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.
Massive White Oak tree
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.
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