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.
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
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010