www.landscape.org The Remarkable World OF INSECT GALLS By Joe Boggs, Assistant Professor, Commercial Horticulture Educator, Ohio State University Extension / OSU Department of Entomology A RICH HISTORY ever, abnormal gall-like growths that are Galls found on trees are remarkable caused by something else can also occur plant structures that have been observed, on trees. The growths may be stimulated studied, and utilized since ancient antiq- by exposure to chemicals such as her- uity. The Greek naturalist Theophrastus bicides or even misdirected plant hor- (372-286 B.C.) wrote about “gall-nuts” mones circulating within the tree. Burls of Syria. The Roman naturalist Pliny the are a good example. Elder (23-79 A.D.) wrote about medical Fungal cankers on trees are some- uses of gall extracts. Gall ink, which is a times mistaken for galls and vice versa. mixture of iron and gallic acid (3,4,5-tri- The big difference is that cankers involve hydroxybenzoic acid) extracted from oak the death of plant tissue through the fun- Larger Oak Apple Gall galls, was used for centuries for art, liter- gal infection; galls involve the growth of ary works, and official documents. plant tissue. The development of callus The Dead Sea Scrolls were written in tissue to close wounds created by can- iron-gall ink. Leonardo da Vinci wrote his kering fungi adds to the confusion. notes with it, J.S. Bach composed with it, Various fungal fruiting structures are Rembrandt and Van Gogh drew with it, occasionally mistaken for galls, particu- and the Magna Carta was penned with larly the shelf-like growths sprouting out gall ink. Indeed, much of our history was of trees that are produced by bracket literally written in gall ink. The U.S. Dec- fungi. These structures are the mac- laration of Independence, the Constitu- roscopic evidence of the microscopic tion, and the Bill of Rights; collectively wood decay fungi digesting the tree from referred to as the “Charters of Freedom” within. Their activity may be dramati- by the U.S. National Archives, were cally disclosed when trees fall; galls do penned on parchment using iron-gall ink. not cause such damage. The brackets With such a rich history, it could be are solely composed of fungal tissue with Not a Gall - A Burl on Black Walnut assumed that galls are well understood. the purpose of producing fungal spores, In fact, despite galls being one of the thus the name “fruiting structures.” most obvious and unusual of plant struc- Continued on page 40 tures, there remains much that we do not know. The identification of the gall- makers behind many of our most note- worthy galls remains a mystery. Indeed, scientists continue to debate exactly how galls are formed; so far, no scientist has ever grown a plant gall without help from a natural gall-maker. WHAT ARE PLANT GALLS? Not a Gall - A Fungal Canker The name “gall” may conjure a range of images; some accurate, some inaccu- Not a Gall - Dryad Saddle Bracket on Redbud Produced by Fungus Produced by Polyporus Botryosphaeria dothidea rate. True plant galls are produced under the direction of a living gall-maker. How- squamosus The Landsculptor – February 2017 39 www.landscape.org The Remarkable World OF INSECT GALLS Continued from page 39 are composed of a chaotic mass of unor- ganized plant cells that have been modi- This article will focus on galls pro- fied by bacterial DNA. duced by insects. However, there are many organisms that can induce the formation of true galls including microor- ganisms such as viruses, bacteria, fungi, nematodes, and eriophyid mites; as well as macroorganisms such as insects. In- deed, some of the most obvious plant galls are produced by eriophyid mites (order Trombidiformes, family Eriophy- Larger Oak Apple Gall Opened idae) and various rust fungi such as the to Show Gall Wasp Larva cedar-apple rust fungus, Gymnosporan- gium juniperi-virginianae. extrafloral nectaries. The nectar attracts other insects, such as ants and wasps, Bacterial Crown Gall on Euonymus which, in turn, protects the developing immature gall-maker from predators. THE WONDERS OF The biting or stinging insects may also INSECT GALL GROWTH protect the galls themselves from her- Gall-making insects direct the growth bivores determined to feed upon the of highly organized gall tissue by produc- nutrient-rich plant gall tissue. The ability ing chemicals that turn plant genes on for gall-makers to use chemicals to turn and off at just the right moment to direct plant genes on and off to produce such plant cells to form highly organized and complex plant structures speaks to why predictable plant structures. Indeed, insect insect galls are so fascinating. gall structures are so predictable that the The gene-switching chemicals exuded Maple Spindle Galls