Euphorbia (?) infected with grubs?

[stephenAZ]

I have this euphorbia (not sure which kind) that until recently was 3 healthy spires the same height. The other day one of them completely rotted at the based and fell over. I cut off the rotted part and replanted it (the short one now), but while I was digging (ground very soft) I found a couple of these grubs. Is this an insect infestation? (see attached images)

[mcvansoest]

If they were in the plant then maybe but if just the soil there are plenty of critters that make their home there especially in soil that gets regular water without it being an infestation.