i have a large 60 year old oak tree that rains 'sap' from aphids and scales throughout the growing season covering the house, cars and all the other plants with black sticky soot. Any good ideas how to control this?
In this area, oaks are not usually plagued by piercing-sucking insects. Nevertheless, chances are that the tree is stressed or over-fed with nitrogen fertilizer. Is the tree in a lawn? Does the lawn receive fertilizer? Some trees may be infested for a few years running, but healthy trees usually don't stay infested. Other than a few species, including Liriodendron (tulip poplar) and the smooth-leaved Tilia (limes), most trees seldom sustain the kind of insect attack you describe. The black stuff is actually sooty mold, which feeds on the deposited honeydew (honeydew is nothing more than tree sap that exits the rear end of the insect). Some people advocate carefully pressure-washing the stems of an infested tree in winter to physically dislodge the overwintering scale insects. A dormant oil spray applied in January or February will smother both overwintering scales and aphids, and their eggs, but this and pressure washing may be impractical on a large tree. Otherwise, improve the health of the tree and do what you can to encourage biodiversity. Ensure the tree is deeply watered in summer. Do not encourage soft growth (e.g., by pruning, frequent shallow watering or the application of nitrogenous fertilizers). Encourage bird visits (small birds, like sparrows, chickadees and hummingbirds feed voraciously on such insects) by keeping cats indoors and providing a safe drinking water source for them.
