University of Manitoba - Faculty of Agricultural & Food Sciences - Insect and Mite Galls
Insect and Mite Galls

Problem type: Insect

Name of problem: Insect and Mite Galls

Plant name(s): Maple, oak, basswood, hackberry, willow, chokecherry, ash, elm as well as other tree and shrub species

Symptoms / Characteristics:
Galls are colorful and unusual deformities of plant tissue ranging in shape, size and appearance and can form on leaves, buds, petioles, flowers, twigs and branches. Galls may be green to yellow, red, brown, tan or black in color and they may be fuzzy, waxy or hard.

Insects and mites are the most common organisms forming galls on plant tissue. There are thousands of gall types formed on all species of plants, most insects being specific to the plants they attack. Galls are produced when the insects or mites feed or lay eggs. Insect saliva can be secreted into the plant, initiating plant growth hormones to produce abnormal cell development, resulting in gall formation. Most galls are produced on new growth or actively growing tissue and are first noticed by late spring when insects are becoming active and laying eggs. Eggs usually hatch in a short period of time and the young will leave the gall in search of new plant tissues to feed on. Many galls are not seen until after the insect or mite has left the gall. The galls may continue to develop even after the pest has left the gall.

Some commonly seen galls occur on oak species, which are produced by a cynipid wasp. One example is the oak bullet gall wasp, which forms a cluster of circular galls on oak branches. Maple and basswood species are commonly seen with numerous raised bumps on the upper leaf surfaces. Maple bladder galls often have a brilliant red color making them very conspicuous. Ash trees are commonly seen infested with the ash flower gall mite, which creates galls on flowers that become very conspicuous in the winter. Generally, mites causing gall formation overwinter under bark tissue where they remain until new plant growth begins developing in the spring.

Control / Preventions:
The majority of galls are only a cosmetic problem and do very little harm to the plant. Population levels can vary from year to year, and will usually subside with natural controls. Once galls are formed, they cannot be cured, but only removed by handpicking or pruning. This can control the mites from overwintering and laying eggs.

Generally, chemical control is not recommended and is very difficult. Insecticides need to be applied before the galls are formed and usually when eggs are being laid. If heavy infestations persist over several years and plant growth is slowed and appearance is unattractive, an insecticide may be applied. A dormant oil or lime sulphur application can be made in the fall or early spring while the tree is still dormant (before bud break). Malathion or Sevin can also be used when buds are turning green in the spring, followed by a second application two weeks later. Make sure all pesticide labels are read and followed carefully.