The aphids will not cause your tree to turn. They are just something that happens and will soon be gone. They can be knocked off with a strong spray of water. I am unable to enlarge the photo showing the trunk of your tree but from what I can see, there does not seem to be a 'flair'. The bottom of the trunk should not be straight like a telephone pole but rather flair out. If it does and I cannot see it, good. If not it is possible you have some encircling roots. That can happen when a tree is planted (or transplanted) too deeply. It has been in place long enough now where those unnatural roots have grown and the trunk has grown. They can compress the trunk and gradually kill your tree. The first thing you can personally do is remove soil from around the base of the tree until you find the first real roots (ignore the fine hair roots). Look to see if they are going in a circle. I am giving you a link to a publication on Stem Girdling Roots and another one on how to find an arborist. The pub on SGR is rather technical but you can skim it but make sure to look at the photos. There are leaves that match yours. If you feel this may be your problem, you may wish to have a certified arborist make an on-site diagnosis. http://www.extension.umn.edu/garden/yard-garden/trees-shrubs/img/guide-stem-girdling-roots.pdf   http://www.treesaregood.org/