I am not sure what type of grub or fat larvae worm you found but there is no spray to use on the soil to kill them. The best advice I can give you is to replant those begonias in fresh potting soil and watch for signs of the pest . It could be that the spring feeding cranefly laid the eggs in the fall and the grubs are hatching now. The larvae of cranefly have tough gray to brown skin and difficult to cut in half. So these larvae are called leather jackets due to their tough skin. By the start of summer these larvae will have changed into adult cranefly that look like large mosquitoes and then they fly away and no longer feed on plant roots. Keep growing, Marianne Binetti