Two species of aphids make birch trees their active feeding and breeding territory: the European birch aphid and the common birch aphid. European birch aphids are yellow in color while common aphids are larger and green in hue.
Aphids excrete honeydew as they feed and move around the tree foliage. Their honeydew draws sooty grey-black mold to the site, exacerbating their damage. Aphids can be managed with manual removal, biological controls and insecticides as a last resort.
Washing the Aphids Off
Spraying off the bugs with a strong stream of water from a garden hose can be effective in killing and displacing the pests quickly. Spray the underside of all infested foliage thoroughly and repeat as needed.
Introduce Biological Controls
Introduce natural predators of aphids to your birch trees. Ladybugs, lacewing beetles and parasitic wasps all hunt and feast on aphids and can get a population under control cleanly with little effort on your behalf other than buying the friendly insects and setting out the box in the tree.
Reduce Nitrogen Fertilizer Use
Scale back use of nitrogen-rich fertilizers, as this only serves to produce more food for the aphids. Switch to a granular slow-release fertilizer product to control the bursts of new foliage until the aphid problem can be contained.
Spray with any of the many anti-aphid insecticide products on the market such as dormant oil spray, insecticidal soap or neem oil. Apply according to label directions on a wind- and rain-free day when bees or other beneficial insects are not actively foraging in the area.
If the aphids seem to be congregating on a few key areas in the tree, prune those groups of foliage away and destroy them, to reduce the aphid population.