By Jorob
United States 
Last year our apple trees were badly infested with woolly aphids. I have been told that it is a good idea to use "dormant oil" on all the bark in winter months to kill the eggs. However, I have also read that the aphids lay eggs in the roots. Is dormant oil effective, and is there any way I can apply it as a root drench?
9 Jan, 2017
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Never heard of Woolly aphids living on the roots of trees before. Interesting. Just read up on their life cycle.
9 Jan, 2017
Yes they go underground and infest the roots. The roots get so weakened that a tree can topple over. A very hard pest to control. If you have any other trees nearby such as elm which are infested and not on your property to control you have a big problem. Since you are in the USA, I would suggest that you contact your state or university agricultural extension agent for advise on control of this pest and for local knowledge of this pest infestation in the area where you live.
10 Jan, 2017
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No, dormant oil can't be used as a root drench, sadly. The only way that you can get all of the aphids living on the roots is to use a systemic insecticide. I would wait to do this until after blooming is done, to protect bees, and you may want to discard this year's crop. To keep them from coming back next year, use copious amounts of earthworm castings, and compost around the dripline in summer and fall. That will boost the tree's natural defenses to prevent re-infestation.
9 Jan, 2017