Unusual bumps or growths often appear on
leaves and stems of deciduous trees such as oaks, maple and hackberry.
These growths are caused by insects and are generally not considered serious
to the health of the tree.
How are Galls Formed?
Galls are natural plant tissue similar to a tumor in animals.
Either mechanical damage or salivary secretions from the mouths of insects
or spider mites trigger the production of normal plant growth
hormones. This causes
localized cells to increase in size and or number resulting in a growth
called a gall.
Gall formation usually occurs during times of fast growth in late spring
when new leaves, shoots and flowers are being formed. The insect or mite
that caused the gall develops within the gall and eventually eats its way
out leaving a small exit hole.
Several organisms are capable of causing galls on plants. The most common
are eriophyid mites, gall midges, gall wasps, certain
aphids, and psyllids.
Although many species of plants are attacked, oaks are by far the most
commonly affected tree. Over 800 different types of galls are known to
affect different oak species.
Common Types of Galls
Leaf Galls - These growths appear on the leaf stem (petiole)
and leaf blade. They appear as blisters, nipples or erineums (hairy, felt-like
growths) on the upper or lower leaf surface.
Stem and Twig Galls - These range from slight swellings
to large knots on twigs and stems.
Bud/Flower Galls - These growths may alter the size and shape
of buds and flowers.
Galls and Plant Injury
Generally, insect galls do not seriously damage plants. Since the gall
itself is plant tissue, it will require moisture and nutrients just like
any other part of the plant. However, except in the case of extremely small
plants, this is not a problem.
Since they do not adversely affect tree growth, control is generally
not recommended for this type of gall.
insecticides may be used in extreme cases but the timing is
critical and these applications are not always effective. Sprays must be
timed for the exact time of insect activity for the specific insect or
mite that is causing the gall. Once the gall begins to form, it is too
late for that year.
Maple Bladder Gall - An eriophyid mite causes bladder or
pouch-like growths on the leaves of silver and red maples. Galls form on
the top of the leaves and may turn yellow to red in color. In severe cases
much of the leaf surface will be covered. The damage is cosmetic and does
not affect the overall vigor of the tree. No treatment is recommended.
Maple Spindle Gall -
Sugar maple is the primary host for this
gall also caused by eriophyid mites. Galls are purplish and spindle-shaped
on the upper leaf surface. The damage is cosmetic and does not affect the
overall vigor of the tree. No treatment is recommended.
Hackberry Blister Gall - Two types of psyllids cause these
growths on the leaves of hackberry trees. Blister galls form on the top
leaf surface and nipple galls form beneath the leaf.
Trees are generally not damaged by light infestations. Several years
of heavy leaf damage may cause a loss of tree vigor.
Control measures in most cases are not warranted. If severe leaf damage
has occurred for several years, acephate (Orthene) may be applied as the
leaves are expanding (opening up from the bud) in early spring to reduce
Oak Galls - As mentioned, over 800 different galls have
been shown to form on the many species of oak trees. Common galls of many
sizes appear on leaves. Generally, an insect egg has been laid inside the
gall and the larva eats its way out eventually.
Again, these galls are not detrimental to the tree and treatment is
not recommended. On small seedling trees, an insecticide spray at the time
the leaves are unfolding may reduce the numbers of galls.
Maple Velvet Galls - A tiny mite will cause the appearance
of a velvet covered growth on the underside of leaves of maple, hickory
and some other trees. They are often bright crimson or orange in color
and draw people=s attention.
The damage is cosmetic and does not affect the overall vigor of the
tree. No treatment is recommended.