Rotten bulb

Hi guys, really Need your Help !!!
i bought a new Calla lily (zantedeschia ) plant few weeks ago. had a watering schedule every 3/4 days, soil was never overwatered. Today found white tiny bugs at the button of the pot (I know it's a sign of ovewatering), plus some leaves went yellow. I Took the plant out of the pot (soil was dry-ish) but the bulb itself began to rot and was covered in white bugs.
Please Help, what should I do To save the plant ???
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In the sticks near Peterborough