Abstract: 【Aim】 The study aimed to screen a highly pathogenic Metarhizium anisopliae strain against Orthaga achatina larvae, to determine its virulence by feeding larvae with crude toxin and to assess the lymphocyte responses in the larvae. 【Methods】 The virulence of M. anisopliae strains and crude toxin to O. achatina larvae was assessed using mortality-time probability method and TDM model, respectively, and the changes of lymphocytes in treated larvae were observed through a microscope. 【Results】 When various strains are compared, Ma1291-2 strain exhibited the strongest virulence against O. achatina larvae, causing a corrected mortality of 99.8%±2.6%, the cadaver rate of 86.9%±1.3% and the half lethal time (LT₅₀) of 6.29 d after inoculation with (1.0±0.5)×108 spores/mL for 11 d. The crude toxin levels produced by different strains were significantly correlated with the corrected mortality and LT₅₀. A time-dose-mortality model was employed to estimate parameters of time and dose, showing that Ma1291-2 strain and its crude toxin had a strong lethal effect on O. achatina larvae during 6-7 and 3-4.5 d after treatment, respectively. The concentrations of total hemocytes, plasmatocytes, spherulocytes and oenocytoids reached the maximum at 2 d after the larvae were fed with the crude toxin of Ma1291-2 strain and then declined. The concentration of granulocytes rapidly increased at 2 d after the larvae were fed with the crude toxin of Ma1291-2 strain, and reached a peak at 3 d after treatment. However, the concentration of prohemocytes did not show any significant change during the first three days of treatment, but decreased significantly at 4 d after treatment. Rupture of plasmatocytes and granulocytes, and melanization of spherulocytes and oenocytoids were observed during 3-4 d after treatment with the crude toxin of Ma1291-2, but prohemocytes did not significantly change. 【Conclusion】 The results show that the immune response of larval lymphocytes to the crude toxin is the strongest during 2-3 d after treatment, and the crude toxin is toxic to lymphocytes. This study proves potential applicability of this pathogen in the development of biological control for O. achatina.

Key words: Metarhizium anisopliae, crude toxin, Orthaga achatina, pathogenicity, hemocyte, immune response