Galls are abnormal swellings of plant tissue caused by insects, bacteria, fungi, mites, and nematodes. Certain moth caterpillars, beetles, flies, aphids, and small wasps are well known to cause galls. In fact, insect-induced galls are the most common galls.
Galls are caused by the abnormal growth of plant cells and serve as a means of obtaining food and shelter for the developing insects. There are hundreds of different kinds of galls, each characteristic of the insect or mite producing it.
How and When Galls are Formed
Galls are thought to be formed when chemicals are secreted by the insect and these react with certain plant chemicals to produce the abnormal plant growth. This may occur when adults lay eggs in the plant or when eggs hatch and the young begin to feed on the plant. After the gall develops, the insect becomes confined within its "house" and feeds only on gall tissue.
Galls usually form in the Spring as new plant growth is developing and the gall forms rapidly. In a matter of weeks some galls may break open and release their "house dwellers." Other galls may hold their form and keep the insect or mite until the following Spring.
Where are Galls Formed?
Galls are found most commonly on stems and leaves, but also occur on flowers, fruits, and on the trunk. Other locations may be leaf shoot terminals, leaf petioles, and roots. Galls that occur on leaves, flowers, and fruits drop off when these plant parts are shed, but galls on stems, trunks, and roots usually remain as growths for more than a year.
Gall Damage
Galls may cause concern to homeowners from fear that these abnormal swellings may increase to the point that plant health is threatened. Insect-induced galls, for the most part, do not result in noticeable or measurable plant injury. After galls are formed, there is nothing that can be done, except for small isolated trees and shrubs, such as pussy willow. In this case, pruning may be of value in reducing the number of galls next year. Certain kinds of galls may be prevented by spraying the trees in late winter to kill the overwintered insect or mite.
Oak Galls
There are hundreds of kinds of galls that form on the leaves, twigs, and branches of various oaks. Most of them are caused by tiny wasps. The adults emerge in Spring and travel to the leaves and twigs where they deposit eggs. The young hatch from the eggs, then mature in the galls that form around them. Most kinds of galls on oaks rarely affect the health of the trees, but some, such as the gouty gall, the homed oak gall, and the oak potato gall, may affect the health as well as the appearance of the tree by killing the branches.
Maple Bladder Gall
The soft maple tree is often attacked by the maple bladder- gall mite that causes small, wart-likegrowths on the foliage. These growths are first green then turn red and finally black. They occur singly or in clusters and may be so abundant that the leaves become crinkled and deformed. The galls seldom, if ever, affect the tree's vigor.
Hickory Galls
Most galls on hickory are caused by aphid-like insects. The galls are hollow, green in June, and turn black in July. They appear on leaves, stems and small twigs. In June, the insides of the galls are lined with tiny shiny "plant lice" of varying sizes. The galls range in size from a small pea to more than 1/2-inch in diameter.
Beaked Willow Gall
The work of a small fly, the willow beaked gall rnidge, produces a large oval-shaped gall usually on the lower shoots of pussy willow. The young of this fly overwinters in a channel within the galls.
