ILLUSTRATED GUIDE TO THE PLANT GALLS OF THE ROEMER ...

SUNY Geneseo Journal of Science and Mathematics 1(1), 2000: 16-32 16

ILLUSTRATED GUIDE TO THE PLANT GALLSOF THE ROEMER ARBORETUM

AT SUNY GENESEO

KATHRYN ALEYA WEISS

Abstract- The Roemer Arboretum on the SUNY Geneseo Campus is an excellent area to study the formation ofgalls on a variety of plants. This paper was produced from field and literature research on galls and gall-inducing

organisms during a directed study, and is a catalog and guide for students. I observed and sketched the galls in theArboretum over a semester. Gall producers change from year to year however, the most common ones, including

those found on Oak trees, are listed here. The purpose of this guide is to provide a reference for future workstudying galls. Future experiments in the spatial distribution of galls would also be well suited to the Arboretum.

IntroductionA gall is an abnormal growth or

tumor on plant host tissue caused by theirritation of an insect, bacterial, or fungalparasite. Gall-inducing insects lay their eggsin the spring on the surface of plants, orinsert their ovipositor directly into the plant.Throughout the spring, summer, andsometimes even into the fall, the presence ofthe insect causes the plant to produce aprotective gall full of nutritive tissue that canvary in sizes from microscopic to as large asan apple. The parenchyma cells of theplant’s mesophyll are induced to form a gallby both the insect’s actions (Giuntoli 1985)and chemicals produced by the insect. Theinsect egg hatches into a larva, and usuallyfeeds on the nutritive plant tissue during thewinter before pupating. Parasitism by otherinsects also occurs. Some species of insectslay their eggs in galls that have already beenformed. The parasite organism will thenfeed on the original gall maker. I focused onidentifying the primary gall maker in thisstudy. The following spring, the new adultexcavates a perfectly round exit hole in theprotective gall and emerges to mate and beginthe cycle again. Some insects emerge the

same year they are formed. Galls caused byfungi produce spores that are carried by thewind to other plants.

The exact mechanism of gallformation is not known. The mechanicalirritation of insect larvae growing within theplant may induce the formation of the gall,which is essentially scar tissue. Theirritation may be exacerbated by viralsymbionts in the growing insect’s saliva.(Giuntoli 1985) As the insect feeds on thegall, its actions cause the gall to grow larger.The insects also produce chemicals thatinduce the formation of galls. It has beentheorized that insects can mimic the plant’sown growth hormones in order to induce gallformation. One such hormone is IndoleAcetic Acid (IAA). (Giuntoli 1985)

Interestingly, some insects have theability to become phloem sinks within theplant, channeling some of the plant’snutrients directly to the growing larvae.(Larson 1997) The insect takes advantage ofsource-sink relationships in the plant, andthus becomes a competitor with the plantfor resources.

Gall insects are plant parasites, or inthe best cases, merely commensalists. If the


host plant is not harmed by the presence ofthe gall, then the relationship is acommensalism. (Giuntoli 1985) However, ifthe plant suffers because of a particular gall,then the insect is considered a parasite.Despite their wide distribution, galls usuallydo not have a large detrimental effect on theirhosts unless they are highly concentrated onone plant. Gall formation may stunt thegrowth only on the particular part on whichit grows. For example, goldenrod stems thatcontain galls usually have fewerinfloresences at the top than theircounterparts in the same area without galls.(Felt 1940) Too many galls on one host willharm the plant; it is in the best evolutionaryinterest of the gall not to totally destroy itshost because of the shelter and nutrients theplant provides over a long-term basis. If thehost dies, the insect, bacteria, or fungusliving within it will also die. Therefore,feeding on plant tissue must be gauged as tostimulate growth of the plant without killingtoo many plant cells. (Felt 1940)

The most common sites for galls arefruit, seeds, stem tips, shoot tips, andfoliage. Notice the trend in location; thesites that galls form on are those that areundergoing the most rapid growth. Usingthe plants own meristematic tissue, the gall-maker can quickly induce the formation ofthe rich nutritive tissue in the gall. Gallssuch as Oak Bullet galls (illus. #4) andCrown Gall bacteria are also formed on twigsand branches, but they are usually muchharder and thicker than galls formed on othertissue.

