Hillsborough County Extension
5339 County Road 579, Seffner, FL 33584
(813) 744-5519
http://hillsborough.extension.ufl.edu
Live Oak (Quercus virginiana) Gall Maladies in Nursery
Production
Shawn Steed
Multi-County Environmental Horticulture Extension Agent II
Situation
Many gall organisms affect live oak trees. These consist of wasps, flies, and diseases. The most
economically important and destructive are the oak gall wasps. Galls are a biological response from plants in
relation to an elicited antagonistic parasite that causes a reaction from different plant tissues. Galls on oaks are
generally not very problematic and are mostly aesthetically damaging in a production setting. However, galls can
be quite serious in number and amount of damage as to create long term problems that can affect the potential sales
of heavily infected trees. Some trees may have damage to structural branches and central leaders that may inhibit
sales of heavily damaged trees. This occurs especially at the early stages of growth of one gallon or three gallon
trees. A one or three gallon tree whose central leader or structural branch gets stem galls at an early stage are
particularly susceptible. When the gall wasps hatch out, the oak will heal the wounds and the ensuing callus
formation will form a split stem or holes in the tree that will create an unmarketable tree unless the callus of the
wounds can fuse and heal over time.
A relatively benign leafy oak gall (Andricus quercusfoliatus
(Ashmead)) affecting the terminal shoot (Steed 2010).
Leafy oak galls on stems. Notice the spider webs associated with
galls. One of the beneficial organisms associated with live oak
galls.(Steed 2010).
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Illustrations of Damage
This figure shows a parasitized stem as host to the stem gall wasps. Notice the discolored wood in the cut surface and the larval chambers. Also in figure note the concave bumps along the top side of this shoot (right of cut surface), these are other brood chambers for
developing wasps. As these wasps emerge the oak heals these wounds from within and as the callus grows it will split the stem. If this is
a structural branch or a central leader and the visual appearance is bad, a loss of sales may result. (Meretely 2010)
A close up of cross section of stem parasitized by a gall forming wasp which is visible in the circular opening of the brood chamber
(Meretely 2010).
Stem galls on live oak shoot. Opposite side shows a leafy gall
(Steed 2010).
Two different types of stem galls on the same shoot (Steed
2010).
Symptoms
Many of the signs of non-economically damaging live oak galls are medusa-like symptoms at shoot-tip
terminals, or witches broom shoot tips, to small vase like structures on the undersides of leaves. Some of the more
damaging symptoms are the stem galls that infect small shoots and branches. These can leave very unsightly to
structural defective scars as the tree tries to heal from the wounds caused by the gall wasps. Some of these wounds
can be confused for mechanical injury or disease on stems.
Control
As with any other pest in a production setting, it is important to scout for signs and symptoms of this pest
and a determination must be made if treatment is necessary. Economic threshold for injury to the trees must be determined to decide when spraying will be appropriate. Dr. Eileen Buss entomologist from the University of Florida
suggests this when considering pruning and mowing infected branches to control gall wasps.
Pruning and dropping or chipping infested branches is not enough to control these wasps
- if pruning is done soon before the gall insects emerge, they could still come out and reinfest trees.
If damage is approaching economic threshold for making
pesticide applications, some biology must be determined before
treating. Dr. Buss has this to say...
Because we lack so much information on
the life cycle of each gall wasp, we can only talk
with some generalities. The optimal timing for control seems to be from just before bud break in
March to early shoot elongation. Most gall makers
(that I have worked with) are active during this
time. We've had success killing the wasps of the
bullet gall (Disholcaspis querucusvirens) before
they lay eggs into dormant buds in December with
Bifen XTS (or Onyx). I'd probably put it out in midMarch and follow up again after a month. It should
have a month's worth of residual, at least. Keep in
mind that it will also kill natural enemies, so if you
have a biocontrol program going on, it'll suffer from
a pyrethroid application.
Stem lesions from gall wasps (Steed 2010).
Control Continued
There are numerous biocontrol agents at work on live oaks that may go unrecognized. There are many small spiders that associate within and about gall structures as habitats. There are also lace wings, ladybugs, and natural
predator mites that might be benefiting the grower without notice within the canopy. Spraying may increase the
incidence of other pests not currently at economically damaging numbers for the grower. There are also many
kinds of beneficial wasps that parasitize the gall wasps (Frankie, et.al. 1978).
One way to identify timing on the emergence of gall wasps in the spring would be to put out sticky cards
around gall infected wood that has not yet hatched. These can be checked to find out when gall wasps are hatching
from the gall structures to start spraying. This will reduce the numbers of potential of egg laying adults that can
infest the next season.
Gall wasp larvae within a cross section of live oak stem (Mertely
2010).
Closeup of gall wasp larvae (Mertely 2010).
Further reading
Insect Galls of Floirda. SP 343. Hodges, A. Buss, E. Mizell, R. 2006. University of Florida, Institute of Food and
Agricultural Sciences Communication Services.
The Mealy Oak Gall on Ornamental Live Oak in Texas. MP-1315. 1978. Gordon W. Frankie, David L. Morgan,
Mike J. Gaylor, James G. Benskin, Wayne E. Clark, Hal C. Reed and Philip J. Hamman.
Photo Credits
James Mertely, Ph.D. Gulf Coast Research and Education Center.