Drought stress modulates indirect defense via bottom‐up effects in tomato and wheat

BACKGROUND: Biotic and abiotic factors induce bottom-up effects that can be used to enhance indirect plant defenses in agroecosystems. However, the feasibility of integrating biological control with drought-induced plant defense remains largely unexplored. We tested under laboratory conditions how three water regimes (optimal, moderate and severe drought stress) affect plant traits and the attraction of natural enemies, in infested and uninfested tomato and wheat plants. Plant morphological parameters and expression of defense-related genes were measured. The olfactory responses of Encarsia formosa and Cryptolaemus montrouzieri to tomato volatiles, and Aphidius colemani to wheat volatiles were investigated. RESULTS: Moderate to severe water regimes significantly reduced stem diameter on both systems and overregulated ASR1 and PIN2 on tomato. PR1 was overexpressed only under severe stress on wheat. Multiple olfactory responses among the tested natural enemies were observed. Tomato plants under moderate drought stress significantly attracted E. formosa without infestation. C. montrouzieri significantly preferred moderately water-stressed plants when no pest occurred, but the predators chose optimally watered plants over highly drought-stressed plants under pest infestation. A. colemani showed significant preference towards optimally watered plants without the host presence, except when compared to infested moderately drought-stressed plants. CONCLUSIONS: Our results suggest that water stress and pest infestation activate plant defense mechanisms with multiple olfactory consequences on the associated beneficial arthropods. These findings could be used for implementing biocontrol strategies within the IPM context under a changing climate scenario. © 2026 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.