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(515) 294-9915
5/15/00
Contacts:
Donald Lewis, Extension Entomology, (515) 294-1101,
drlewis@iastate.edu
Elaine Edwards, Extension Communication Systems, (515)
294-5168
Yard and Garden Column for the Week Beginning May 19
A Lot of Gall
By Donald R. Lewis
Extension Entomologist
Iowa State University Extension
It is a weird but wonderful place out there in the landscape. Take galls, for example. Galls are deformities found on the leaves, stems and flowers of trees, shrubs and other plants. Plant galls come in a fascinating variety of strange forms, textures and colors. . Some are irregular, bumpy or warty; others are smooth and spherical. Some galls sport thick growths of fuzz, hair or spines. Some don't.
Gall formation is an equally bizarre biological puzzle. Galls result from an intricate interaction between two living organisms. Gall-makers, the organisms that initiate the process, are tiny, highly specialized wasps, flies, mites, aphids or microbes. Their attack on a host plant unleashes a unique response. The plant abruptly changes the course of normal growth and modifies growing tissue into a special swelling that surrounds the tiny insects and mites.
The mystery of gall formation was not understood until recent times, even though galls have been written about since the days of the Roman Empire. Plant growth-regulating chemicals produced by the gall-maker interact with the plant's own hormones to produce galls in young, newly developing tissue.
Thousands of species of insects and mites produce galls on hundreds of species of plants. Each gall-maker attacks specific tissues on a specific plant. The resulting distortion is a distinctive gall that is different from galls caused by other species.
Benefits to the gall-maker
Gall-maker "parasites" do not destroy the plant, they modify it for their own purposes. The gall provides some protection from adverse weather, predators and parasites. Galls are rich in proteins and carbohydrates. Thus the gall-maker lives a sheltered, luxurious life surrounded by a ready source of food in which it grows, develops and even reproduces. The new adult gall-makers emerge from the galls in the summer or the following spring.
Despite their unsightly appearance, most insect galls do not seriously affect the vigor of healthy plants. Occasionally, a heavy or prolonged gall infestation on small branches or roots may weaken or kill portions of a tree. Leaf galls are aesthetically displeasing and may cause some premature leaf drop but they do not directly kill the plant.
Galls cannot be "cured" after they have formed. That is, spraying or treating does not make them go away. Preventive treatments applied before the galls form may be effective but are not usually practical. Control with insecticides is not recommended.
Common Insect Galls in Iowa
Ash Flower Gall. These one-inch, brown, spherical swellings of male flowers on green ash are most obvious during the winter because they hang on the trees. Green galls present during the summer are less noticeable.
Hackberry Nipple Gall. The presence of these quarter-inch long cylindrical "warts" on the leaf underside is a distinctive aid in the identification of the hackberry tree. Galls remain green all year but issue an annoying, gnat-like psyllid in the fall.
Hickory Petiole Gall. This large, globular swelling of the leaf stem is filled with a squirming mass of small aphids. The gall appears to stunt tree growth in some years. Unfortunately, there is no cure.
Maple Bladder Gall. This is probably the best known of all galls. The erect sacklike growths on the upper leaf surface are common on silver maple trees. The 1/8-inch long galls start out green, turn bright red and finally black. Successive generations of gall-maker mites are produced and new foliage continues to produce galls until about July.
Succulent Oak Gall. This gall appears on new pin oak leaves as a half-inch diameter, smooth, round sphere. The gall is hollow with a small kernel in the center that contains a single, developing wasp gall-maker larva.
Oak Bullet Gall. This is only one of hundreds of stem galls found on the twigs of oak trees. Bullet galls are hard, pea-sized spheres clustered along the side of the twig.
Horned Oak Gall. This very hard, woody expansion of the oak twig is irregular in shape and as large as a golf ball. Tubular projections grow from the mature gall when the gall-maker wasps are ready to emerge.
Grape Phylloxera. This common gall from the eastern United States almost changed the course of world history. The irregular wartlike galls often cover the leaves of grapes. No harm comes to our native, wild grapes. However, the phylloxera insect was accidentally introduced from the United States into the grape growing regions of France in 1859 and by 1870 had brought near-collapse to the French economy. Americans eventually provided the practical solution that rescued the French wine industry from crushing disaster. Restoration of the French wine culture to its former prosperous condition helped pay the enormous debt of the Franco-German War and aided the French people in their experiment with their new republican form of government. Who says insects can't change history? Vive-la-France! Vive-la-Bug!
ml: isugarden
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