You may have noticed a strange white webbing on your Post Oak and other associated trees this summer.  In some cases, the white webbing is falling to the ground like snow.  The wooly aphid is a sucking insect that lives on plant fluids and produces a filamentous waxy white covering which resembles cotton or wool.

The adults are winged and move to new locations where they lay egg masses. The larvae often form large cottony masses on twigs, for protection from predators. These aphids originated in Japan.
In flight they have been described as looking like "flying mice", and are given nicknames like "angel flies" or "fluff bugs".

Woolly aphids generally are not much cause for alarm, although they can cause rather unsightly damage to plants, which is particularly a problem for growers of ornamentals. Woolly aphid damage is aesthetic and rarely impacts the overall health of larger established trees. In most cases natural biological controls, such as lacewings, lady beetles, hover flies, and parasitic wasps keep woolly aphid populations below numbers that can damage trees, despite the appearance of distorted leaves.

Once leaf deformation begins, it is too late to prevent damage. Treatments can kill the woolly aphids but distorted leaves will remain on the tree for the entire growing season even after the aphids have moved on. Fortunately distortion of the leaves does not damage the health of the tree. The damage is primarily aesthetic despite peoples' concerns.

Removing honeydew from plants is not necessary or recommended as it does not harm plants.

Treating these aphids for the health of plants is usually unnecessary. However, in cases where the honeydew is a significant issue, treatments can be applied after the leaves have started forming (or as soon as honeydew problems are noticed) to kill the aphids and reduce the honeydew that is produced.  At the first of September, it is a little late to apply any treatment.