Because galls are formed from theplant’s own tissue, a gall takes its form notonly from the organism that initiated itscreation, but also the location of the gall onthe plant. For example, many galls that formon leaves are fuzzy because leaves often

have tiny hairs; the gall is an abnormalgrowth of the plant’s own cells. Anotherexample is the oak bullet gall. Even thoughother galls on oak trees tend to be fairly soft,bullet galls are very hard because they formfrom the tough cells that make up twigs, andare just an aberration of those cells.

Some gall wasps do not actuallycreate their own galls, but lay their eggs inthe already formed galls of other insects.During the winter, the predatory wasp larvawill feed on the original grub inhabitant.Thus, in the spring, many of the insectsemerging from the galls are not the architectsof the galls, but other insects that have takenadvantage of a gall’s protection and foodsupply. The predatory wasps are notcompletely safe in their new home either.Many species of birds can recognize certaingalls, and peck holes inside to reach the tastyinsects. A hole made by a bird can beidentified by the rough edges around thehole. The hole is quite large on the outersurface, then gradually tapers down as itnears the center of the gall.

Identifying even microscopic gall-makers is not as hard as it seems, becauseeach gall-maker produces a uniquely shapedgall. In fact, many times it is easier toidentify the tiny insect or organism by thegall it produces, rather than by its ownanatomical features, as the characteristics arequite specific. (Giuntoli 1985) However,this method of identification has led todifferent names for the same insect becauseof differing host species and life cycle. Thisaspect can make identification difficult.Galls are usually found in patches; you willprobably find a number of similar galls in anarea, or none at all. However, thedistribution of galls is spatially dependent.Galls may be clumped in a small local area


such as a stand of trees, but evenlydispersed overall throughout a forest.

Study Site and ProceduresAll observations and drawings of galls weremade in the Roemer Arboretum on theSUNY Geneseo campus. The Arboretum isa 20-acre area of land that was previouslygrazing land, and is now specifically setaside for field study. It is an excellent placeto find galls. The large old trees show a“wolfed”, or outspread growth form, and soleaf and twig galls are easier to reach fromthe ground. Also, since the area isundergoing secondary succession after beingpastureland for many years, there are manysmall trees and shrubs. Rapidly growingplant tissue is where most galls are formed.Many of the plants in the Arboretum areoften hosts, and a number of interesting gallsare easily found on a walk. Since I was not

doing a statistical survey, I did not have toworry about a random sampling, but insteadconcentrated on covering as much of the areaas possible. I paid special attention to theOak trees because so many different typesof galls are found on these trees. I used thebasic tour map to find the locations of thelarger oaks. There is a group of White Oaksjust after the bridge at the entrance to theArboretum. There are Red Oaks as well asWhite Oaks on the lower poison ivy trail,and near the study plot. I went out aboutonce a week to gather galls while the weatherwas good. My study lasted from Septemberto December 1999. Since the leaf galls aremade of the same tissue as leaves, they didnot keep well, and so I did not preserve thegalls after I sketched them. To identify thegalls I found, I relied on the books includedin the Reference section. Plant Galls andGall Makers by Ephraim Porter Felt wasparticularly useful because it contained a keyto many galls. I also accompanied Dr. Briggson a walk through the Arboretum. Hehelped me identify trees and other plantsthat commonly housed galls. I spoke withDr. Vasey about the anatomy and behaviorsof the causal insects, especially EurostaSolidaginis.

