Are you sure they aren't vine weevil grubs. There have been a lot of questions on these beasties over the years and I had my begonia corms eaten by them during the winter when they were (so I thought) drying safely in the garden shed. They love peat based compost so change it to compost without peat and use new soil each time you plant the corms.

This ^^^^^^

I found vine weevils merrily chomping their way through 2 of my begonias last year. if the maggots have a reddy brown head and bristly body then that is the most likely culprit.

you can by a preparation to water on the plants to kill them. but as said the peaty based compost is the main reason they are so prevalent. but they also infest gritty soils with sedums in them.

If I find tubers or bulbs in this state then I also give them a wash in a copper sulphate solution, to kill off any infection, then dry them off before storage. Copper sulphate is a bactericide and avilable, under various trade names, from the garden centres.