Caffeine-induced strong-to-slight hormesis in Drosophila ananassae: Physiological and developmental responses across two exposure regimes.

Caffeine induces concentration-dependent biphasic or hormetic effects in insects, yet its impact on Drosophila ananassae remains unexplored. We evaluated adult physiological traits (survival, fecundity, lipid content) under adult-only exposure and developmental traits under combined short-term parental adult and continuous offspring developmental exposure across a caffeine concentration gradient (0.6-4.8 mg/ml). Low concentrations (0.6-1.8 mg/ml) increased survival (LT50) approximately 1.6-fold over control (Kaplan-Meier; log-rank P < 0.01), moderately increased fecundity and adult lipid content. Proportions of pupae eclosion were near control, and eggs hatched were slightly but significantly higher than control (P < 0.05) at lower concentrations. Higher concentrations (≥ 3.0 mg/ml) were detrimental, decreasing survival (̴2.5-folds), fecundity, and lipid content, and progressively impairing performance. The proportion of pupae eclosion declined significantly beyond ≥3.6 mg/ml. Developmental duration increased for 2nd and 3rd larvae and pupae as concentration increased, indicating inhibition of post-embryonic development, whereas early embryonic and first-instar durations were unaffected. Mixed-model ANCOVA indicated significant treatment, isofemale line, and treatment × line interaction (P < 0.001), whereas dry body weight remained a non-significant covariate. After body size standardization, lipid content remained positively associated with fecundity across isofemale lines (R2 = 0.88-0.91, P < 0.001). Hormesis Index demonstrated stimulation for adult traits at low concentrations and inhibition at higher concentrations, with transition near 1.8 mg/ml. API showed improved integrated adult performance at lower concentrations, whereas DPI remained near control-equivalent levels or was slightly higher before both indices declined beyond 3.6 mg/ml. These findings suggest concentration-dependent caffeine hormesis, with lower concentrations increasing adult traits and maintaining near biological-maximum developmental performance under combined exposure, whereas higher concentrations impair physiological and developmental traits.