Toxicity Changes in Different Passage Strains of Beauveria bassiana to Monochamus alternatus through Different Media

Two strains B5 and Bxs with high toxicity and two strains B11 and BZ with low toxicity to Monochamus alternatus were screened from 15 strains of Beauveria bassiana.Among these,B5,Bxs and B11 were isolated from M.alternatus,BZ was isolated from Dendrolimus sp..These 4 strains were used as the origin strains B0.Strains H1,H2,H3 and H4 were obtained by repeat passage through the larvae of M.alternatus, while strains R1,R2,R3 and R4 were obtained by repeat passage through the PPDA media.Toxicity of origin and passage strains was measured.Results showed that after being cultured on PPDA meida,the virulence decreased step by step between sucessive generations,irrespective of the high-virulent strains(B5 and Bxs) or the low-virulent strains(B11 and BZ). For each of the strains,significant differences in LT50 were observed between the mother generation and the filial different generations.Virulence of strains isolated from M.alternatus larvae had different change way from those not isolated from M.alternatus larvae,though all of them were cultured with the same batch of M.alternatus larvae.For example,the virulences of B5,Bxs and B11 strains isolated from M.alternatus larvae tended to increase after being reared for several generations,and the LT50 differed significantly between each of the generation.However,the virulence of BZ isolated from Dendrolimus larvae had no obvious difference between different generations.The difference in virulence changes in the B.bassiana strains may be related to specialization with their host insects.