Hi, I have just been in my garden planting some plants and I found these. I don't know if the larvae are the same as the pupae? The larvae are about 16mm long and the pupae about 10mm. The soil was quite damp but not wet as it was close to a pond. They were all about 7 to 10 cm below the surface.
They are probably from a beetle, the gray ones are actually the larvae and the white ones the pupae. Beetles are the largest group (order) in the animal kingdom, and with largest I mean most animal species on Earth belong to the beetle order (I remember that from zoology class at University). So it is hard to exactly tell you which species this is.
I think this is the larvae of one of the water beetles, possibly Dytiscus marginalis - they like water, but do spend time outside for various reasons, and prefer to do that near the water, preferably in damp soil. They're nothing for you to worry about in terms of your plants. Information and image of larvae (scroll down) here https://en.m.wikipedia.org/wiki/Great_diving_beetle
