The GARDENA gardening expert
Dear Mr W.: There is only one possibility: water it with a systemic insecticide. This is adsorbed by the plant through its roots, and it is then distributed throughout the plant tissue, poisoning the aphids as they suck out the plant cells. I am sure that this isn’t a case of green spruce aphid infestation, as they tend to attack spruces and not firs. No matter: firs have their own aphids ... - One such systemic plant protection agent which is approved for used in private gardens and small gardens is, for example, the “Bayer garden watering agent against pests Calypso”. However, this is really toxic, so please handle it carefully.
As an alternative, you can have a gardener spray the fir with a backpack sprayer as soon as the first infestation damage appears. But it isn’t easy to spray a whole twelve-metre-high tree! For this reason, it would be simpler to administer the active ingredient via the roots.