Hackberry wooly aphid (Shivaphis celti) is an introduced insect which feeds off the sap of hackberry leaves, excreting copious amounts of honeydew in the process. Often honeydew laden areas beneath infestation may be covered in black sooty mold growing off of the excreted sugar mixture.
Often the first observed sign of a hackberry woolly aphid infestation is the sticky honeydew it produces. The aphids also secrete pale wax, which covers their bodies. These woolly aphids on shoot terminals and leaves appear as fuzzy, bluish or white masses, each about 1/10 inch or less in diameter. Winged forms have distinct black borders along the forewing veins and their antennae have alternating dark and light bands.
Check for the insects to confirm that the cause of honeydew is aphids and not the citricola scale. Citricola scale (Coccus pseudomagnoliarum) is the only other honeydew-producing insect that infests hackberry. Citricola scale females and older nymphs (immatures) are brownish to gray, oval, slightly dome shaped, and occur on twig bark from fall through spring. Because the scales are immobile most of their life and their mottled gray to brown color blends in with bark, these scales are easily overlooked. In the spring female scales produce tiny flattened, orangish nymphs that settle and feed on the underside of leaves during spring and summer, then move in fall to overwinter on bark.
Chemical control options are available and infested plants must be thoroughly treated. Contact insecticides and horticultural oils are usually ineffective against aphids with waxy protective coatings; systemic insecticides are sometimes effective in these cases. Aphids are notoriously difficult to control and nearly impossible to eradicate completely. An integrated pest management approach is usually the most effective. Predatory/parasitic insect populations usually keep aphid populations in check; insecticide applications that kill beneficial insects may result in aphid outbreaks.