Beyond acute toxicity: Lethal and delayed developmental effects of Nepeta cataria and Piper aduncum essential oils on Spodoptera littoralis

Botanical insecticides are increasingly explored as alternatives to synthetic counterparts; however, their biological effects are often evaluated mainly through acute mortality, with limited attention to delayed developmental consequences. This study investigated the lethal and sublethal effects of essential oils (EOs) from Nepeta cataria L. and Piper aduncum L. on Spodoptera littoralis . The chemical profiles of the EOs were characterized by gas chromatography–mass spectrometry (GC–MS), and acute toxicity was assessed using topical bioassays. Surviving larvae exposed to a fixed sublethal dose (0.020 µL EO insect⁻¹) were monitored for development, body weight, body dimensions, chrysalis formation, and adult emergence. The EOs showed distinct chemotypes: N. cataria EO was dominated by monoterpenoid constituents, particularly 1,8-cineole, whereas P. aduncum EO was characterized by phenylpropanoids, particularly dillapiole. Both EOs induced dose-dependent mortality, with P. aduncum EO showing higher topical toxicity than N. cataria EO, based on median lethal dose (LD 50 ) values of 0.075 and 0.196 µL EO insect⁻¹, respectively. At the sublethal dose, neither developmental duration nor body weight was significantly affected, whereas stage-dependent body-dimension differences were observed in the measured subset of individuals. Most importantly, adult emergence was significantly reduced in both EO-treated groups, while chrysalis formation was not significantly affected. The lowest numerical emergence was observed following P. aduncum EO exposure. These results indicate that EO exposure can impair pest performance through delayed carry-over effects that would be overlooked by acute mortality assays alone. Integrating development and life-history endpoints may therefore improve the evaluation of EO-based pest management tools.