While Sevin may not kill hosta plants, it can potentially harm beneficial insects and affect foliage. Hostas have few problems than cannot be treated by proper sanitation, so chemicals like carbaryl --- the active ingredient in Sevin --- should be used only as a last resort.
Sevin is harmful if inhaled or splashed in eyes or on skin, possibly fatal if swallowed and a possible carcinogen, according to its Material Safety Data Sheet. It is toxic to fish, can kill bees in substantial numbers and may damage foliage when it's wet or humidity is high.
Of the dozens of pests that Sevin is effective against, cutworms, ants, sowbugs, exposed thrips, earwigs, grasshoppers and Japanese beetles are hosta pests. However, Sevin is not listed for use on slugs, the most common pest, or for deadly root and foliar nematodes.
Sevin and Hostas
Resort to Sevin for infestations of species on which it is effective, but never use it on flowers, where bees can pick it up. Avoid applying Sevin on delicate foliage, particularly in high humidity or after rain or irrigation. Surface application of granular Sevin in insect hiding places, like mulch or in barriers around hostas, is suggested as a safer alternative by the University of Wisconsin Extension.