Abstract Aims The control of the common cockchafer Melolontha melolontha using the entomopathogenic fungus (EPF) Beauveria brongniartii is one of the most successful biological control systems. This study aimed to identify factors influencing the outcome of laboratory bioassays, which are crucial early steps in the development of biocontrol products, by using this system as a role model. Methods and Results We combined spray and injection applications of conidio- and blastospores of the host-specific pathogen B. brongniartii BIPESCO2 (Bip2) and the generalist EPF Metarhizium brunneum Ma 43 and applied the treatments to cockchafer adults and larvae. Furthermore, the mycotoxin oosporein was tested alone or with Bip2 blastospores, as well as Bip2 conidiospores, in immersion, spray, and injection treatments of larvae. The most efficient spore suspension was applied to different larval body parts and to their food. Bip2 and Ma 43 infected adults frequently, but larvae resisted topical spray applications. Injection treatments revealed that adult cuticles offered limited protection, whereas the larval cuticle acted as an effective barrier. Larval thorax and legs, with articulations and intersegmental membranes, were more susceptible than the abdomen. Oosporein synergized with blastospores in larval immersion treatments, but alone had no effect. We propose that oosporein’s antibiotic activity disrupts the larval cuticle microbiome, facilitating infection. Conclusion Contrary to the assumption that laboratory bioassays overestimate EPF performance under field conditions, we found the opposite. We therefore argue that more elaborate studies are required for realistic evaluation of candidate biocontrol agents, considering host–pathogen traits and test conditions.