Results and DiscussionList of GallsCommon gall name Gall inducerAsh leaf gall Lasoptera fraxinifolia gall midgeCedar-Hawthorne (Apple) rust Gymnosporangium globosum fungus Cedar stage (illus. #1) Hawthorn stageGoldenrod gall (leafy) (illus. #2) Rhopalomyia solidaginis midge fly


Goldenrod gall (spherical stem) (illus. #3) Eurosta solidaginis gall flyGoldenrod spindle gall Gnorimoschema gallaesolidaginis mothGrape: phylloxera gall Phylloxera vitifoliae aphidHawthorn: bud gall Gymnosporangeum fungusOak: bullet gall (illus. #4) Disholcaspis globolus gall waspOak: leaf midrib fuzzy gall(clustered) Biorhiza mellea gall waspOak: leaf midrib gall (illus. #7) Callirhytis flavipes gall waspOak: transverse leaf gall Andricus singularis gall waspOak: peach gall Cynips capillata gall waspOak: saucer gall (button gall) (illus. #5) Neuoterus umbilicatus gall waspOak: spangle gall (illus. #6) Cecidomyia poculum midge flyOak: striped peach gall Cynips dimorphus gall waspOak: hedgehog gall Acraspis erinacei gall waspOak: hairy oak gall Acraspis villosa gall waspOak: spiny vase gall Xanthoteras gall waspsOak: spotted oak apple gall (illus. #8) Cynips centricola gall waspOak: wooly fold gall Cecidomyia midge fliesRose bud gall Diploepis sp. gall waspRose: mossy gall Diploepis rosea gall waspWillow blister gall Oligotrophus salicifolius gall midge

Dichotomous Key for Oak Galls in the Roemer Arboretum

1. Found on leaves 3.Found on twigs or at base of leaves 2.

2. Brown, round, and hard; found attached to twigs Oak Bullet Gall (illus.#4)Thin-walled, spotted, fibrous inside; attached totwigs or base of leaves Spotted Oak Apple Gall illus.#8)

3. Found directly on veins 6.Found anywhere else on leaf 4.

4. Flat, round 5.Any other shape 10.

5. White color Oak Spangle Gall (illus.#6)Brown color Oak Saucer Gall (illus.#5)

6. Found on main vein Oak Leaf Gall (Main Vein) (illus.#7)Found on any other vein 7.

7. Oblong, fuzzy Wooly Fold GallRound, fuzzy 8.

8. Peach colored, clustered 9.Brown Fuzzy Oak Gall


9. Striped Striped Peach GallPlain Peach Gall

10. Has spiny or wooly emergences 11.Smooth Transverse Oak Leaf Gall

11. Spiny emergences 12.Wooly appearance Hairy Oak Leaf Gall

12. Round Hedgehog GallVase-shaped Spiny Vase Gall

Guide to the Galls of the RoemerArboretum

Ash Leaf Gall (Causal organism:Lasoptera fraxinifolia gall midge)

Tiny (0.1 centimeters) fuzzy yellowgalls are found on the underside of Ashleaves. Multiple galls form on each leaf, butthe galls are spaced out upon the leaf. Thereis an indentation in the center of the gall, andthe entire gall resembles a tiny crown. Thisgall occurs along the leaf veins as well as thespaces between veins.

Cedar-Hawthorn (Apple) Rust (Causalorganism: Gymnosporangium globosumfungus) Illustration #1

Cedar stage: Light brown gall withrounded bumps all over. The gall may havelobes and has a large range in size, anywherefrom 1 to over 4 centimeters in diameter.They are most often found on the ends ofbranches. In the spring, the galls have horn-like projections. When it rains, the gallabsorbs moisture, and the projections enlargeinto gelatinous, spore-bearing horns.(Hutchins 1969) In the fall, the horns aremissing and the rather lumpy shape of thegall can best be seen.

Hawthorn Stage: The spores releasedby the cedar stage are carried by the wind tohawthorn trees, where small projectionssurrounded by rust-colored spots appear onthe leaves. The spores will later be released

from the leaves, and blow back toward thecedar trees. (Hutchins 1969)

Cedar trees also share a similarrelationship with apple trees, transferring afungus called cedar-apple rust in the sameway.

See also: Hawthorn bud gall.

Leafy Goldenrod Gall (Causal organism:Rhopalomyia solidaginis midge fly)Illustration #2

This gall looks like a rosette, and iscomposed of a large cluster of stunted leavesat the top of the stem. Goldenrods thathouse these insects produce fewerinflorescences. The gall itself is containedwithin the cluster of leaves. Look carefullyat the tops of goldenrod stems in thesummer and early fall to find these galls.They are much larger than normal clusters ofleaves.

