Also, aphid feeding on flower buds and fruit can cause malformed flowers or fruit. The honeydew that drops from these insects can spot the windows and finish of cars parked under infested trees. A fungus called sooty mold can grow on honeydew deposits that accumulate on leaves and branches, turning them black. The appearance of sooty mold on plants may be the first time that an aphid infestation is noticed. The drops can attract other insects such as ants, that will feed on the sticky deposits.
Some aphids are very important vectors of plant viruses.
However, it is seldom possible to control these diseases by attempting to kill the aphid vectors with an insecticide. Aphids carrying viruses on their mouthparts may have to probe for only a few seconds or minutes before the plant is infected. Resistant varieties or sequential plantings may be helpful in reducing problems with some viruses that attack annual plants. They deposit several wingless young on the most tender tissue before moving on to find a new plant. The immature aphids or nymphs that are left behind feed on plant sap and increase gradually in size.