| Summary: | The interactions between the entomopathogenic fungus <i>Beauveria bassiana</i> Balsamo (Vuillemin) (Hypocreales: Cordycipitaceae) and the entomopathogenic fungus <i>Isaria fumosorosea</i> (Wize) Brown and Smith (Hypocreales: Clavicipitaceae) were examined on young adults of <i>Sitophilus granarius</i> (L.) (Coleoptera: Curculionidae) and <i>S. oryzae</i> (L.) (Coleoptera: Curculionidae). Conidial suspensions of these entomopathogenic fungi were applied both separately and in combination, at three dosages, 10<sup>4</sup>, 10<sup>6</sup>, and 10<sup>8</sup> conidia/mL. Mortality of experimental adults was recorded daily for 15 days. An overall positive interaction between the pathogenic microorganisms was observed. Mean weevil mortality caused by the separate acting fungi, <i>B. bassiana,</i> ranged from 26.7% to 53.3% and from 36.6% to 63.3% for <i>S. granarius</i> and <i>S. oryzae</i>, respectively. The respective values for <i>I. fumosorosea</i> were 20.0%−53.3% and 46.7%−66.7%. The combined treatments showed a distinct interaction between the pathogens; for <i>S. granarius</i>, the interaction between the pathogens was additive in all combinations, whereas, for <i>S. oryzae</i>, the interaction was additive in seven and competitive in two of the combinations. Applying both entomopathogenic microorganisms may offer a method for weevil control that could be more effective than using each pathogen alone.
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