Monoterpenoids: Eco-Friendly Slug Control Solutions for Sustainable Agriculture

Slugs pose a significant threat to agriculture, causing substantial crop damage and economic losses. Traditional molluscicides harm nontarget organisms and endanger ecosystems. Environmentally friendly molluscicides that ensure safety for humans and animals and promote sustainable agriculture are urgently needed. This study is the first to explore the fumigant toxicity, contact toxicity, and repellent effects of four monoterpenoids (geraniol, citral, geranyl nitrile, and citronellal) against Philomycus bilineatus. Compared to metaldehyde, monoterpenoids exhibited stronger contact toxicity, leading to rapid slug elimination (within 4 h), while metaldehyde caused death in slugs over a longer period of 48 h. Among the monoterpenoids, citral has weaker contact toxicity compared with the other three compounds, and geranyl nitrile (GN) and citronellal have stronger fumigant toxicity than the other two compounds. In terms of repellence and antifeeding effects, citral has the weakest toxicity. GN and citronellal stand out with excellent molluscidal activity (LC50 values of 0.10 and 0.14 g/L, respectively). The computational results based on the density functional theory (DFT) indicate that appropriate energy gaps, electrophilicity indices, and molecular softness correlate with the high molluscidal activity of monoterpenoids. From the computational results of this study, GN and citronellal, which have a moderate energy gap (approximately 0.200 eV), show the best effectiveness in controlling the slugs. In addition, unlike metaldehydes, which have a long half-life and limited biodegradability, these monoterpenoids have low toxicity and are easily degradable in the environment, making them more environmentally friendly. These findings underscore the potential of monoterpenoid compounds as eco-friendly alternatives for managing slugs in agriculture and gardens, thereby reducing reliance on traditional chemical pesticides.