I am very curious, however, as to why a lack of behavioral reaction to narcotics indicates that ant suffering is morally neutral.

The question of pain in insects is incredibly complicated, so please don't take my glib example as anything more than that.

But if ants don't have something analogous to opiods, then that would indicate that pain is never "bad" for them, which would be an (non-conclusive) indication they don't suffer.