| Summary: | <i>Monochamus alternatus</i> is a serious trunk-boring pest. The isolation and utilization of entomopathogenic fungi to manage <i>M. alternatus</i> is important. Here, a new strain GQH6 of <i>Metarhizium robertsii,</i> isolated from the Loess Plateau, was identified morphologically and molecularly. The virulence of the strain GQH6 against the third-instar larvae of <i>M. alternatus</i> was studied. Then, the pathological process, including symptom observation and histopathological observation, was also researched. The corrected mortality was 100% at 10<sup>9</sup> and 10<sup>8</sup> conidia/mL, and 88.89 ± 5.88% at 10<sup>7</sup> conidia/mL. The LC50 was 1.93 × 10<sup>6</sup> conidia/mL and the LC90 was 1.35 × 10<sup>7</sup> conidia/mL. And the LT50 of the strain GQH6 was 3.96 days at 10<sup>9</sup> conidia/mL, and 4.99 days at 10<sup>8</sup> conidia/mL. These virulence indices showed high virulence against <i>M. alternatus</i> larvae. In addition, the symptoms of the infected <i>M. alternatus</i> larvae were obvious. After one day, dark spots appeared and increased in number. By four days, white mycelia appeared. Finally, the larvae body became green. Similarly, the histopathological changes after infection were obvious, mainly manifested in muscle tissue rupture, adipose tissue fracture and midgut disintegration. These results demonstrated that the <i>M. robertsii</i> strain GQH6 isolated from the Loess Plateau was highly virulent against <i>M. alternatus</i> larvae of the third instar.
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