Nope. “is” is correct. The preposition “of” removes “his filthy bachelor items” from the singular/plural discussion. Trust me, I had my knuckles rapped a number of times for these types of errors. Old guys remember these things. Not that it matters all that much, except when it rains and my knuckles ache
Is this near a marsh? They could be marsh mellow. Even though they have hard shells, they’re a bunch of softies….