Predation on the egg of Coelostoma stultum (Coleoptera: Hydrophilidae) by the alien species Deroceras laeve (Mollusca: Gastropoda: Agriolimacidae)

Slugs are polyphagous herbivores that damage various crops. The genus Deroceras is primarily herbivorous; however, certain species are known to feed on various types of organic matter. In this study, we documented field observations of predation by Deroceras on eggs of the water scavenger beetle Coelostoma stultum at the waterside of paddy fields in central Japan and identified the predator species by DNA barcoding. Molecular phylogenetic analyses revealed that the slugs found sympatrically with C. stultum are Deroceras laeve, whose two haplotypes are widely distributed throughout the Palearctic and eastern margin of Asia. In two of three observed predation events, D. laeve thrust its head into the egg cocoon and preyed on eggs while C. stultum females were spinning. In the other event, no females spinning egg cocoons were observed. Due to the relatively large body size of the slug, it may push the female beetle away and prey on eggs. Considering the native distribution range of these two species, invasion of D. laeve has resulted in a new combination of predator–prey relationships in Japan between D. laeve and native C. stultum. It is a matter of concern that slugs may establish predatory pressures that threaten C. stultum.