Q: We have two birch trees on the side of our house. My neighbor’s driveway is under them. They are dripping something on their cars. Does this mean they are diseased?
A: Birch trees sometimes become infested with a big crop of aphids, which drip their undigested honeydew onto nearby surfaces. Tell the neighbors you’ll apply some imidacloprid (Bayer Tree & Shrub, Bonide Systemic Granules, etc) in March to control them. I have more details and pictures at xrl.us/birchaphid.