In the complex interplay among parasitic wasps, their insect hosts, and pathogenic microbes, the system involving Sclerodermus guani (Hymenoptera: Bethylidae) (a parasitoid wasp), Monochamus alternatus (Coleoptera: Cerambycidae) (the pine sawyer beetle, its host), and Beauveria bassiana (Hypocreales: Cordycipitaceae) (a fungus) presents a unique scenario where wasp offspring develop within a nearly sealed host gallery. This nursery is vulnerable to fungal invasion, often introduced by the foraging female wasps or M. alternatus itself, creating a three-way interaction where the fungus can infect both M. alternatus and S. guani. To assess how the route and timing of fungal exposure impact the S. guani population, we simulated this system by introducing different concentrations of B. bassiana either directly to the female wasps or to M. alternatus prior to parasitism. We further examined the effect of exposure timing by applying the fungus at different developmental stages of the S. guani offspring. Key population parameters, including the reproductive capacity of female wasps, the survival and developmental fitness of S. guani offspring and the germination period of hyphae, were measured. The results indicated that the most severe damage to populations of S. guani occurs when its host, M. alternatus, is infected by B. bassiana. Among the various developmental stages, S. guani offspring exhibited the greatest vulnerability during mid-to-late larval stages, whereas the egg and pupa within cocoon stages demonstrated a higher tolerance. We conclude that both the pathway and the timing of fungal exposure are critical factors influencing its impact. These findings provide valuable insights for optimizing the integrated use of biological agents in pest management, informing strategies that mitigate adverse effects on beneficial parasitoid wasps.

Pine wilt disease, caused by Bursaphelenchus xylophilus and vectored by Monochamus alternatus, is a devastating forest disease. The parasitoid Sclerodermus guani and entomopathogenic fungus Beauveria bassiana are effective biocontrol agents against wood-boring pests, but their compatibility remains poorly understood. We exposed adult female S. guani to three concentrations of B. bassiana conidia (1 × 10⁵, 1 × 10⁶, and 1 × 10⁷ conidia mL⁻¹) and assessed parasitoid mortality, infection, reproduction, and parasitism capacity against M. alternatus larvae. Mortality and infection rates increased significantly with concentration and exposure time. At 30 days post-exposure, corrected mortalities were 72.47%, 91.30%, and 100% for the three concentrations, respectively. Time–dose–mortality analysis yielded LT₅₀ values of 27.69, 20.00, and 5.92 days, confirming faster kill at higher doses. Fungal exposure reduced offspring production and development at high concentrations, yet fungus-carrying females still successfully parasitized M. alternatus. Importantly, low fungal concentrations caused only minor reductions in parasitism success and offspring survival. These findings demonstrate that the compatibility between B. bassiana and S. guani is strongly concentration-dependent. Therefore, optimizing fungal dosage is essential to balance fungal dispersal and parasitoid fitness in fungus-carrying biocontrol systems. Our study provides valuable insights into ecological interactions between entomopathogenic fungi and parasitoids, facilitating integrated pest management strategies for wood-boring pests.