Description
Abnormal warts, swellings, and knots on the leaves, twigs, and
branches of trees and shrubs are usually galls. There are hundreds
of kinds of galls--large and small, conspicuous and inconspicuous--
but each is characteristic of the organism that produced it.
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Figure 1. Larvae Inside Wooly Fold Gall on Red Oak
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Figure 2. Wooly Fold Gall on Red Oak Caused by a Fly
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The
gall-producer always remains faithful to a particular kind
of plant. Most of the common galls are due to abnormal cell
growth stimulated by insects and mites, but some galls are
caused by bacteria, fungi, and nematodes. Still others are
caused by aphids, mites, or fly larvae. On oak trees particularly,
the growth of galls of many shapes is stimulated by a large
number of species of small wasps belonging to the Cynipidae
family.
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Life
History
The habits of the gall-maker are as varied as the galls. In
general, the galls provide a home for the insect, where it can
feed, lay eggs, and develop. In the case of the cynipid galls,
the small wasps lay eggs on the developing buds and shoots.
These hatch into legless grubs that cause the galls to develop.
After development, the galls are deserted by the insects. |
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Figure 3. Oak Hedgehog Gall
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Figure 4. Linden Tree Gall
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Figure 5. Potato Gall on White Oak
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Figure 6. Phylloxera Gall on Hickory
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The numerous galls (green, reddish, or black) seen on the
uppersides of maple leaves are stimulated by microscopic mites
that overwinter under the bud scales of the trees. As soon
as growth starts in the spring, the mites move to the leaves
and stimulate gall growth. They feed, lay eggs, and live inside
these galls.
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Figure 7. Ash Midrib Gall
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Figure 8. Small Oak Apple Gall on Red Oak
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Damage
The damage caused by galls is difficult to record or to estimate.
The presence of galls does not indicate that the plant is
diseased and is likely to die. The damage is localized; with
a few exceptions, is not considered serious enough to justify
spraying. Occasionally, however, galls develop in such numbers
that the leaves become distorted and lose their natural beauty,
or some may fall prematurely. Cynipid galls sometimes kill
oak twigs and branches. The hickory gall aphid may seriously
damage hickory twigs.
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Figure 9. Gouty Vein Gall on Sugar Maple
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Figure 10. Phylloxera Globosa Galls on Walnut
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Figure 11. Gall on Bur Oak Caused by a Wasp
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Chemical
Control
Usually control for galls is not needed or recommended.
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