Melissa,
Wasps, bees, etc. emerge full-grown. But they can be smaller than normal (or deformed) if they have had a tough diet as a larva; that's unusual, though. I'll have a think about your 1/3-size one.
Voles, etc.
Spreading underground: Well, Cupressus trees roots are about the most aggressive root spreaders I have come across. I once dug up a small cube of border soil (where I was having great difficulty getting anything to grow). The surface of the soil looked normal, but as soon as I hosed the soil, I began to reveal the mesh of roots. By the time I had finished hosing, I was left with something like a birds's nest or Shredded Wheat! Bermuda grass is also extremely good at spreading underground. You probably want something that spreads but gives other plants a chance. Mentha? Perhaps not sufficiently garden-worthy. I wonder if others that spread and root on/near the surface may stand a chance, eg. Rodondo creeper, Lippia nodiflora, Tecoma (Tecomaria) capensis, Viola odorata. And consider the stoloniferous form of Ranunculus asiaticus, as discussed
here already.
I have been trying to think of plants that may not only be
unpalatable to voles, but which may also
deter/repel them. My mind goes to camphor tree (Cinnamomum camphora), but (although I have seen some healthy specimens in Cyprus), that is too agressive, and the vapours inhibit plants as well.
There is plenty of advice/info online, eg.
- "Narcissi (daffodils) are poisonous to voles, and they will not eat them. By planting them throughout the planting beds, the garden becomes less attractive to voles."
"Rosemary deters voles. I once planted a few tulip bulbs around a rosemary plant. The voles ate all the bulbs in the garden bed except the ones near the rosemary plant."
[http://thesagebutterfly.blogspot.com.cy/2012/03/vole-in-garden-control-methods.html]
- Crown imperial repels voles
[https://en.wikipedia.org/wiki/List_of_pest-repelling_plants]
Mike