cottonwood flowerbud gall (7812 bytes)

Oh, the Gall of it!

Information from Colorado State University Cooperative Extension

Those strange lumps and odd-shaped cones on landscape trees may look grotesque, but seldom are damaging to plant health.

Such abnormal growths are called galls and are the result of a wound, infection by a microorganism or by the feeding or egg laying of certain insects and mites.

Certain insects and mites, when feeding or laying eggs, secrete chemicals similar to plant growth hormones. These chemicals can cause the plant to develop strange features or abnormal. These galls usually start in the spring or early summer when the plant is growing rapidly. Young insects develop and feed within the growing gall, often emerging and moving on before the gall is noticed.

In Colorado, the most important gall-makers are:

Click on highlighted words below to view pictures.

  • Eriophyid or gall mites which make a variety of galls. These include finger-like pocket galls on leaves, irregular growth on ash and cottonwood flower buds (see above) as well as irregular leaf curls that resemble herbicide injury.
  • Psyllids, which produce the common nipple gall on hackberry leaves.
  • Gall-making aphids that produce the lumpy galls on cottonwood and aspen, and that also cause distorted leaves on ash, aspen and cottonwood trees.
  • Gall wasps that produce a wide range of galls. Most all insect galls found on oaks and roses are produced by gall wasps.
  • Gall midges that cause a variety of swellings on the leaflets and needles of such trees as honeylocust and pinyon pine. Swelling of flower parts and fruit also may be the work of gall-making midges.

The creatures that cause these galls vary from year to year. Seeing many galls one summer does not mean you will see many every summer.

Galls can be very difficult to prevent with insecticides because sprays must coincide with egg-laying by the adult insects.

After the gall starts to form with the eggs inside, they are protected from contact insecticides. This, together with the fact that galls rarely threaten a plant's health, has led Colorado State University Cooperative Extension entomologist, Whitney Cranshaw, to recommend against control of galls under most circumstances.

Photo: Judy Sedbrook

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Date last revised: 01/05/2010