It is an eye-catcher: A driveway dotted with thousands of tiny discs, as if bumblebees golf students just graduated and tossed their woolen tam-o-shanter hats. The gardener looks up for the source, and then back down at the oak leaves that have fallen.
We see marks on the leaves and it's clear the pom-poms were once attached there.
The pom poms are galls, structures formed by the oak leaves in response to irritation by insects. The culprits are gall-making wasps -- but don't be misled by the name. These wasps are harmless to people, very tiny relations of the stinging species.
In each gall a wasp is incubating. It might emerge, fly up and lay its eggs at the base of an oak leaf bud. When those eggs hatch the cycle can begin again.
There are many kinds of gall maker, each calling forth from its oak a living compartment distinctive in size, color and shape. This gall is probably oak spangle gall, formed when the cynipid wasp Neuroterus quercusbaccarum is in residence. Spangle galls can vary in color, maturing to this greenish brown and ranging to bright red.
What these leaf gall makers have in common is virtual harmlessnes. That is, even in a gall-rich year, the host tree loses only the barest amount of leaf surface and energy to the gall makers.
So we can file this away in the "no action required" bin.
We estimate the plant's loss of energy-producing surface -- the amount of leaf tissue lost on an average leaf, and the proportion of damaged leaves to the whole.