Spherical Goldenrod Gall (Causalorganism: Eurosta solidaginis gall fly)Illustration #3

This is the most common gall foundon goldenrod plants. Gall flies lay their eggsin the center of the stem, causing it tobecome bulbous and enlarged. They are thesame color as the stem, and may have tinyleaves attached to the outside. These ball-


shaped galls can be collected in the wintersince the insect affects the tracheid cells, andthe internal structure of the entire stem ischanged. This means that during the fall andwinter, the bulbous forms of the galls caneasily be seen on the stems after the leaveshave died (senesced). Each gall usuallycontains one insect that emerges in thespring, boring a round hole out of the gall.However, many species of birds prey on thewasps by pecking holes into the galls. Gallsthat have been preyed upon by birds can beidentified by the large, jagged, roughly roundholes made by the birds that taper downuntil the insect is reached. See theillustration for examples of insect-made andbird feeding holes. The smaller, round holeon the right was made by the emerginginsect. The larger, jagged hole was made bya bird looking for a meal. Goldenrod galls arealso parasitized by gall wasps, who lay theirown eggs inside already-formed galls. Theyoung wasp will eat the fly larvae and thenemerge in the spring. So, even though theshape of the gall it produces can identify thecausal organism, the insect that emerges isnot always the insect that produced the gall.

Goldenrod Spindle Gall (Causalorganism: Gnorimoschemagallaesoligaginis moth)

This gall is formed on the stem ofgoldenrod plants and is elliptical in shape.In all other respects it looks very much likethe gall produced by Eurosta. The femalemoth lays an egg on the stem. It soonbecomes a larva that crawls up the stem andburrows in. The larvae is believed to excretea “cecidogenetic substance” (gall producing)that then causes the elliptical gall to form.(Meyer 1987)

Grape Phylloxera (Causal organism:Phylloxera vitifoliae aphid)

Grape Phylloxera galls are one of thefew types of gall that can be economicallyimportant and destructive. Too manyPhylloxera galls has seriously harmed fruitoutput and the wine industry of France inthe past. (Hutchins 1969) The insects creategalls on both the roots and the leaves,damaging both. Dozens of galls will form onone leaf, seriously depleting the leaf’snutrient supply and photosynthetic ability.

Hawthorn Bud Gall (Causal organism:unknown, a fungus, perhaps closelyrelated to the one that causes Cedar-Hawthorn rust)

The tips of hawthorn buds becomeenlarged when this gall is present. The toughpetioles are also enlarged and project out.The gall is the same dark color as thebranches and is greater than a centimeter inlength. This gall may be related to theCedar-Hawthorn rust, but the causalorganism is not known.

Galls on Oak TreesThe Oak family supports the

greatest diversity of gall producers, over 800species of insects and mites form galls.Bacteria, fungi, and viruses also form galls.Gall wasps are the most common gallproducers on Oak trees. Tiny Cynipidwasps, less than 1/8 inch in length affectroots, flowers, acorns, and especially leavesand twigs. (Day 1996) These wasps, whichwill not sting humans, have complex lifecycles of alternating generations. Differentgenerations may produce galls that lookcompletely different. In the past, scientistsdid not recognize the different generations asthe same insect, and so gave them differentscientific names. See the description of the


hedgehog gall for an example of a wasp lifecycle.

Galls formed in the fall are usuallymuch harder than those formed in the spring.They fall to the ground and the wasps maynot emerge until the next year. In somecases, the insects may not emerge for up tofive years. (Hutchins 1969)

Oak Bullet Gall (Causal organism:Disholcaspis globolus gall wasp)Illustration #4

Bullet galls are found on small oaktwigs that are usually less than a centimeterin diameter. The galls are spherical and veryhard, and may be found in pairs or evengroups of three. They are the same color asthe twigs and are covered in tiny bumps.Multiple exit holes can be found on thelarger galls.

