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What are insect galls? Galls are abnormal growths that can arise in all parts of a plant resulting from the work of usually immature insects and other organisms. In a way, they are basically "plant tumors." Unlike human tumors, galls usually do not injure their hosts to the point where the entire plant is debilitated. The few injurious galls appear only to attack pears, wheat, grapes, and roses.

There are over 1500 species of gall producers. However, most galls are produced by plant mites, gall midges, and gall wasps. These creatures produce galls to provide food and shelter for themselves. Galls can be simple deformities consisting of a rolled leaf edge or a pouch-like growth on the plant, or complicated structures made out of seemingly unrelated plant tissue that are highly organized.

The principal gall producers include:

1. Plant mites - microscopic, pale yellow or translucent organisms with slender, pear-shaped body and transverse ridges or lines. The gall producers are in their larval stage and have 4 legs, while adult mites have 8. Mites produce simple galls ranging from leaf deformities such as pouches or pockets with erineum that it winters under.They're not actually insects, but they managed to wheedle their way onto this page with claims that they'd be terribly lonely if we had an entire 'Mite Galls' page devoted to them. How could we resist?

2. Aphids and plant lice - soft-bodied insects with sucking mouth parts. These insects produce complicated galls, wintering on the bark, then hatching out in the spring and attacking a bud to form their galls.

3. Gall midges - small delicate flies that are about 1/4 inches long, and have antennae. The maggot larvae are what produce the galls. Gall midges winter in galls and emerge in the spring.

4. Gall wasps (also known as Cynipids) - Wasp larvae that are formed usually on oaks.

Although the number of gall producing organisms are numerous, there are only a few host plants available for them to inhabit. This severely limits the types of trees these insects can inhabit. As a result, gall producers are very plant specific, and most of them reside among willows, oaks, goldenrod, and asters. Galls vary in shape, size, and complexity. The known types of galls are classified into categories:

Types of Galls
1. Blister galls
Blister-like swellings of leaves.
2. Bud galls
Deformities that originate in buds that varies from aborted buds to large swellings of bud structure.
3. Bullet galls
Nearly solid, unicellular or monothalamous galls produced by species of gall wasps. Mostly on oak twigs. Looks like bullets.
4. Cecidomyia
General term applied to any species referring to gall midges or Itonididae which cannot be readily assigned to a more closely defined genus.
5. Erineum
Hairy or pile-like growths upon leaf surfaces produced by plant mites.
6. Flower galls
Many gall insects live in aborted or deformed flowers or masses of flowers.
7. Fruit galls
Galls that create a marked deformity on fruits and seeds.
8. Leaf galls
All deformations definitely associated with leaves.
9. Leaf spots
Leaf galls that look more like marked discoloration rather than swellings or deformations.
10. Many-celled/polythalamous
Adjective for galls containing two or more young or larvae in more or less separated cells.
11. Oak apples
Term used for particular familiar large galls on oaks. Produced by gall wasp, genus Amphibolips.
12. One-celled/monothalamous
Adjective for galls inhabited by one or more larva(e) in one cell or cavity.
13. Pouch galls
Simple, pouch-like deformities caused by deformations in the leaf surface. Created mostly by gall mites and aphids.
14. Roly-poly galls
Easily recognizable by loose, usually oval cell with a large cavity. Smaller than oak apples. Mostly on oaks.
15. Root galls
Galls on roots of plants. Variable in size, shape, and location on root.
16. Rosette galls
Specialized typed of bud galls usually found at bud tip. Generally consist of central cell surrounded by a rosette of partly developed leaves.
17. Stem or twig galls
Deformations on twigs and stems, usually affecting the entire circumference of structure.
18. Subcorticol galls
Galls just under the bark of (usually) one side of stem or twig. Irregularly shaped.

Gall production is a highly varied process. Gall producers attack every part of the plant, taking advantage of developing plant parts that have a lot of plastic tissue. Very basically, a gall is produced by an insect feeding on it. The feeding stimulates plant growth without killing too many cells. The insect then deceives the plant tissues adjacent to the area it is feeding on and causes it to grow abnormally. However, multiple variations of this basic principle exist among the diverse population of gall producers.

Some gall insects attack definite portions of its host without directly affecting structures of other parts, except maybe in the case of deformity arising indirectly from a failure of certain plant organs to develop normally.

Other galls occupy cells in normal stems or twigs and produce little or no enlargement of infested part of the pant. These indeterminate galls form the deformity through stimulation or irritation of plant cells where gall producers feed. The cells adjacent to those are directly affected.

Determinate galls are highly complicated galls that develop in such a way as to show little connection between the deformity and the original plant parts from which it grows from. Buds, bark, and leaves are particularly susceptible to these types of galls because of their abundance of sufficient meristenatic or plastic tissue. The stimulus or irritation of the gall maker dominates the tissues of the plant part. The stimulus changes the entire structure and arrangement of the developing plant cells, and also increases the number of plant cells in that region.

Although galls are basically tumors on plants, most are do not affect the vegetation in a detrimental way. These gall makers take particular care not to kill their hosts, as the amount of plants that will allow them to inhabit their tissues. The only major harm galls pose is one of degrading the aesthetic value of the plant. Galls cause no real economic injury to humans.

Interestingly, some galls are useful to human culture. The Cynips gallae-tinctoriae has been used as a source of ink. These galls contain 50 to 65 percent tannic acid and are very useful as ink or in dyes for wool. The Greeks used the Cynips theophrastea gall for lamp fuel. Honeydew producing galls are used to attract bees and flies in the agricultural industry. Some galls have been known to be used for food and medicine.