Implementing a predator-mediated pathogen dissemination system can meet the goal of simultaneous biocontrol of harmful mites and other pests. But for the system to be effective, it must satisfy the requirement of high lethality of the pathogen to the target pest(s) with low lethality to predatory mites in a suitable environment. Prior testing showed that the fungal entomopathogen Isaria fumosorosea (Wize) Brown & Smith exhibited high toxicity to aphids yet low toxicity to the predatory mite Neoseiulus cucumeris (Oudemans). In practical application, the humidity conditions of conidia carried by predatory mites to plants are not necessarily constant. Therefore, we set two different humidity conditions to observe the changes of spore attachment quantity and the differences of germination and infection between predatory mites and aphids after their bodies were stained with spores. Under 50±1% relative humidity (RH), predatory mites cleaned most spores in a short time; 24 hours after dusting, the number of spores (mean ± SD) on body was (0.64±0.10)×10³, just (4.43±0.35)% of that at 0h, but spores on aphids were kept (16.58±1.48)% ((6.46±0.91)×10³ spores) of that at beginning. There was a significant difference in the percentage of spores retained on aphids and predatory mites (p=0.000<0.01). Humidity could affect the cleaning behavior; under 95± 1% RH (the suitable humidity for conidia germination), more spores persisted than that under the 50 ± 1% RH condition; at 24h, the numbers of spores on each predatory mite and aphid were (1.00±0.10)×10³ and (10.02±2.87)×10³. Here, under a scanning electron microscope, we found that spores on the aphid's body germinated 36 hours after dusting under 95± 1% RH, and the I. fumosorosea mycelium had spread after 48 hours; many new mycelium and spores were produced after 72 hours. In stark contrast, few spores germinated on the mite's body, thus leading to very little damage to its surface. Different cleaning behavior and different morphological structure of body wall can both affect the attachment and infection of I. fumosorosea to aphids and predatory mites. In the future, chemical substance, enzyme and other factors should also be researched to explain the difference of infection of entomopathogenic fungi to different species.