I have a gall aphid problem, having removed over 300 from one tree?

In the United States, we can apply a systemic insecticide to the soil near the stem in the fall. The tree will absorb the product and translocate to the crown resulting in control of gall aphids on most tree species the following spring. The systemic insecticides are not available for landscape use in Canada. Applying horticultural oil in late winter may suppress eggs that overwinter on the tree. Applying an insecticidal soap spray just prior to insect emergence may also help suppress the pest.