Transverse Oak Leaf Gall (Causalorganism: Andricus singularis gall wasps)

These oak galls are round, protrudefrom both the top and bottom of the leaf inthe spaces between the veins, and grow onRed Oak. The gall itself is sandwichedbetween the outer membranes of the leaf,and is smooth and green, about 1 – 1.5centimeters in diameter. Exit holes are foundon the underside of the leaf. Light brownand smooth on the upper surface of the leaf,the gall changes to an even lighter shade onthe underside, punctuated with small blackdots. They are closely related to oak applegalls (discussed later.)

Oak Peach Gall (Causal organism:Cynips capillata gall wasp)

Peach galls form in clusters along aleaf’s midrib, on the underside of the leaf,and look very much like small peaches.They are usually light pink in color and

about 0.5 centimeters in diameter. Mostcommonly, they form on White oaks. Theymay be plain or striped; the Striped PeachGall is formed by a Cynips dimorphus gallwasp.

Oak Saucer Gall (Button Gall) (Causalorganism: Neuoterus umbilicatus gallwasp) Illustration #5

Saucer galls are brown and found onthe upper surface of oak leaves. The gall iscircular with raised edges and an indentationin the center. White marks can be found onthe underside of the leaf opposite the gall.They are usually a little less than 1centimeter in diameter.

Oak Spangle Gall (Causal organism:Cecidomyia poculum midge flies)Illustration #6

Spangle galls are tiny and white, andwhen they cover the leaf, they look likesequins or spangles. They are usuallyclustered on the underside of the leaf. Theyare round, raised on the edges, and indentedin the middle except for a tiny bump in thevery center where the midge larvae is. Exitholes are seen near this bump. Each galltypically contains one fly, but there may bemany galls covering each leaf.

Oak Hedgehog Gall (Causal organism:Acraspis erinacei gall wasp)

These galls are found on the leaves ofwhite oak, and are round with short spinesevenly spaced around the exterior. Usually,they are about _ inch in diameter and greenin the spring. In the fall, they turn brown.Inside each gall lives one to eight wasps.Acraspis wasps have a complicated life cycleof alternating generations. In the spring,male and female wasps mate, then the femalelays her eggs in oak leaves. These eggs will


cause the large hedgehog gall to form slowlyduring that summer. In late autumn, theinsects emerge from the hedgehog galls.They are all females with smaller wings and,through parthenogenesis they lay their eggson oak leaves, causing tiny galls on the budscales that last through the winter. Thewasps that then emerge in the spring areboth male and female, and the cycle beginsagain. (Hutchins 1969) One explanation forthe all-female generation in the cool weatheris that it might be difficult to find matesduring this time; if the females can lay fertileeggs without mating, the problem is solved.(Hutchins 1969)

Oak Leaf Midrib Fuzzy Gall (Causalorganism: Biorhiza mellea gall wasp)

These galls are green, and found inclusters on the upper surface of oak leaves.They are small (usually about .25centimeters in diameter) and round, and arecovered with tiny fuzzy dots that are veryclose together. They are soft and fleshy.

Oak Leaf Midrib Gall (Causal organism:Callirhytis flavipes gall wasp) Illustration#7

This gall occurs singly, and is aswelling of the leaf’s midrib. Galls are hardand green, and are usually centered on theunderside of the leaf. They are usuallyfound in the spring, (Felt 1940) but persistin the leaf until it senesces (dies and dropsoff the tree) in the fall.

Hairy Oak Leaf Gall (Causal organism:Acraspis villosa gall wasp)

This gall is hairy and green in thespring, and turns a rusty brown color in thefall. It looks like a little ball of wool. Theyare usually a little over one centimeter indiameter and are found on the upper sides of

the leaves. The insect larvae is hiddenwithin the wooly mass.

Spiny Vase Oak gall (Causal organism:Xanthoteras gall wasps)

Spiny Vase galls are complexstructures that resemble tiny vases and arecovered with tiny red or yellow spines.These unusual galls are found in clusters onoak leaves. They form in the late autumn.

