Monday, August 11, 2014

Oak Galls on White Oak Trees in Northern Virginia Counties

Homeowners in six Northern Virginia counties have expressed concern about defoliated white oak trees.  Forestry officials have conducted aerial and ground surveys and determined that, while this occurrence is fairly widespread, it is concentrated in and around the hills of western Fauquier County and adjacent Loudoun County.  Portions of Prince William, Culpeper, Orange and Rappahannock counties are also affected.

The culprit appears to be a very tiny insect known as a gall wasp. There are many species of gall wasps, particularly those that affect oak trees. This type of insect injects eggs into plant tissue, which forms a swelling or ‘gall’ around the injection site. Inside a hollow space within the gall, the developing egg hatches into a larva, and ultimately emerges from the gall as an adult wasp, repeating the cycle one or more times each year depending on the species. Each species of gall wasp specializes on a particular host and plant part, and each species produces a unique gall. Thus, there are a wide variety of plant galls that differ in shape, size, color, texture and the part of the plant affected (leaves, twigs, buds, flowers, etc).  

While gall wasps are a normal component of every forest ecosystem, they are generally kept under control by other insects and are not typically abundant enough to cause serious damage to trees and shrubs. However, in rare instances they can become so abundant that their galls can cause noticeable damage.

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