Produced species of the gall-maker can be identi- by gall-makers can only act upon plant by the Eriophyid Mite Vasates fied by the gall structure alone without cells that are capable of becoming other aceriscrumena the need to see the gall-maker itself! types of plant cells; a process known as The chemicals used to direct gall growth include replicas of plant hormones (only plants can actually produce true “plant hormones”), or chemicals that may be nothing like plant hormones but the plant's response is the same as with plant hormones; these are called “hormone ana- logs.” The gall-forming process is usually initiated by the female when she injects these gall-inducing chemicals into the plant along with her eggs. The eggs them- Oak Bullet Galls Exuding Nectar Cedar-Apple Rust Fungal Gall selves may ooze gall-inducing chemicals with Telial Horns on Juniper and once the eggs hatch, the interaction between the plant host and gall-maker per- Insect galls are constructed entirely sists with the immature gall-makers con- of plant cells. This is unlike fungal galls tinuing to exude chemicals to direct plant which are an assemblage of fungal cells growth to suit their needs. intertwined with plant cells. Insect galls The resulting galls provide both a tend to be constructed of highly orga- protective home and nourishment for nized, sometimes specialized plant cells. the next generation of gall-maker. Some This is unlike bacterial crown galls pro- types of insect galls are comprised of duced under the direction of the bacte- complex plant structures and may in- Stinging Insects Attracted to rium, Agrobacterium tumefaciens, which clude functional plant organs such as Bullet Galls by Nectar 40 The Voice of Michigan’s Green Industry. www.landscape.org Oak Bud Gall Growing from a Leaf Bud Acorn Plum Gall throughout the growing season and are Locations of Meristematic Tissue found on all tree stems, galls can be formed from these cells anytime during differentiation. These unique plant cells the growing season and anywhere on a are called meristematic cells and are found live stem. Horned oak galls produced by both in plant buds and in the cambium the wasp, Callirhytis cornigera, are a good located within plant stems between the example of stem galls that arise from phloem and xylem. The meristematic cells cambial cells. Unfortunately, this is one in the buds are precursors to stem or leaf of the few galls that can cause apprecia- cells; it's how trees gain height. The cam- ble damage to their host trees. The galls Mature Acorn Plum Gall bial meristematic cells give rise to the phlo- may completely surround and girdle the em and xylem; it's how trees gain girth. stem and gall tissue may invade the xy- Under the influence of chemicals exud- lem to choke off the flow of water and ed by a gall-maker, the meristematic cells nutrients. As the result, the stem beyond that were originally destined to become the gall often dies. leaf cells begin marching to a different drummer. Once the errant leaf cells fall A FEW GALL WONDERS under the chemical spell of a gall-maker, The dominant insect gall-makers in- there is no turning back; they will become clude members of just three insect or- gall tissue. This means that gall formation ders: Hymenoptera (wasps, sawflies); cannot occur once meristematic leaf bud Hemiptera (aphids/adelgids, phylloxer- Acorn Plum Gall Opened to Reveal cells are committed to becoming leaf tis- ans, psyllids); and Diptera (midge flies). an Overwintering Wasp Larva sue; it's one reason the leaf-gall season There are more than 2,000 species of begins in the spring! However, once the insect gall-makers in the U.S.; however, lies: Cynipidae (“gall wasps”) and Ceci- galls start growing they will continue to three quarters belong to only two fami- domyiidae (“gall midges”). Of the over grow even after the leaves fully expand. 800 different gall-makers on oaks; over The descriptively named oak bud gall 700 are gall wasps. In other words, when produced by the gall wasp, Neuroterus trying to identify an insect gall, there is vesicula, is formed when a single bud is a high probability it was produced by a directed to become a reddish-brown, ball- gall wasp or gall midge and if the gall is like gall that is only slightly larger than a found on an oak, it was most likely pro- normal bud. Oak bullet galls produced by duced by a wasp. the gall wasp, Disholcaspis quercusglobulus, The acorn plum gall (a.k.a. oak acorn are another good example of a gall-maker galls) that sprouts under the direction hijacking tree buds for their own purposes. of the gall wasp, Amphibolips quercus- Stem galls that arise from cambial Horned Oak Gall Growing from juglans, from acorn caps is one of the tissue present a different scenario.