Burning Leonardo bootloader onto ATMega32u4 via USB, not ISP?

I'm just curious, is an ISP interface necessary if I want to burn the Leonardo bootloader onto an ATMega32u4? Or can I do so purely using the Arduino IDE and the USB port?

zenwebb:
I'm just curious, is an ISP interface necessary if I want to burn the Leonardo bootloader onto an ATMega32u4?

Yes.

zenwebb:
Or can I do so purely using the Arduino IDE and the USB port?

No.