Galls are abnormal plant growths caused by various organisms (insects, mites, nematodes, fungi, bacteria, and viruses). This article will deal with galls caused by the feeding or egg- laying activity of insects and mites. Because of their unusual forms and colors, galls often cause homeowners to become concerned. However, galls seldom threaten plant health and their numbers are highly variable from season to season. For those reasons, control is generally not suggested.
How are galls formed?
Galls are formed by insect/mite feeding or egg-laying activity. Either mechanical damage or salivary secretions (introduced by insects and/or mites) initiate increased production of normal plant growth hormones. These plant hormones cause localized plant growth that can result in increases in cell size and/or cell number. The outcome is an abnormal plant structure called a gall.
Gall formation generally occurs during the accelerated growth period (late spring) of new leaves, shoots, flowers, etc. Mature plant tissues are usually unaffected by gall-inducing organisms. The gall-making organism develops inside the gall and the gall continues to grow as the insect/mite feeds and matures. Once gall formation is initiated, many galls will continue to form even if the insect dies. In addition, most galls are usually not noticed until they are fully formed and remain on plants for extended periods of time.
Galls and plant injury
Galls are growing plant parts and require nutrients just like other plant parts. Its possible that galls steal vital plant food and adversely affect plant growth. This is most likely a problem when galls are numerous on very young plants. Injury may also occur if galls are numerous on branches or if abundant
for several consecutive years. In most cases, however, galls are not abundant enough to harm the plant.
Most galls do not adversely affect plant health. Therefore, management is generally not suggested to protect plant vitality. Chemical applications are an option, but are often ineffective since the precise timing of sprays is critical. To be effective, sprays must be timed to coincide with initial insect/mite activity before gall formation begins. Once galls start to form, they conceal the causal organism and it is too late for treatment. For insects/mites that overwinter on the host plant, horticultural oil applications can be made before insect/mite activity begins in the spring.
Ray Ridlen is an agriculture/horticulture educator for the Oklahoma County Extension Service. His column addresses frequently asked horticulture questions. For more information, call 713-1125.