Cellular immune reactions of the sunn pest, Eurygaster integriceps, to the entomopathogenic fungus, Beauveria bassiana and its secondary metabolites

Abstract

In this study, five morphological types of circulating hemocytes were recognized in the hemolymph of the adult sunn pest, Eurygaster integriceps Puton (Hemiptera: Scutelleridae), namely prohemocytes, plasmatocytes, granulocytes, adipohemocytes, and oenocytoids. The effects of the secondary metabolites of the entomopathogenic fungus Beauveria bassiana on cellular immune defenses of Eurygaster integriceps were investigated. The results showed that the fungal secondary metabolites inhibited phagocytic activity of E. integriceps hemocytes and hampered nodule formation. A reduction of phenoloxidase activity was also observed. The data suggest that B. bassiana produce secondary metabolites that disable several immune mechanisms allowing the fungus to overcome and then kill its host. This characteristic makes B. bassiana a promising model for biological control of insect pests such as E. integriceps.

Figure 1.

(a–e) Light microscopy of Eurygaster integriceps hemocytes: (a) a prohemocyte with a large nucleus (thin arrow) and a thin cytoplasm; (b) a plasmatocyte exhibiting a spindle shape and cytoplasmic elaborations; (c) a granulocyte filled with the typical granules in the cytoplasm (arrow) and large nucleus (arrow); (d) an adipohemocyte with lipid droplets spreading in the cytoplasm and specific cytoplasm elaborations; (e) an oenocytoid with a round eccentric nucleus and crystalin inclusions. Magnification 40× with the exception of (b) (60×). Bar = 5 µm, with the exception of (b) (3 µm). High quality figures are available online.

Figure 2.

Phagocytosis of Beauveria bassiana spores by plasmatocytes. In vivo effect of secondary metabolites on the E. integriceps phagocytic activity on B. bassiana spores. Mean ± standard error, N = 30, *p < 0.05 vs. control. High quality figures are available online.

Figure 3.

Nodule formation in Eurygaster integriceps adults 12 hours after inoculation by Beauveria bassiana spores. Scale bar 10 µm. The magnification of images is 40×. High quality figures are available online.

Figure 4.

Double reciprocal plot to show the effect of different concentrations of Beauveria bassiana secondary metabolites on the phenoloxidase acyivity of Eurygaster integriceps adults (I/V_max = intercept on the I/V_o ordinate, -I/K_m = intercept on the negative side of the I/(S) abscissa). EtoH was used as control solution. High quality figures are available online.

Figure 5.

Phenoloxidase isozyme profiles. (1) Molecular marker, (2) control, (3) hemocyte lysate sample after 12-h injection of 15% concentration of secondary metabolite. Samples exposured to Beauveria bassiana secondary metabolite for 12 h versus control. High quality figures are available online.