Tricounty News

Those strange growths on leaves are galls



Homeowners are reporting many strange and interesting growths that resemble needles, woolly balls, seeds, velvet patches and more on the leaves and twigs of trees and shrubs. Most of these abnormal plant growths are galls induced by egg laying or feeding of insects and mites in early spring. The gall-making insect or mite develops inside the gall and it continues to increase in size as the organism inside feeds and matures. Galls can assume many colors, shapes and sizes and each species of gall maker produces a unique gall.

Maple bladder galls are very common on the leaves of silver and some other maples. The seed-like galls on the upper surface of the leaves are produced by eriophyid mites that overwinter on their host tree and initiate gall formation as the leaf buds open in the spring. Spindle galls, sometimes called finger galls or needle galls, are very common this spring on the leaves of plum, cherry, linden and some maple trees. These galls are also produced by eriophyid mites and appear as slender projections on the surface of the leaves. Other common galls formed by mites are the ash flower gall that was very abundant last year and the erineum or velvet galls found on maple, viburnum, birch and linden. Erineum galls are very common on the leaves of compact cranberry viburnum this year and appear as a bright red carpet-like growth on the leaves.

Some species of aphids are capable of producing galls. An aphid-induced gall that causes considerable leaf drop in poplar and cottonwood trees some years is the poplar petiole gall. The gall appears as a swelling on the leaf stalk and occasionally on the leaf blade itself.

The nipple like galls on the lower surface of hackberry leaves is caused by a psyllid.  Because of their very small size, the hackberry psyllids can pass through window screens and may become a nuisance in the fall when they are seeking shelter for the winter.  Hackberry blister gall is also caused by a psyllid and appears as numerous blister-like galls on the upper surface of the leaves.

It is estimated that there are over 700 different kinds of galls that may be found on oak trees. The hard marble-sized growths that often appear on the twigs of oak trees are oak bullet galls produced by a very small cynipid wasp. Oak apple gall, oak jumping gall and hedgehog gall are some unusual galls found on oak trees produced by cynipid wasps. Hedgehog gall is an oval, spiny gall that may be bright yellow, orange or red and appears on the major leaf veins of bur and white oak. Oak apple galls are very large round galls that are formed on the petiole or midrib of the leaves of red, pin and other species of oak.

Jumping oak galls are interesting because each seed-like gall contains a single wasp larva that will actually cause the gall to move when it is rubbed off the leaf. An interesting gall that is occurring on some oak trees this year is called the wool-sower gall which consists of a cotton-like ball about an inch or so in diameter with reddish seed-like structures in the cotton mass. It is also produced by a cynipid wasp.

The eastern spruce gall and Cooley spruce gall are caused by an aphid-like insect called an adelgid. The eastern spruce gall occurs as pineapple-shaped galls at the base of new shoots on Norway and white spruce. If galls are abundant each year the stems may be weakened and control may be necessary. Cooley spruce gall occurs at the tips of new growth of Colorado blue spruce and white spruce, but rarely causes any damage to the trees.

Galls may cause some leaves to curl and drop and affect the plant cosmetically, but there are only a very few of them that have an adverse affect on the health of the tree or shrub on which they are found.  There are chemical control options available, but timing is essential as once the gall begins to form it is too late for treatment. There really is no treatment necessary and the galls should be tolerated.