Sounds like they have too much moisture. Try increasing the drainage by mixing some sand and gravel into the soil and/or relocating them to a southeasterly facing side of the house, or under some sort of protective overhang such as extended eaves or the edge of a carport, etc. You could also try some winter protection from the rain, such as removable row covers. The maggots may be secondary to the root rot, or just a coincidence and not related at all.

I don't know about the maggots but when my semps rot from poor drainage combined with cold temperatures they start turning somewhat translucent from the center spreading out until all leaves are mushy and translucent. Because of all the recent rain here my semps are covered. We'll see if I lost some anyway to wet and cold when I get home. When I lived in zone 6 with lots of snow I never lost a semp, winter or summer. Here I have to baby them winter and summer. The only times they are happy are fall and spring.

Hi, the root rot might be right, but it sounds like vine weevil to me. These are fat maggots with brown heads, they are particularly fond of living in pot plants and feed on the roots, until the plant literally falls out of the ground. I have had vine weevil on semps..my suggestion is if they have any base of the plant to put them into dryish sand and hope they grow new roots.In the UK you can either get nematodes (ie biological control) or use provado. Both give protection for about 6 months. Nursery's over here now have to use this kind of treatment or whole stocks are lost to the little blighters. Hope this helps, Mark