In recent years, biological methods have been used widely for controlling stem-boring pests in forests. In this study, the larvae of Xylotrechus rusticus (L.) (Coleoptera: Cerambycidae) were infected with 4 strains of 2 species of Beauveria (Hypocreales: Cordycipitaceae). Larval detoxifying and protective enzyme activities were measured at different times after infection. The results showed significant differences in the pathogenicity of the 4 strains. The most virulent strain, Beauveria bassiana (Bals.-Criv) Vuill. strain BbCC01, caused 93.3% mortality, with an estimated LT50 of 4.69 d. The detoxifying and protective enzyme activities in larvae infected with strain BbCC01 changed significantly. To resist the infection, the activities of the detoxifying enzymes (carboxylesterase, glutathione S-transferase, and acetylcholinesterase) first increased and then slowly decreased. The activities of the protective enzymes (catalase, superoxide dismutase, and peroxidase) showed a similar trend. The enzyme activities peaked at 72 to 96 h post-infection. In this study, the B. bassiana strain BbCC01 proved to exhibit high virulence towards X. rusticus larvae. This research provides important information regarding the biological control of X. rusticus and elucidates the patterns of detoxifying and protective enzyme activities in the susceptible larvae.