Galls on Oak Leaves
Tom Ingram: Ask A
Master Gardener
Sunday, June 23, 2019
Q: I am noticing these round things about the
size of a golf ball that appear to be falling from my oak tree. What are they?
CS
A: What you are referring to is an Oak Apple
Gall. They are called Oak Apple Galls because they kind of look like small apples.
These curious growths are caused by a small wasp called a gall wasp. When the
gall wasp lays its eggs, it injects a toxin into the host plant, which
stimulates the tree to produce a protective cover around the eggs.
When the eggs reach
larval stage, they begin to feed and develop inside the gall. When ready, they
either emerge to create another generation or overwinter inside the gall,
depending on the time of year.
A variety of gall
types can form on leaves, twigs or branches. Typically, these galls do not harm
the tree; however, a large outbreak could disrupt nutrient flow within a twig
resulting in twig die-back. But this is the exception, rather than the rule.
There is another type
of gall that can be problematic that is the result of gall midges. The gall
midge lays its eggs on oak leaves and injects a toxin causing the leaves to
form a gall that looks like a leaf with curled up edges.
Unfortunately, the
larvae of the gall midge are the preferred food for the larvae of the oak leaf
itch mite. Oak leaf itch mites can be problematic in that they are microscopic,
and their bite can irritate our skin. In the height of an outbreak of oak leaf
itch mites, it would not be unusual for potentially 300,000 mites to fall from
the tree each day. That’s right, I said each day. Their bites can be painful,
and some people will develop red swollen bite marks.
Unfortunately,
treating your tree with some sort of pesticide is usually not an option due to
expense and the fact that the insects are living inside the gall, which makes
it hard for pesticides to reach them.
If you experience an
oak leaf itch mite outbreak in your yard, the best advice is to stay away from
the tree or wear long-sleeved shirts, long pants and a hat when working
outside. That sounds like fun in an Oklahoma summer, doesn’t it?
You can best control
these leaf galls through good sanitation: disposing of leaves and galls that
fall to the ground. If a branch has a lot of galls and you can reach it, you
could prune that branch and dispose of it.
The good news is that
large outbreaks are rare in our area, but if you come in from working in the
yard and feel a bite but don’t see anything, it just might be an oak leaf itch
mite.
Garden tips
- Vigorous, unwanted limbs should
be removed or shortened on new trees. Watch for forks in the main trunk
and remove the least desirable trunk as soon as it is noticed.
- Most varieties of mums are more
productive if “pinched back” now. Either pinch off with fingers or cut to
remove an inch or so of limb tips above a leaf. This results in the growth
of new limbs and a fuller plant. Do not pinch after mid-July or it will
interfere with fall blooming.
- Watch for tiny, sap-sucking
insects called aphids on roses, perennial flowers, shrubs and vegetables
(especially tomatoes). They produce a sticky substance called “honeydew.”
Many can be dislodged with a hard spray from your garden hose, or two
applications of insecticidal soap will usually greatly reduce any aphid damage
to your plants.
- Crapemyrtles are one of the few
shrubs that should be planted in the middle of summer. Growth of new roots
of these plants occurs best with summer soil temperatures.