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.

Chemical 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.

 Common Galls

 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 populations.

 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.  


Note: We have provided some general information and observations on this topic aimed at the home gardener. Before you take any serious action in your landscape, check with your state's land grant university's Cooperative Extension Service for the most current, appropriate, localized recommendations.


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