Microbe applications to soil lead to prey‐mediated effects on predatory mite with potential for a detrimental biocontrol outcome

BACKGROUND: Beneficial microbes are increasingly applied as biocontrol agents to enhance soil vitality and crop productivity. However, given that alterations in soil conditions can propagate across trophic levels, it is essential to evaluate the broader ecological effects and biosafety of soil-applied microorganisms on higher trophic organisms. This study investigates the impact of two beneficial soil-applied microbes, Bacillus cereus and Chryseobacterium cucumeris, on the soil-borne plant pathogenic fungus Fusarium oxysporum, cucumber, the two-spotted spider mite Tetranychus urticae and its predator Phytoseiulus persimilis. RESULTS: Both bacterial agents were effective against F. oxysporum and demonstrated the ability to be transmitted across plant-herbivore-predator trophic levels. Notably, B. cereus was vertically transmitted from adults to eggs in P. persimilis, whereas C. cucumeris was only detected in the digestive tract. Bacillus cereus had no significant effects on fitness of both mite species. In contrast, C. cucumeris significantly reduced survival and reproduction of T. urticae and survival of P. persimilis adults, but not egg production. Greenhouse trials confirmed that soil application of C. cucumeris around the roots of cucumber, and subsequent uptake in the foliage could significantly suppress the survival and reproduction of both mite species. CONCLUSION: This study demonstrates that soil-introduced biocontrol microbes can transmit across trophic levels and may exert non-target effects on predators. The findings underscore the importance of considering indirect ecological risks and biosafety implications for beneficial invertebrates when deploying microbial biocontrol agents in integrated pest management systems. © 2026 Society of Chemical Industry.