The main thing tree owners need to remember about gall formation is that in many cases, it does not damage trees. Consequently, trees infested with leaf galls do not need pesticides and other types of chemical controls.

Galls cause worry for tree owners each year, but in truth, most of these odd-shaped growths are more of a curiosity than a concern.

This is the time of year people frequently begin to notice galls on their trees. Galls have long been a part of tree life in Missouri and elsewhere in the central U.S. Some galls are small, pea-sized growths on leaves. Others appear on twigs or stems and are persimmon-size and sometimes larger. Many galls appear on oak trees, but other types of trees and other plants, as well, have been known to have galls.

These tumor-like growths are examples of parasitism. Galls are the end result of what happens when gall-making insects or mites manipulate the growth process of a tree for their own benefit. Galls usually form during the accelerated growth period of new leaves, shoots, and flowers in late spring. Insects or mites damage plants by chewing on them and the salivary secretions of these insects causes plants to increase production of plant growth hormones. Higher hormone production results in increased cell size or cell numbers. These abnormal cell growths are what are known as galls.

Mature plant tissues are usually not affected by gall-inducing organisms. The gall keeps growing as the gall-making insect feeds and grows inside the gall. Several types of common oak galls in this area are caused by wasps in the Cynipidae family of insects, which are – appropriately enough – often called gall wasps. Some wasps are so tiny they are hard to see and are easiest identified by the type of gall they cause to form on a tree.

The basic process of gall formation is this: In spring, a female gall wasp lays an egg in the developing plant material (either leaf or woody part) of a tree. A wasp larva hatches from this egg and begins to feed on the surrounding plant material. Hard plant material – the gall – forms around the feeding larva. This mass of hardened vegetative material creates a barrier between the larva and the rest of the tree, but it also serves as a cozy shelter for the larva to complete its development. Also, the larva manipulates the physiology of the plant to the extent that the gall tissue nearest the insect has increased nutrients. Some insects emerge from the gall in late spring or summer while other insects continue to develop inside the fallen gall and emerge the following spring. (It should be noted that Cynipid wasps are not the only gall-formers found in Missouri; mites and even fungi also forms galls on trees and plants, too.)

Some types of galls occur periodically and their frequency is driven by a larger-than-usual over-wintering population of the types of insects specific to those types of galls. Long periods of snow cover and/or consistent cold winter temperatures (no unseasonable warm-ups followed by freezes) allow higher survival of some types of gall-making insects in winter and this, in turn, may lead to larger gall infestations the succeeding spring. (It should be noted that winter weather patterns are only one of several factors that affect over-wintering insect populations.)

Since ancient times, galls have fascinated humans. Galls of some type of oak trees were mixed with other materials to make iron gall ink, a printing material that was used into the 20th century. Along with this functionality came folklore. It was once thought galls could predict bad times, depending on whether a larva or full-grown insect was found when the gall was cut.

Even stranger than their purported fortune-telling capabilities was a long-time association between galls and waterfowl. In 1755, in the Natural History of Norway, Danish naturalist Erik Pontoppidan wrote:

“It is said that a particular sort of geese is found in Nordland (a region of Norway) which leave their seed on old trees and stumps and blocks lying in the sea and from that seed, there grows a shell fast to the tree, from which shell, as from an egg, by the heat of the sun, young geese are hatched and afterwards grow up, which gave rise to the fable that geese grow on trees.”

Potoppidan dissected a number of galls and found nothing but larval insects. Despite his research, the belief in “tree geese” – also called “barnacle geese” – continued in parts of Europe into the 19th century.

Moving from fiction back to fact, the main thing tree owners need to remember about gall formation is that in many cases, it does not damage trees. Consequently, trees infested with leaf galls do not need pesticides and other types of chemical controls. The best tactics to get trees through gall stress or other types of problems are the usual tree-care procedures – mulch around the tree’s base, water during drought and avoid wounds to the trunk caused by mowers and trimmers. Raking and removing or burning fallen leaves may help reduce over-wintering fungi or gall insects, depending on the species of gall-maker.

Information about tree care can also be found at www.missouriconservation.org

Francis Skalicky is the media specialist for the Missouri Department of Conservation’s Southwest Region. For more information about conservation issues, call 417-895-6880.