What Are Plant Galls and What Causes Them?

Plant galls represent abnormal growths or swellings that appear on various parts of plants, including leaves, stems, roots, and flowers. These are modified plant tissues. Galls display remarkable diversity in appearance, varying greatly in shape from spherical, bullet-shaped, or flower-like formations to irregular lumps and knobs. They also come in a wide range of colors, often appearing red, green, yellow, or black, adding to their distinctive appearance.

How Galls Develop

Gall formation begins when an external organism stimulates the plant’s tissues, triggering abnormal growth. This process often involves the inducing organism manipulating the plant’s natural growth hormones, particularly auxins and cytokinins. These hormones regulate cell division and enlargement; their disruption leads to uncontrolled cell proliferation and expansion, forming the gall.

The gall is composed entirely of plant tissue, but its development, including its unique shape, is actively directed by the gall-inducing organism. Chemicals secreted by the inducer, for instance through saliva during feeding or injected during egg-laying, initiate and sustain this growth. Within the developing gall, specialized nutritive tissues rich in nutrients are formed, providing a concentrated food source for the inducer.

Organisms That Induce Galls

Insects and mites are the most frequent culprits in gall formation. Among insects, gall wasps are prominent inducers, creating a diverse range of galls, particularly on oak trees. Other insect groups include gall midges, aphids, and psyllids. Mites, including eriophyid mites, are also common gall-formers, often causing distinct pouch-like or hair-like growths on leaves.

These organisms induce galls for their survival and offspring protection. The gall shields developing larvae from predators, parasites, and harsh environmental conditions. It also provides a consistent food supply through its specialized nutritive tissues. Beyond insects and mites, other organisms like fungi, bacteria, and nematodes can also trigger gall development. For example, the bacterium Agrobacterium tumefaciens causes crown galls by transferring genetic material into plant cells. Certain rust fungi and root-knot nematodes also induce abnormal plant structures.

Ecological Role of Galls

Galls are generally not severely detrimental to the overall health of mature host plants. Most galls cause minimal damage, primarily presenting as an aesthetic concern. However, in cases of heavy infestations, on young or stressed plants, or when galls form on branches and girdle them, they can impede nutrient and water flow, potentially causing localized dieback or, in rare instances, plant mortality. Some gall-inducing organisms can also have significant economic impacts on agricultural crops.

For the inducing organism, galls offer a protected microenvironment with a stable temperature and humidity, along with a dedicated food source. Galls also play a role in broader ecosystem biodiversity, serving as microhabitats for a variety of other organisms, including inquilines (species that live within the gall but do not induce it), parasites, and transient insects. Potential indirect benefits for the host plant include increased frost resistance or the attraction of beneficial organisms.