usb .... wallwart ..... power to shield

[hitsware]

I have a synth plugged into the Arduino shield on a Mega.
I have been using it stand alone with wallwart PS, keyboard, and TV type monitor (works great ! )
I want to switch to running it from PC via USB .
The USB powers the Mega itself, but doesn't power the shield.
1) Is there a way to make the USB power the shield also ? (prefered)
2) If not will it burn something with both USB and wallwart connected ?