That is beetle damage on your hostas. Not slugs or snails.
Malathion will work just fine and Sevin dust will do the job as well and may be a little easier to apply unless you get some malathion ready to go in an aerosol can. Sevin may stick around a little longer, if rain doesn't wash it off, and provide a bit more residual activity. You do not have to "blanket" every leaf with Sevin dust. I find putting it in a recycled plastic shaker bottle, like one used for spices with multiple holes in the top, works well and allows you to get a light dusting. As long as you can see the (light) coating of dust you've applied plenty.
Soap solutions only work if you contact the insects directly. The soap is not necessarily toxic to most insects nor does it have much repellent effect. Soap works by reducing the surface tension of water allowing crud to be washed off your hands - or to wet the insect's "spiracles" - their breathing apparatus which are actually a passive system of "tunnels" from the exterior of their exoskeletons leading to trachea in the abdomen where oxygen exchange occurs - and they suffocate - or rather drown. It takes a fair amount of soap solution to do the trick and thorough wetting is required.
Always use pesticides, in this case insecticides, in accordance with label directions. (I have worked in the crop protection business for 45 years.) Bryan
