Climate overrides prey availability in shaping the decadal population dynamics of two sympatric lady beetles in monsoon farmlands of central China

Background Under global warming, insect responses vary across regions, yet the population trends of dominant lady beetle species in central China remain poorly understood. Whether abiotic forcing (climate) or biotic interactions (prey availability) dominates their long-term population dynamics is unresolved. Methods We monitored lady beetle populations using black-light traps during March–November, 2015–2024, at a fixed farmland site in Luohe, Henan, China. Trap data were integrated with local meteorological records and aphid catches from a suction trap. Results A total of 21,572 individuals were collected, comprising Propylea japonica (10,511) and Harmonia axyridis (11,061). P. japonica showed pronounced interannual variation (range: 65–4,258 individuals) and exhibited three seasonal peaks in early June, mid-July, and late September. Its first capture date advanced significantly (−3.5 d yr −1 , P = 0.045), whereas H. axyridis phenology remained stable, generating clear temporal niche partitioning ( P. japonica dominant June–July; H. axyridis dominant August–October). H. axyridis catches increased significantly over the decade ( R 2 = 0.69, P = 0.0028). Stepwise regression identified May precipitation as a shared positive predictor for both species, while temperature effects were species-specific: June temperature was a positive predictor for H. axyridis , and previous-November temperature was a positive predictor for P. japonica (both models explaining >90% of interannual variance). Aphid catches showed no positive association with either predator; predator peaks lagged behind aphid peaks by ~48 days in spring, and summer predator maxima coincided with the seasonal aphid trough. Conclusions Climatic factors, rather than prey availability, are the dominant correlates of population dynamics for these two species, which respond asymmetrically to shared climatic drivers. The decoupling of predator activity from aphid densities indicates a limited numerical response, suggesting that abiotic forcing can outweigh trophic interactions in this monsoon-governed system. Conservation biological control should integrate long-term climatic monitoring rather than relying solely on prey-driven paradigms.