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While sitting on my grandparents’ porch last
Sunday, I noticed numerous ball-shaped objects in their oak trees. Looking
closer, I found that almost every limb of the 10-year-old trees contained
gouty oak gall. A plant gall is an abnormal swelling that develops
within tissues that have been colonized by a parasitic organism. The gall
forms as a result of a reaction to the parasitic attack on the
plant’s cells. The organism’s saliva stimulates overgrowth or
cell proliferation in the host plant. The resulting structures are
distinctive galls that are highly species-specific. Galls vary in size, shape, color, texture, and
location on the plant. Some appear as bumps on leaves; others are large,
warty growths on stems. Common galls include ash flower gall, hackberry
nipple gall, maple bladder gall, maple spindle gall, gouty oak gall, horned
oak gall, and oak-leaf gall. To cause a gall, an organism must feed or lay eggs on
the plant while the plant parts are growing rapidly, usually in early
spring. Galls may occur on leaves, stems, flowers, seeds, fruits, or roots.
Many types of organisms — including nematodes,
bacteria, fungi, eriophyid mites, beetles, psyllids, true bugs, wasps,
aphids, and midges — cause gall. Gall-making insects and mites generally spend most of
their lives feeding and growing inside the gall, which provides both a
protected enclosure for the offspring and, in the inner wall of the growth,
a food source. Each gall-maker is specific to the kind of plants it
will attack. More than 2,000 different plant galls are caused by insects
and mites, but because most galls do not cause damage resulting in economic
loss, there’s been little effort devoted to their study and not much
is known about these formations. Approximately 750 different galls have been
identified on oak trees alone. Gouty oak gall and horned oak gall are very
noticeable and noteworthy. Horned oak gall occurs on pin, black, and water
oaks; gouty oak gall affects pin, scarlet, red, and black oak species. Both
of these galls can be debilitating and even kill young trees. Approaches to
management of these two galls are limited: If possible, trim affected twigs
and branches out. Affected trees can be fertilized in mid to late fall to
encourage healthy growth. Most galls do not harm a healthy established tree or
shrub. Leaf galls can cause leaf distortion, and some leaves may fall
prematurely. Although the galls detract from the beauty of the foliage, the
infested leaves are usually able to carry out photosynthesis. Twig galls are more serious and are generally harmful
to a tree or shrub. Gall formation on the twig disrupts the twig’s
function, and the portion of the twig beyond the gall may die. A tree
heavily infested with twig galls can over time show general decline in
vigor and may occasionally die. Pruning out twig and stem galls while they
are green is sometimes an effective means of control. (Small holes in the
gall indicate that the inhabitants have escaped.) Because they spend such a small part of their lives
outside their galls, little is known about the life cycle of many
gall-makers, and so chemical control is generally not feasible. In
addition, on tall trees gall-makers are hard to reach with insecticides.
The timing of chemical control must be initiated before gall formation
begins. Once a plant has been infested with galls, chemical sprays
won’t make them go away. For more information on galls, go to Iowa State
University’s “Insect Galls on Trees and Shrubs” Web page,
www.extension.iastate.edu/ Publications/IC417.pdf.
Jennifer Fishburn is a horticulture educator with the
University of Illinois Extension Sangamon-Menard Unit. Contact her at
www.extension.uiuc.edu/Sangamon.
This article appears in May 17-23, 2007.
