Study On Pathogenicity Of Beauveria Bassiana To Chilo Suppressalis And Synergistic Effect Mixed With Bacillus Thuringiensis And Tetrachlorantraniliprole

The Chilo suppressalis is one of the frequently occurring important pests of rice in China,and with a wide distribution,and has been ranked as one of the"top ten pests and diseases"that jeopardize food production in China,seriously threatening food production security.At present,the control of C.suppressalis in China is still mainly chemical control,and green control techniques such as biocontrol are the development trends of agricultural pest control in the future.To determine the biocontrol potential of Beauveria on C.suppressalis larvae,this paper studied the differences in pathogenicity and virulence of three Beauveria strains from different origins,the control efficacy of the screened strain on overwintering larvae and the effect on the growth and development of surviving larvae were determined,the effects of low temperature on the growth,germination and spores production of Beauveria,and synergistic effect of Beauveria mixed with Bacillus thuringiensis and low toxicity chemical insecticide tetrachlorantraniliprole on C.suppressalis in the hope of providing a theoretical and experimental basis for the highly effect ive application of Beauveria to control C.suppressalis.The following main findings were obtained for this paper:（1）One B.bassiana strain of highly pathogenic on C.suppressalis larvae was obtained,and its virulence on larvae of different insect ages and suitable stage for field control were clarified.Three Beauveria strains of Bb-YM,Bb170428 and NEAU30305 have been identified to have pathogenicity and virulence on C.suppressalis by bioassay.Among them,Beauveria bassiana Bb-YM showed higher virulence on C.suppressalis larvae and both low and high instar larvae,but there were differences in virulence,with the strongest virulence on 3^rd instar larvae(LC₅₀=3.61×10⁵spores·m L^-1),followed by 2^nd instar larvae(LC₅₀=8.25×10⁵ spores·m L^-1),thus establishing a suitable period for field control of C.suppressalis in this strain as the 2^nd-3^rd instar larval stage.The corrected mortality of C.suppressalis larvae treated with strain Bb-YM at 1×10⁹ spores·m L^-1concentration more than 90%,showed good application control potential.（2）Treated the overwintering larvae of C.suppressalis in late autumn with strain Bb-YM,significantly reduced the pupation rate and adult emergence rate of the overwintering larvae was determined and greatly reduced the effective insect source amount in the following spring.After harvesting in the autumn field,the overwintering larvae in rice culms was treated with strain Bb-YM at LC₅₀(8.25×10⁵ spores·m L^-1)and LC₈₀(8.66×10⁷ spores·m L^-1)concentration,up to the following yeaer end of emergence,larvae corrected mortality were 80.90%and 92.09%,muscardine rates were 62.32%and 75.36%,contemporary larval pupation rates were only 14.49%and 5.80%,and adult emergence rates were only 13.04%and 2.90%,which can significantly reduced the number of winter sources and have high control efficacy.Although the larval and pupal stages of the surviving individuals with the Beauveria treatment did not differ significantly from the non Beauveria control,but significantly shortened their pupation and eclosion periods.Beauveria treated overwintering larvae had a reduced female to male sex ratio.（3）It is clear that low temperature conditions exert different degrees of effects on Beauveria growth and development.Beauveria strain Bb-YM,Bb170428 and NEAU30305 from different species could germinate,grow and sporulate under 5-15℃,thus confirming that Beauveria could infest the overwintering larvae of C.suppressalis at low temperature conditions（such as fall or spring）,but can have a significant inhibitory effect on spore germination rate,colony growth and spores production,the lower the temperature,the stronger the inhibition,which is hardly to germinate or sporulate at 5℃.（4）It was determined that the mixture of strain Bb-YM with Bt64,tetrachlorantraniliprole had a synergistic effect on C.suppressalis larvae.The mixing of strain Bb-YM with strain Bt64 could obviously improve the insecticidal activity and pathogenicity on C.suppressalis larvae and reduce the lethality time.Its synergic ratio was elevated with increasing concentrations of strain bt64 spores,but the LT₅₀ was shortened The synergic ratio of strain Bb-YM mixed with Bt64 LC₅₀ and LC₇₀ was 2.14 and 3.97,and the LT₅₀was reduced by 3.65 d and 4.25 d.The mixed strain Bb-YM with the recommended dosage of tetrachlorantraniliprole at 1/3,1/5,1/7 showed a significant insecticidal effect on C.suppressalis larvae,and the synergistic ratios is 494.90,179.63,and 62.96,the synergistic effect was significant.The LT₅₀ significantly shortened and significantly increased the insecticidal speed.For C.suppressali control in outdoor bucket grown rice,the strains Bb-YM LC₇₀+Bt64 LC₅₀and Bb-YM LC₇₀+tetrachlorantraniliprole 15 000×treated C.suppressali larvae that the actual control efficacy was 76.52%and 80.35%,the control efficacy was not significantly different（P>0.05）compared with the recommended dosage of tetrachlorantraniliprole at 3 000×（84.51%）and was extremely higher than the same concentration strain and tetrachlorantraniliprole alone（P＜0.01）,had a significant synergistic effect,and greatly reduced chemical pesticide use level.