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Willow Galls

David J. Shetlar
Ohio State University Extension

Willow GallThe pussy willow is a very interesting ornamental plant that is enjoyed for its early spring flowers. However, owners of these plants frequently become alarmed at the sight of strange-looking growths on the twigs and leaves. Most of these unusual growths are called galls. These and other galls are formed on all the willows.

What are Galls?

A gall is any unusual growth of plant cells which have been stimulated by chemicals secreted or injected by an insect, mite, nematode or disease agent. As an example, gall wasps insert an egg into developing plant tissues, and at the same time, it injects some chemicals formed in its body. The chemicals stimulate the plant cells into a rapid and abnormal division and growth of plant cells in the immediate vicinity of the egg. The egg, as it hatches and continues developing into a larva that also secretes fluids from its body cells. Thus, the surrounding plant tissue continues to grow abnormally into a characteristically shaped gall. The size, shape, and form of the gall is determined by the gallmaker, and also by the plant it selects. The galls are always identical for each gallmaker, season after season.

There are, in general, two types of galls - open and closed. Open galls are produced by insects with sucking mouthparts and mites. The gall always has a hole, slit or other opening which is tightly sealed when the young insects or mites are developing. However, when the insects or mites mature, the gall dries and the opening is enlarged sufficiently for the pests to escape. Closed galls are produced by gall wasps, and beetles that have chewing mouthparts. Either the larva or the new adult gallmaker chews a hole through the gall tissues in order to exit the gall.

Identity of the Growths on Willow

There are several common growths which are found on willows in Ohio. The willow beaked-gall affects the twig tips and buds by causing a slight swelling tipped with a sharp point. The willow cone gall produces a large, pine cone-shaped, gall on the terminal of branches. The willow blister gall and fleshy gall appear on the leaves. 

The Willow Beaked-Gall Midge

The willow beaked-gall is not considered to be a very serious problem. The galls do little to no harm to the well-being of the pussy willow plant.

The willow beaked-gall is caused by a tiny fly like gnat called a midge, Mayetiola rigidae (Osten Saken). The willow beaked-gall midge remains as a mature larva within the gall over winter. These larvae pupate in the early spring and the black with orange coloring, adults fly in April. After mating, the females lay eggs in the newly expanding willow buds. The eggs soon hatch and the larvae burrow deeper into the tissues. This causes the tissues to swell noticeably by mid-May. The gall is almost completely expanded by July but the tiny leaves still attached make them difficult to see. Eventually the gall hardens, turns a reddish-brown and the small leaves drop off. The terminal branch bud remains to form a beak-like structure.

Willow Cone Gall

The willow cone gall is caused by another midge. The adults appear in late April into early May. They deposit single eggs into the swelling terminal buds. After the egg hatches, the bud ceases to elongate. Instead the leaves expand into a mass of flattened scales in the form of a developing pine cone. The larvae remain in the gall for the entire season and during the winter. In early spring the larvae pupate.

The willow blister gall is a grayish-white and red, somewhat round and fuzzy growth on the upper and lower surfaces of willow leaves, especially pussy willow.

The galls are caused by a tiny mites called eriophyid mites. Eriophyid mites are microscopic and rather carrot-shaped. They spend the winter on the willow bark and branches and in the spring they come out of hibernation to move to the developing leaves. Their feeding and presence on the tender leaves causes the leaf edges to swell, wrinkle and curl over to form a protective chamber. Additional pouch-like galls may occur on the entire leaf surface. The summer is spent feeding and reproducing. When large numbers of eriophyid mites develop, willow leaves may become completely curled and badly distorted.

Fleshy Willow Leaf Gall

These appear as thickened lumps of tissue on the leaves of several willows. They start out green in color but often turn red and brown. Several galls may be present on a single leaf. The galls are formed by a sawfly, Pontania proxima (Lepeleter), a type of primitive wasp.

The adult sawflies emerge as the willow leaves are expanding and they insert eggs into the expanding tissues. This causes the leaf tissue to swell into a chamber which nourishes and protects the developing grub. Little is known from this point on but most gall making sawflies emerge by fall and overwinter in surrounding leaf litter.

Control Hints

In general, plant galls are not a serious threat to the life of a plant. They do cause some unsightliness when large numbers appear. Controls are not normally recommended for willow galls.

Strategy 1: Mechanical Control - Simply prune out the willow beaked-galls as they are noticed. If these are pruned out in the fall or winter, be sure to destroy them by crushing or burying. The adults will probably still emerge even if the galls are simply thrown to the ground. The same control method can be used for the willow cone gall.

Strategy 2: Chemical Control - Though this is not recommended for willow galls, several chemicals are registered for management of gall midges, gall wasps and eriophyid mites. Chemical controls are rarely effective because they need to be applied before the galls form. Since the gallmaker lays its eggs over a several week period, several sprays would need as the developing leaf and stem tissues expand. See Bulletin 504 for currently registered insecticides labeled for control of gall wasps or gall midges.

NOTE: Disclaimer - This publication may contain pesticide recommendations that are subject to change at any time. These recommendations are provided only as a guide. It is always the pesticide applicator's responsibility, by law, to read and follow all current label directions for the specific pesticide being used. Due to constantly changing labels and product registrations, some of the recommendations given in this writing may no longer be legal by the time you read them. If any information in these recommendations disagrees with the label, the recommendation must be disregarded. No endorsement is intended for products mentioned, nor is criticism meant for products not mentioned. The author and Ohio State University Extension assume no liability resulting from the use of these recommendations.

Information provided by the Ohio State University Extension Service