On root mealies, I've found the only successful treatment is Bayer Advanced Tree and Shrub (BATS). I make sure the soil is moist and then water with a weak solution of BATS. Recommended dose is one tablespoon per gallon of water.
Marathon is another trade name for Imidacloprid, it’s also in Bayer Advanced Plant Care, Tree & Shrub.
I usually only treat once, when moving them into winter shelter in the fall. Through transpiration, it will also kill surface mealies or other critters.
If they are only on the surface of the plants, it is quick and easy to use a Qtip in alcohol and torture the fuzzy little critters until they cry "uncle".
