Slugs are significant agricultural pests and act as vectors for zoonotic parasites. However, current molluscicide options are limited and associated with substantial environmental risks. This study investigates the role of aldehyde dehydrogenase (ALDH) in the biosynthesis of farnesoic acid (FA), a key intermediate in the sesquiterpenoid hormone pathway, in two slug species: Philomycus bilineatus and Laevicaulis alte. Transcriptomic analysis revealed that both species possess conserved sesquiterpenoid biosynthetic pathways, yet they exhibit distinct levels of ALDH gene expression and differences in FA content. RNA interference (RNAi)-mediated gene silencing was employed to validate the potential of these candidate genes as targets for molluscicide development. Structural modeling of ALDH proteins using AlphaFold2 demonstrated notable divergence in the architecture of their active sites, suggesting species-specific enzymatic properties. Citral, a known inhibitor of ALDH, significantly reduced FA production in vivo and exhibited contact toxicity against both slug species. The lethal concentration 50 (LC₅₀) values were determined to be 378.2 g/L for P. bilineatus and 85.2 g/L for L. alte, respectively. Molecular docking analyses indicated that citral binds within the conserved substrate-binding tunnel of ALDH, potentially inhibiting the oxidation of farnesal. These findings establish ALDH as a critical enzymatic target for disrupting endogenous hormone biosynthesis in slugs and support the development of novel, eco-friendly molluscicides targeting the sesquiterpenoid pathway.