I can't believe no one has yet mentioned using Coca-cola. That was
recommended to me about fifteen years ago for the love-bugs in Florida. It
seemed to do a good job of dissolving the smeared/dried-on bug guts. Just
put some in a spray bottle, moisten the surface, then cover with a cloth
soaked with more Coke. After a few minutes, wipe it clean.

Unlike some of the other solvents, this should have no adverse impact on
your decal adhesive.

Joe