A sticky substance is dripping from my tree onto my car. What is it?

That sticky substance coating your car (and everything else under the tree) is honeydew — a sugary waste product excreted by sap-sucking insects. It's not sap from the tree itself, but rather the byproduct of an insect infestation in the canopy.

Common honeydew-producing insects:

• Aphids — the most likely culprit. Tiny (1-3mm) soft-bodied insects clustered on leaf undersides and new growth. Many species, many colors.
• Scale insects — look like small bumps on twigs and branches. Soft scales produce copious honeydew; armored scales do not.
• Whiteflies — tiny white-winged insects that flutter when disturbed. Common on citrus and ornamental trees in warmer climates.
• Leafhoppers and psyllids — small jumping insects that also produce honeydew.

Secondary problems from honeydew:

• Sooty mold — a black, crusty fungus that colonizes honeydew-covered surfaces. It blocks sunlight from leaves but doesn't directly infect the tree.
• Ant activity — ants "farm" aphids and scale, protecting them from predators in exchange for honeydew. Seeing lots of ants marching up your tree? They're likely tending an aphid colony.

Solutions:

1. Blast aphids off with a strong stream of water — this is often enough for mild infestations.
2. Encourage natural predators — ladybugs, lacewings, and parasitic wasps all feed on aphids and scale.
3. Horticultural oil or insecticidal soap sprays work well for accessible canopies.
4. Systemic insecticides (soil-applied imidacloprid) are effective but should be used cautiously — they can harm pollinators if applied to flowering trees.
5. Address ant colonies that may be protecting the pest population.

For more on diagnosing pest problems, see Common Tree Pests and Signs of Insect Damage on Trees.