Q. The lovely old White Oak tree in our yard appears to be dying. The oak leaves are turning brown and dropping. I have enclosed a sample in hopes that you can tell me what is wrong with my white oak tree, and what I can do to save it.
A. The sample revealed white oak leaves that had been damaged by a small, non-stinging cynipid wasp, Neuroterus saltatorius, also known as the jumping oak gall wasp. The leaves were badly off color or entirely brown, and completely stippled with small brown “spots” about the size of poppy seeds. Those “spots” are actually galls that contain a single wasp larva.
These wasps overwinter as mature larvae or pupae in the soil around host plants. The wasps that hatch in early spring are all female, and they promptly lay eggs in the opening buds of white oaks. Blister-like galls form on the leaves in response to this egg-laying activity. Both male and female wasps hatch from these eggs; they mate, and the females lay eggs in the leaves. The resulting galls develop in five to six weeks, appearing as small brown “seeds” on the undersides of the leaves.
When the galls mature, they drop to the ground. Once on the ground, larval activity causes the galls to jump around, hence their common name. Entomologists theorize that this movement allows the larvae to fall into crevices in the soil where they will be protected from cold winter temperatures.
Although jumping oak gall wasps are present every year, most of the time their numbers are low enough that the damage goes unnoticed. But periodically environmental conditions allow large numbers of them to survive and reproduce, resulting in very noticeable damage to affected trees. They may be bad for a year or two, but then go back to their normal low profile.
Oak trees support many different gall-making insects. They generally do not cause life-threatening damage to trees and chemical control is unnecessary. Jumping oak galls can cause premature defoliation and that is stressful for affected trees.
To protect affected trees from additional stress, water during hot, dry weather and mulch to conserve soil moisture, moderate soil temperatures, and to protect trees from lawn mowers and weed whackers. Remember that two to three inches of mulch is sufficient to achieve those benefits. More than that is too much.
Raking and removing fallen leaves can reduce the number of overwintering larvae and may help prevent the problem next year. Chemical control is not generally recommended for jumping oak galls, despite what seems to be tremendous damage. Although white oaks are the primary host for jumping oak galls in our area, other species of oaks serve as hosts in different parts of the country.