Many insects induce hypertrophies (a condition of abnormal rapid cell division and cell enlargement) in host tissues during feeding or to complete their life cycle. These hypertrophies are generally referred to as galls, and are caused by a wide variety of insects. Gall-forming insects release plant growth-regulating chemicals that alter normal plant growth and development. Most gall-forming insects are host species-specific and are often named for their host. Galls can be formed on virtually any host tissue including leaves, buds, flowers, cones, shoots, twigs, branches, and main stems. There are hundreds of gall-forming insects, most of which cause little if any serious harm to their host plants. However, because galls may be large and conspicuous, they often cause concern.
The causal agents of galls are usually not observed; larvae inside of galls are difficult to identify. Diagnosis is usually based on the gall symptoms and host species. Galls vary widely in size, shape, color, texture, and longevity and are determined by the host species, host tissue, and the causal agent. In general, galls are tissue swellings caused by rapid cell division and enlargement (gall midges in conifers also cause resin accumulation at their feeding sites, which contribute to gall swelling). Galls can be small leaf spots or bumps (e.g. eyespot galls on maple, dogwood, and yellow poplar), soft and fruit-like (e.g. oak apple galls), carpet- like (e.g. eriophyid galls), woody (e.g. many oak galls), ornamented (e.g. horned oak galls), spiny rose galls), fuzzy (e.g. hedgehog gall, woolly rose gall), cone-like (e.g. eastern spruce gall), well defined (e.g. nipple galls), deformed (e.g. many psyllid galls), or abnormal clusters of buds, shoots, or leaves (e.g. witches brooms). The variations are nearly endless.
Usually not required.