Spotted Oak Apple Gall (Causalorganism: Cynips centricola gall wasp)Illustration #8

These large (2.5 cm or more) roundgalls are found attached to either oak leavesor twigs. They have thin shells and whencut open, reveal fibrous strands radiatingfrom a central core. In this core the wasplarva, or grub, spends the winter. When firstformed in the spring, they are green withbrown spots; during the summer and fall, thegreen parts turn brown, but the distinctivespots are still visible. In the spring the adultwasp will gnaw its way out of the gall, thenmate and lay eggs, continuing the cycle.Look high up the oak branches for thesegalls. They are common, but sometimesdifficult to find.

Wooly Fold Oak Gall (Causal organism:Cecidomyia gall fly)

These galls form on the top side ofthe oak leaf, directly along a vein. The veinbecomes enlarged and fuzzy. Usually thegall is light green, a lighter shade than the leafitself. They are hollow, and inside the gallnumerous midge fly larvae spend the winter.

Rosebud gall (Causal organism:Diplolepis sp. wasp)

Rosebud galls are swellings of thebuds. The female wasp inserts her


ovipositor into the bud (Meyer 1987) andlays an egg. The galls are tough, brown andscaly, and can be found even into the winter,especially once the leaves have been shed.

Mossy Rose Gall (Causal organism:Diploepis rosea wasp)

Mossy rose galls can be foundanywhere on the rose, not just the ends (asis the case for the rose bud gall.) Their greencolor turns to brown in the fall. The gallconsists of a central portion that containsthe larvae, and “supple, branchedemergences” (Meyer 1987) that completelycover the core.

Spiny rose galls, caused byDiploepis bicolor are also formed on rosebushes and blackberry bushes. They arecovered with long, slender spines and rangefrom light green in color to pinkish. Nonewere found in the Arboretum this season,but it is very likely that they would formhere.

Willow blister gall (Causal organism:Oligotrophus salicifolius gall midge fly)

Galls on willows are found in theleaves, transverse to both sides of the leaf.They are green with fuzzy orange dots,spherical, and the portion of the leaf past thegall to the tip dies and turns brown. Thegalls are formed next to the leaf vein, andhave a relatively straight side adjoining thevein. A large (0.18-cm) exit hole is apparenton the underside of the leaf.

AcknowledgementsI wish to thank my mentor, Dr. GeorgeBriggs, for his help in researching and writingthis guide, and also Dr. Carey Vasey.

Literature CitedDay, Eric. 1996. Galls. Virginia Polytechnic

Institute Cooperative Extension,Blacksburg, Virginia. 3 pp.

Felt, Ephraim Porter, D.Sc. 1940. PlantGalls and Gall Makers. ComstockPublishing Company, Inc., Ithaca,New York, USA. 364 pp.

Giuntoli, Nick Morgan. 1985. Wasp Gallson Oak Leaves—The Anatomy of anEcological Relationship. [M.A.thesis]. San Jose State University,San Jose, California, USA. 58 pp.

Hutchins, Ross E. 1969. Galls and GallInsects. Dodd, Mead & Company,New York, New York, USA.

Larson, Katherine C, Thomas G. Whitham.1997. Competition between gallaphids and natural plant sinks: plantarchitecture affects resistance togalling. Oecologia 109: 575-582.

Meyer, Jean. 1987. Plant Galls and GallInducers. Gebruder Borntraeger,Berlin, Germany.

291 pp.


Cedar-Hawthorn RustCaused by Gymnosporangium globosum


Leafy Goldenrod GallCaused by Rhopalomyia solidaginis


Spherical Goldenrod GallCaused by Eurosta solidaginis


Oak Bullet GallCaused by Disholcaspis globolus


Oak Saucer GallCaused by Neuoterus umbilicatus


Oak Spangle GallCaused by Cecidomyia poculum


Oak Leaf Midrib GallCaused by Callirhytis flavipes


Spotted Oak Apple GallCaused by Cynips centricola

ILLUSTRATED GUIDE TO THE PLANT GALLS OF THE ROEMER ...

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