Resistance evolution to basil essential oil: how selection pressure shapes tolerance and fitness costs in Aphis craccivora.

Essential oils (EOs) are increasingly proposed as sustainable alternatives to synthetic insecticides, yet their long-term evolutionary effects remain poorly understood. This study examined how two contrasting selection regimes with basil essential oil (Ocimum basilicum; estragole-rich chemotype) selection regimes, a stepwise escalation regime (+ 15% per generation) and a repeated constant-exposure regime with stabilisation phases at F7-F8 and F14-F15, influenced tolerance evolution and associated fitness costs over 15 generations. Stepwise selection led to limited tolerance (nymphs RRF10/F0 = 1.62) but severe fitness costs, with fecundity declining to 7.0 ± 0.55 nymphs/female at F9 and demographic collapse by F12. Conversely, repeated selection promoted higher tolerance without detectable initial fitness costs (F10 nymphs RR = 2.51; fecundity = 19.8 ± 0.80, comparable to the control). However, this apparent absence of fitness costs was transient; by F15, intensified exposure to 12.59% BEO reduced fecundity to 12.0 ± 1.12 nymphs/female despite sustained tolerance (nymphs RR = 2.06; adults RR = 2.60). All resistance ratios remained below 3.0. These findings demonstrate that EO tolerance follows non-linear, regime-dependent trajectories before conventional resistance thresholds are reached. Exposure structure, rather than dose intensity alone, shapes the stability and fitness cost of adaptation, underscoring the need for evolutionarily informed EO deployment in sustainable pest management.