Q: Last summer, I noticed some small white, fluffy, cotton balls on my apple trees. Is it aphids or some sort of a fungus?
A: I think you’re seeing wooly apple aphids. They suck sap from the twigs on the tree but they also attack the roots of apple trees. The waxy filaments that cover individual aphids protect them from predators.
The good news is that they are not completely immune from beneficial insect attacks.
One of the best ways to attract good insects is to plant sweet alyssum, Lobularia maritima, underneath the trees. This spring-flowering plant is very attractive to insects like syrphid flies, which love to eat aphids.
You can buy alyssum plants at a nursery, but I think the easiest thing to do is to plant seeds in late April when the soil is warm. The plants will die in cold weather but they usually self seed. If you want to be sure you have plants every year, plan on seeding lightly each spring.
Also consider spraying the trees with horticultural insecticidal oil (click for sources) in winter. Oil suffocates aphids without harming good insects.