Influence of Beauveria bassiana on the cellular defense response of the beet armyworm, Spodoptera exigua

Abstract Fifth instar Spodoptera exigua larvae were found to be highly susceptible to hemocoelic challenge of low dosages (50–500 cells/larvae) of Beauveria bassiana blastospores. At higher dosages (5 × 10 3 –5 × 10 4 cells/larvae), fungal challenge caused cessation in larval development and death within 2–3 days postinjection. A dosage of 5 × 10 2 blastospores/larvae, producing synchronous larval mortality within 72 hr, was selected for phagocytic studies. Total and differential hemocyte counts revealed that infection by B. bassiana caused a dramatic reduction in the major hemocyte class, the granulocyte, by 36 hr postchallenge. Fungal infection was also observed to inhibit filopodial formation and spreading of granulocytes by 24 hr postchallenge. At intervals during the injection cycle, the phagocytic competence of circulating hemocytes were evaluated with a second injection of fluorescent-labeled fungal cells. Results of these assays demonstrated that as the disease progressed, an increasing number of hemocytes were unable to phagocytose labeled fungal cells. Interestingly, at the 36 hr intervals, hemocytes were able to phagocytose a portion of fluorescent-labeled cells but did not recognize the in vivo circulating hyphal body cells. In summary, B. bassiana appears to possess a multifaceted capability for both suppressing and eluding the cellular defense response of S. exigua larvae.