The root aphids I dealt with had a couple different forms... a white crab/spider looking thing, a flyer that was similar to a fungus gnat and a thing that looked kinda like the pic you posted. If you are getting any yellowing (the leaves I saw looked like everything from a sulfur deficiency to a nitrogen deficiency, but mostly a magnesium deficiency). Here's what we did that got rid of them for over a year ago...
Bayer Tree & Shrub (has Imid like Merit, just lower dose)
followed by Spectracide (main ingredient is Triazicide, maybe that can be located somewhere local to you?)
followed by
Azamax drenches once a week
a p-bomb was let off during that too
If I had to do it over, honestly, I'd skip the Merit. The body count was way higher from the Triazicide than a week's worth of bodies from the Bayer. I'll totally head to Home Depot for you if that's the route you pick (no one around me sells Merit that I know of). PM me if you'd like me to help.
Incidentally, the Spectracide killed a fungus gnat problem too.