Question
The leaves of our California Valley Oak trees are covered with a white powder and the leaves are curled and dropping earlier than usual.  Some leaves appear to have a small larva on bottom that looks like aphids.  The white sticky powder is very messy on the deck and patio furniture.  Will this hurt the trees and is there a treatment for it?  This is the first summer we have noticed this dust on the trees.  Thank you for your assistance.

Answer
Sounds like cottony aphids, an insect.
These are insects that suck the plant juices from the plant and secrete a substance called honeydew which is high in sugar content and sticky. They can harm the trees if the population get high enough but the sap dripping is the major problem. The repeated defoliation will harm the trees growth.

Chemical Control - Contact Insecticides Numerous contact insecticides are registered for aphid control. Since aphids are often placed under considerable pesticide pressure in field crops and greenhouses, they may be resistant to certain categories of insecticides. Therefore, if you do not obtain reasonable control, consider rotation to another insecticide. Contact insecticides currently registered for aphid control include: acephate (Orthene),  bifenthrin (Talstar), chlorpyrifos (Dursban), diazinon,  malathion, nicotine sulfate, pyrethrum, rotenone, resmethrin, and tetramethrin + sumithrin.


Chemical Control - Systemic Insecticides Several systemic insecticides are useful in aphid control. Aphids have sucking mouthparts and are thus very susceptible to pesticides located in the plant vascular system. Some of the systemic insecticides also have contact activity. Systemics injected or applied to the ground are less harmful to beneficial insects. Systemic insecticides include: acephate (Orthene), Imidacloprid (Bayer Advanced Garden Tree and Shrub Insect Control) is a newly available systemic insecticide that can provide aphid control on trees for several months following application to the soil.
Check with your local nursery for the Bayer product and the amount to use. This will depend on the size of the tree.