Insect
and Disease Fact Sheet Compliments of New Century
1-877-79TREES
Common Oak Galls
Galls
are irregular plant growths which are stimulated by the reaction between plant
hormones and powerful growth regulating chemicals produced by some insects or
mites. Galls may occur on leaves, bark, flowers, buds, acorns, or roots. Leaf
and twig galls are most noticeable. The inhabitant gains its nutrients from the
inner gall tissue. Galls also provide some protection from natural enemies and
insecticide sprays. Important details of the life cycles of many gall-makers
are not known so specific recommendations to time control measures most
effectively are not available.
Gall
makers must attack at a particular time in the year to be successful.
Otherwise, they may not be able to stimulate the plant to produce the tissue
which forms the gall. Generally, initiations of leaf galls occur around “bud
break” or as new leaves begin to unfold in the spring.
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Oak Apple Gall |
Jumping Oak Gall |
Twig and Stem Galls
Twig and
stem galls, such as the gouty oak gall and horned oak gall, are solid, woody
masses that can girdle branches or make them droop from the sheer weight of the
heavy growths. The galls can grow to more than 2 inches in diameter. Horned oak
galls can be found on pin, scrub, black, blackjack, and water oaks while gouty
oak galls occur on scarlet, red, pin or black oak.
These
galls have a long and complex development that takes two or more years to
develop. The first stage is a blister-like leaf gall that occurs along larger
leaf veins. The second stage is a knotty twig gall that is started in
mid-summer and becomes fully mature in 1 to 2 years. Adults emerge in the
spring. Gouty oak twig galls are smooth; hormed oak
galls have horn-like projections. One female wasp can emerge from each horn.
Generally,
insecticidal control is not satisfactory because the wasps are physically
protected within the galls. Correctly timing applications to provide effective
preventive control is difficult. Where practical, pruning of infested twigs may
help to reduce the problem on lightly-infested trees. However, pruning is
impractical if large trees are heavily infested. A commercial arborist may be
able to provide assistance with valuable plantings.
Leaf Galls
Leaf
galls rarely affect tree health so control is rarely justified. However, an
application of carbaryl (Sevin)
at bud break may reduce infestations of some galls. It is difficult to spray
moderate to large trees without special equipment and the necessary protective
clothing to protect the applicator from spray drift.
Oak Apple Galls
These
are large (1- to 2-inch diameter) rounded growths that are filled with a spongy
mass. A single wasp larva is located in a hard seed-like cell in the center.
Galls are usually found on the petioles or midribs of leaves. They will dry to
a brown, paper thin wall. Removing and destroying galls before they dry and
wasps emerge from a hole may help to reduce the infestation. While large and
spectacular, they cause no measurable harm.
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First Stage blister-like leaf
galls. |
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Roly Poly Galls This
is a group that is similar in size and appearance to hollow green grapes.
Inside, in a small, loose "seed like" structure, is the larva of a
tiny wasp. These galls seem to appear in place of leaves or reduce the size
of leaves, but they do not affect tree health. |
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Wool Sower
Galls These
are wasp galls that appear on white oak in early summer and resemble toasted
marshmallows. One gall is actually a group of small hairy galls joined at a
common spot on a twig. They can be pulled apart to see seed-like structures
that contain the developing wasps. Vein Pocket Gall and Leaf
Pocket Gall These
galls are caused by the larval (maggot) stages of very small flies called
midges. Vein pocket galls are elongate swellings that occur along mid- and
lateral leaf veins of scrub and pin oaks. The process begins when the unfolding
leaves |
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begin to flatten out. At this time, the small midge lays its
eggs. The tiny maggots move to the veins and begin to feed. In a few days
they are covered by gall tissue and complete development by mid-spring. The
mature larvae drop to the ground and remain there over the winter. A related
midge species causes the marginal folds or leaf pockets that can be found on
red oak or pin oak. Several small larvae may be found inside the swollen
folds of the leaf. Jumping Oak Gall Caused
by a small wasp, they typically appear on the leaves of valley oak and
California white oak. These round, seed-like galls fall off leaves when
mature. One wasp lives inside each gall; the wasp's activity may make the
gall "jump" several inches off the ground. Female wasps emerge in
the spring from galls on the ground and lay eggs in opening buds. After
several weeks, small blister-like galls form on the young leaves. During the
summer, males and females mate and females lay eggs in the mature leaves. |
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Information obtained through
University of Kentucky Department of Entomology
Insect
and Disease Fact Sheet Compliments of New Century
1-877-79TREES