Root aphids have several stages of life, the first stages are in the roots and when the population is advances you will have winged fliers that spread the population. So if I am dealing with root aphids I need to get them at the source, the larvae. Thats why the root drench for root aphids. Aphids are evil, some species can lay eggs and even give live birth to clones asexually. The imidacloprid is a systemic pesticide so it will spread throughout the plant tissue therefore harming insects that would feed off the plant. It does exhibit a knockdown effect (kills rather quickly on contact) therefore it's best to apply directly where the aphids are present, so a dip and drench combined is the way to go. I saw what appeared to be a winged flier in the picture so thats why I was thinking you need to drench as well as dip, either way the population is advanced if you have those crawlers. Do some research on aphids and see how difficult they can be to remedy. Acephate/Orthene 97 is another product that has proven effective, it's smelly stuff lol. You definitely have a battle on your hands thats going to require something stronger than insecticidal soap. Using a multipronged attack is best when dealing with such an advanced infestation. So read up on the options available and decide what your pest management protocol will consist of.