I am not sure that root aphids (or are they really wooly aphids?) qualify as "critters" but they definitely are pests. I used to repot every plant with any sign of the dreaded white fuzz, after spraying the roots with insecticidal soap and alcohol, but now I find myself taking a short cut that seems to be working.
In the past year I have had real problems with root aphids, too many plants to repot and also at times (during the summer) when allionii really do not like to be disturbed, so I now take the top layer of dressing (usually gravel or coarse sand) and spray a 50% alcohol solution around the crown. I keep spraying until the alcohol solution runs out the bottom of the pot. Sometimes I use a small brush dipped in alcohol to make sure that no "critters" can lurk on the stem.
The plants seem to react well to this treatment and I can use it at any time of the year. I still repot the really, really sickly plants, but instead of repotting 20 or 30 affected plants, I am now down to doing 8 or 10.
Are these the critters you have?