How Galls Are Formed Galls are abnormal growths of plant tissue that form in response to a wound, infection by various microorganisms, or the feeding and egg-laying activity of certain Insects and mites. Galls occur on almost any plant tissues. The most common are leaf, stem and flower galls produced by Insects and mites. Galls produced by the hackberry nipplegall psyllid. Severe galling produced by the honeylocust podgall midge.
Galls often are unusual in form, conspicuous, and frequently cause considerable concern.
Galls produced by Insects and mites rarely cause serious threat to plant health. Most gall-maker populations fluctuate from season to season. The occurrence of many in one year usually is followed by few the next. Galls are produced by plant cells stimulated to abnormal growth. Galls from Insects and mites usually result from chemical secretions produced during feeding or egg laying.