Virulence and induction of immune responses by the two isolates of Beauveria bassiana against the larvae of Hyphantria cunea

Virulence of the two Iranian isolates of Beauveria bassiana was evaluated against the larvae of Hyphantria cunea through bioassay and immune responses. The median lethal concentrations (LC50) for AM-118 and BB3 isolates calculated as 8.8×105 and 2.3×106 spore/mL. by using dipping method. The LT50 values were calculated as 3.94 and 5.78 days, respectively for AM-118 and BB3 isolates. The highest numbers of the total hemocytes, granulocytes, and plasmatocytes were observed 3-6 hours post-injection of both fungal isolates. On the other hand, the total number of hemocytes increased from 160×104 to 480×104 cell/mL and the numbers of granulocytes and plasmatocytes increased to 200×104 and 140×104 cell/mL, respectively. The larvae injection by the conidia of AM-118 and BB3 led to the increase of nodule numbers at time intervals of 12-24 hours and the activity of phenoloxidase after 6-12 hours of post-injection from 0.12 to 0.2 U/mg protein. The isolate AM-118 showed the higher virulence compared to BB3 and it caused the more mortality in the lower time and spore concentrations. Moreover, the immune responses of the larvae toward this isolate were lower than BB3 isolate. The current results demonstrate that the two Iranian isolates of the entomopathogenic fungus, B. bassiana, are virulent against the larvae but they induce immune responses indicating physiological conflict between the pathogen and host.