Gall

Sectioned oak marble gall showing central "cell", inquiline chamber, and exit-hole with a possibly parasitised stunted gall specimen.

Insect galls are the highly distinctive plant structures formed by some herbivorous insects as their own microhabitats. They are plant tissue which is controlled by the insect. Galls act as both the habitat and food source for the maker of the gall. The interior of a gall can contain edible nutritious starch and other tissues. Some galls act as "physiologic sinks", concentrating resources in the gall from the surrounding plant parts.[3] Galls may also provide the insect with physical protection from predators.[4][5]

Insect galls are usually induced by chemicals injected by the larvae of the insects into the plants, and possibly mechanical damage. After the galls are formed, the larvae develop inside until fully grown, when they leave. In order to form galls, the insects must take advantage of the time when plant cell division occurs quickly: the growing season, usually spring in temperate climates, but which is extended in the tropics.

The meristems, where plant cell division occurs, are the usual sites of galls, though insect galls can be found on other parts of the plant, such as the leaves, stalks, branches, buds, roots, and even flowers and fruits. Gall-inducing insects are usually species-specific and sometimes tissue-specific on the plants they gall.

Goldenrod round gall made by the fly Eurosta solidaginis

Gall-inducing insects include gall wasps, gall midges, gall flies (e.g., the goldenrod gall fly), Agromyzidae, aphids (such as Melaphis chinensis, Pemphigus spyrothecae, and Pemphigus betae), scale insects, psyllids, thrips, gall moths (e.g, Epiblema scudderiana), and weevils.[6]

Galls produced by insects and mites include:

• Ash flower gall: this gall is caused by a small mite that causes irregular distortion of male flowers. The galls are initially green, then dry and turn brown.
• Ash midrib gall: normally 0.5 to 1 inch long, these galls are succulent and have thick walls. A small cavity within each gall contains one or more small maggots, the larval stages of very small flies called midges. Female midges lay their eggs in very young leaflets during early spring. Gall formation begins soon after the eggs are laid. Specifics of the biology of this insect are not known. The galls probably do not harm tree health.
• Elm cockscomb gall: these distinct galls, caused by an aphid, are about one inch long and about 1/4 inch high. The irregular edge of the gall and its red color at maturity account for the common name. The galls dry, harden and turn brown as they age. Aphids may be seen through a slit-like opening in the underside of the gall. This insect has a complex life cycle—it forms galls on elm in early summer, then feeds on grass roots later in the summer. The galls apparently do not cause significant harm to the tree.
• Hackberry leaf gall: this gall is caused by a small (0.1 inch long) aphid-like insect with sucking mouthparts called a jumping plant louse. The adults spend the winter under bark crevices and can invade houses in large numbers in the fall. Females lay eggs over a long period of time beginning when leaves begin to unfold from the buds in the spring. Feeding by the nymphs that hatch from these eggs causes abnormal plant growth that forms a pouch. The psyllids remain inside the galIs until they emerge as adults in late summer to early fall. There is one generation each year. Heavy infestations can result in premature leaf drop which over a series of years may affect tree health.
• Honeylocust pod gall: this gall is caused by a small fly (midge). The sunburst cultivar appears to be very susceptible to this pest. Infested leaves have globular or pod-like distortions that contain one to several small maggots (0.25 inch long). Infestations begin when females lay eggs in young leaflets. There are five or more generations each year. Infested leaves often drop prematurely and repeated damage can kill small branches. New shoots develop at the base of dead twigs. As a result, the natural shape of the tree may be lost.
• Oak gall: see Oak apple
• Petiole and stipule galls: thick globe-like galls can develop on leaf petioles and stems. Many of these are caused by insects called phylloxerans which are very similar to aphids. The hard, woody galls may remain on the tree for several years. Usually, there is one generation each year and the insects over winter on the tree in the egg stage.
• Willow shoot galls: these swellings on shoots, twigs, or leaf petioles, may be caused by small flies (midges) or small wasps (sawflies). The gall increases in size as long as the immature stages are active. They cause no significant injury. The infestation may be reduced by pruning and destroying the galled areas before the adult insect emerges, usually in late summer.
• Witchhazel gall: this gall is caused by an aphid that passes the winter in eggs laid on twigs of the plant. Feeding by the aphid causes the formation of conical galls on the upper side of the leaf. Each gall, produced by a single aphid, later becomes filled with offspring. Mature aphids with wings leave the galls in late spring and early summer and fly to birch. After several generations there, the insects return to witch hazel to lay the eggs that survive the winter. No galls are formed on the birch.

Rhododendron ferrugineum infected by an Exobasidium fungus.

Many rust fungi induce gall formation, including western gall rust, which infects a variety of pine trees and cedar-apple rust. Galls are often seen in Millettia pinnata leaves and fruits. Leaf galls appear like tiny clubs; however, flower galls are globose. Exobasidium often induces spectacular galls on its hosts.

The fungus Ustilago esculenta associated with Zizania latifolia, a wild rice, produces an edible gall highly valued as a food source in the Zhejiang and Jiangsu provinces of China.[7]

Agrobacterium tumefaciens and Pseudomonas savastanoi are examples of gall-causing bacteria. Gall forming virus was found on rice plants in central Thailand in 1979 and named rice gall dwarf. Symptoms consisted of gall formation along leaf blades and sheaths, dark green discoloration, twisted leaf tips and reduced numbers of tillers. Some plants died in the glasshouse in later stages of infection. The causal agent was transmitted by Nephotettix nigropictus after an incubation of two weeks. Polyhedral particles of 65 nm diameter in the cytoplasm of phloem cells were always associated with the disease. No serologic relationship was found between this virus and that of rice dwarf.

Nematodes are microscopic worms that live in the soil. Some nematodes (Meloidogyne species or root-knot nematodes) cause galls on the roots of susceptible plants. The galls are small, individual and beadlike in some hosts. In other plant species galls may be massive accumulations of fleshy tissue more than an inch in diameter. Some ectoparasitic nematodes (nematodes that live outside the plant in the soil), such as sting and stubby-root nematodes, may cause root tips to swell. Nitrogen-fixing bacteria (Rhizobium species) cause swellings on the roots of most legumes (such as clover, peas and beans). These swellings, called nodules, are easily distinguished from root-knot galls by differences in how they are attached to the root and their contents. Nodules are loosely attached to the root, while root-knot galls originate from infection at the center of the root, so they are an integral part of the root. In addition, fresh Rhizobium nodules have a milky pink-to-brown liquid inside them, while root-knot galls have firmer tissues and contain female root-knot nematodes (creamy white beads less than 1/32-inch in diameter) inside the gall tissues.

Mistletoe can form galls on its hosts.