Prohydrojasmon treatment enhances aphid (Brevicoryne brassicae) biological control by improving the performance of key natural enemies in brassica juncea

Biological control, which relies on natural enemies to suppress insect pest populations, is a cornerstone of sustainable agriculture. However, its efficacy depends on plant-mediated interactions that influence predator and parasitoid behaviour. Prohydrojasmon (PDJ), a jasmonate analogue, primes plant defences against herbivores, but its effects on tritrophic interactions remain understudied. Here, we investigated how PDJ treatment of Brassica juncea L. (Brassicaceae) alters the behaviour and efficacy of three aphid parasitoids (Diaeretiella rapae, Aphidius gifuensis, Aphidius colemani) and two predators (Coccinella septempunctata, Harmonia axyridis) at 24 h (PDJ₂₄) and 48 h (PDJ₄₈) post-treatment. Using parasitoid landing assays, foraging trials, parasitism rate evaluations, olfactometer bioassays, predator preference tests, and volatile profiling, we assessed PDJ’s temporal impact on tritrophic dynamics. Results showed that PDJ₄₈ increased foraging duration and parasitism rates in D. rapae (at 24 h and 48 h) and A. gifuensis (48 h only), while A. colemani remained unaffected. PDJ₄₈ also enhanced H. axyridis settlement and aphid consumption, with predators showing no preference for PDJ₂₄-treated plants. Olfactometer assays revealed stronger attraction of D. rapae and A. gifuensis to PDJ₄₈-induced volatiles, aligning with delayed landing responses (24 h for D. rapae; 48 h for A. colemani) and prolonged foraging. PDJ₄₈ increased parasitoid retention on treated plants, suggesting improved host-location cues, whereas C. septempunctata exhibited no preference. These findings demonstrate that PDJ application at 48 h post-treatment enhances tritrophic interactions by synchronising plant defence induction with natural enemy recruitment, while PDJ₂₄ effects were transient or absent. Our study underscores PDJ’s potential as a sustainable tool to synergise plant resistance and biological control, offering a timed application strategy for integrated pest management in Brassica crops.