What is the ruling on killing harmless insects like butterflies, and does fearing them permit killing them?

Among insects, some have been commanded by the Shari'ah to be killed, such as scorpions, and some have been forbidden from being killed, such as bees and ants, unless they cause harm. The default is not to kill insects about which nothing has been specified, unless they cause harm and their harm cannot be averted except by killing them. Butterflies are not to be killed unless they are harmful and their harm cannot be averted except by killing them.