco,vegetables,cotton,and corn.At present,control of A.ipsilon mainly rely on chemical insecticides,but the long-term use of chemical pesticides leads to insecticide resistance,resurgence,and environmental pollution.Therefore,it is urgent to find environmentally friendly control strategies for the management of A.ipsilon.Entomopathogenic fungi have several advantages such as no pollution to the environment,less likely to develop resistance,and sustainable control of A.ipsilon.Beauveria bassiana is one of the most widely used entomopathogenic fungi.However,the efficacy of fungi itself on control of pests is low.Thus,the combination of botanical pesticides and B.bassiana can improve the control effect against pest insects.In additon,this strategy is also environmentally friendly.At present,there are very few reports on the combined application of B.bassiana and botanical pesticides to control A.ipsilon.In this study,the growth characteristics of nine B.bassiana strains were studied,and the virulence of these fungi against the second-instar larvae of A.ipsilon was determined.Bb324,the fungal strain with highest virulence was obtained.Subsequently,the compatibility of Bb324 with commonly used botanical insecticides was studied,and the control effect of the combination of azadirachtin with Bb324 on A.ipsilon was evaluated.The results are as follows:1.Screening of high-virulent B.bassiana strain against A.ipsilonA total of nine strains of B.bassiana were inoculated on SDAY plates to observe the characteristics of colony and spores.The results showed a great difference in the colony morphology of these strains.The sporulation and spore germination rates of Bbz2 and Bb324 strains were higher than those of other fungal stains（sporulation rates:6.17×10⁷and 5.33×10⁷spores/cm²,respectively;spore germination rates:94.47%and 92.40%,respectively）.Nine strains of B.bassiana were prepared into suspensions with a concentration of 2.0×10⁷spores/m L.The pathogenicity and median lethal time(LT₅₀)of Bb324 to the 2nd instar larvae of A.ipsilon were determined by immersion method.The results showed that Bb324 had the strongest pathogenicity,and the cumulative corrected mortality was 70.37%after 8 days of treatment.The LT₅₀（4.40 d）of Bb324 against A.ipsilon was also the shortest among the test fungal strains.2.Determination of pathogenicity and infection symptom of Bb324 on A.ipsilonFive concentrations（2.0×10⁴,2.0×10⁵,2.0×10⁶,2.0×10⁷and 2.0×10⁸spores/m L） of the spore suspension of Bb324 were used to determine the pathogenicity to the2nd-instar larvae of A.ipsilon by using impregnation method.The results showed that the LC₅₀of Bb324 was 13.86×10⁷spores/m L.The symptom of Bb324 infection to the A.ipsilon larvae was observed daily.In the early stage of infection,the test insect activity was weakened,and the body shrank after death.At 2nd and 3rd day,white hyphae grew on the foot,back and head of the insect;at 4th day,the larvae shriveled and shrunk,and the body surface was covered with hyphae;at 5th day and later,the mycelium on the body surface produced a large number of white conidia.3.Screening of high-virulent botanical pesticides against A.ipsilonThe toxicity of azadirachtin 0.3%SL,matrine 0.3%AS,and veratrine 0.5%SL to the2nd-instar larvae of A.ipsilon was evaluated by leaf dipping method.The results showed that azadirachtin 0.3%SL had the highest toxicity,with a LC₅₀of 3.05 mg/L（active ingredient,the same below）,followed by matrine 0.3%AS,with a LC₅₀of 34.22 mg/L.The toxicity of veratrine 0.5%SL was the lowest,with a LC₅₀of 112.13 mg/L.4.The toxicity and control efficacy of Bb324 alone and in combination with azadirachtin on A.ipsilonFirstly,the compatibility of Bb324 with azadirachtin 0.3%SL was determined.It was found that azadirachtin 0.3%SL had good compatibility with Bb324 at the dose of 0.3mg/L（active ingredients）.The results of laboratory toxicity test also showed that the combination of Bb324 and azadirachtin could significantly increase the mortality rate of the 2nd-instar larvae of A.ipsilon.Furthermore,the control effect of the combination of Bb324 and azadirachtin on A.ipsilon was evaluated by using pot experiment.The results showed that the control efficacy of the combination of Bb324 and azadirachtin was61.95%at 7 days after treatment,which was significantly higher than that of using Bb324 alone,indicating that the combination of the two could improve the control efficacy on A.ipsilon.Taken together,Bb324,the strain with good growth characteristics and high pathogenicity to A.ipsilon,was screened from 9 strains of B.bassiana.Bb324 had good compatibility with the botanical insecticide azadirachtin 0.3%SL.The results of laboratory and pot experiments showed that the combination of Bb324 and azadirachtin could significantly improve the control efficacy on A.ipsilon.Therefore,the results of this study provide an important basis for exploring the combined use of entomopathogenic fungi and botanical insecticides to control A.ipsilon.