Gall-Making Insects - John A. Byers

Cecidomyiidae Plant-Insect galls are better known than the organisms that produce them. Galls are colorful and unusual deformations of the meristematic (growing) plant tissues. The organisms are usually small and difficult to identify. One of the first habits of insects observed by man, although most likely unaware of the origin, was that of gall making. Pliny, about 60 A.D., describes plant galls and their rapid growth. Malpighi, the Italian physician and botanist, in 1686 attempted an explanation in the "De Gallis". It was common knowledge at the time that plant galls had medicinal and curative properties (Theophrastus was the first record in 372-286 B.C.) - although galls are now thought not to have such benefits for humans.
Definition: A plant gall is any abnormal growth caused by another organism (in this case we are speaking of insects, but galls are caused by mites, fungi, viruses, bacteria, and nematodes among others). However, as will be discussed later, the plant interacts with the causative organism and provides the raw materials (proteins, fats, carbohydrates) to construct the gall via abnormal tissue growth).

Insects and mites are the most common organisms that cause galls. These organisms can be called "horticulturists" for they develop new plant structures. Many leaf miners can produce cell proliferations that intergrade into true gall development. The attempt by plants to overcome injury by producing callous tissue is in a sense a kind of gall formation.
pinyon pine green and red stubby galls
At left: Pinyon pine needle stubby galls (green and red from different trees) formed by Janetiella coloradensis, a species of the gall midge family Cecidomyiidae.

The family Cecidomyiidae in the order Diptera (flies) have many species that intergrade between gall making and leaf mining. The tulip spot gall, Thecodipolosis liriodendri, is only a slight thickening of the leaf while the tar-spot gall, produced by a species in the genus Asteriomyia has even less thickening and is little more than a leaf mine. The boxwood leaf miner, Monarthropalpus buxi, makes a leaf mine but the surrounding tissues proliferate.

Tannic acid is one of the main economic products obtained from galls. A gall made by an Eurasian cynipid yields 65 percent tannic acid while American sumac galls yields 50 percent. Dyes are also obtained from galls. Turkey red dye is found in the "mad apple" gall. Natives of East Africa used to use galls as a source of dye for tattooing. However, galls were apparently not useful to American Indians. Pliny states that the Aleppo gall is able to dye hair black. Inks are also made from some galls, such as the Aleppo gall produced by Cynips gallae-tinctoriae on several species of oaks in eastern Europe and western Asia. Laws in some of these regions once stipulated that legal documents be made with inks from this gall. Even the USA Treasury, the bank of England, the German Chancellery, and the Danish Government have specified formulas for inks using the Aleppo gall.

Plant galls have also been used in ancient and even fairly recent medicines. Galls were used in the 1700s in France supposedly to control fevers. Today galls are not used medically except in a few ointments. Galls sometimes have been used as food, or rather as a spice. In the Near East, a gall on Salvia pomifera produced by a species of Aulax is known as the gall of sage or "pomme de sauge". It is aromatic and acidic in flavor and is used with honey for cooking. Most galls taste similar to the host plant. Some of the galls have a higher content of carbohydrate, as will be shown later for pinyon needle galls. Cecidomyiidae Most plant galls, however, are injurious to crops, for example, the hessian fly, clover leaf midge, chrysanthemum midge, and pear leaf blister mite.

The gall-making habit is found in six orders of insects: Coleoptera (beetles), Lepidoptera (moths and butterflies), Homoptera (aphids), Thysanoptera (thrips), Diptera (flies), and Hymenoptera (sawflies and wasps). Felt (1917) listed 1440 North American species including mites in the Eriophyidae. He listed 12 Coleoptera species, 17 Lepidoptera, 60 Homoptera, 701 Diptera, 488 Hymenoptera, and 162 Eriophyidae (mites). In 1940, Felt gave approximately the same statistics.
Cynipidae About 444 of the Hymenoptera were cynipid wasps (family Cynipidae) and 682 were fly midges in the family Cecidomyiidae. Among the Homoptera, about 47 were in Aphididae (aphids), 11 in Psyllidae, and 2 in Coccidae.

Gall insects concentrate on all parts of the plant, buds, leaves, petioles, flower heads, stems, bark, and roots. The part of the plant attacked helps in the identification. Interestingly, gall makers produce the same type of gall on different host plants, but there are exceptions, for example, Camptoneuromyia rubifolia produces a spot gall on smilax but a leaf roll on Rubus. Also, different species working on the same plant usually produce very different types of galls. More than half the families of plants have gall insects. The Cecidomyiidae alone attack 69 plant families and 202 genera.
opened stubby galls showing Cecidomyiidae larvae

Opened stubby galls of Janetiella coloradensis (Diptera: Cecidomyiidae) in pinyon needles to reveal larvae (reduced legs and eyes since don't need them).

Cecidomyiid species are especially well represented on the Compositae (151 insect species). The Cynipidae wasps colonize mainly the oaks, some on rose, and a few other plants. The mites form galls largely on woody plants, e.g. willows, maples, birches, beeches and roses. Sometimes galls can be very numerous, Felt estimated that more than 500,000 cynipid wasps were living on one large oak tree. Psyllid, coccid, and aphid galls are often small and can be very numerous.

The gall serves both as a shelter and a food source. Also, the gall insect is partially protected from parasites and predators, especially general ones. The inner walls of galls are moist or liquid and are usually rich in protein and sugars, much more than the surrounding non-galled tissues. Many inquilines live in insect galls as associates in this microecosystem. There are also parasites and predators of the gall insect(s) and other associated organisms. For example, Hendel listed 31 dwellers in addition to one species of gall maker: 10 inquilines, 16 parasites and 5 brief-visiting species.

There are basically two types of galls: open and closed. The open galls are produced by piercing mouthpart forms like aphids, psyllids, coccids and mites. For example, the pear leaf blister mite Eriophyes pyri, the witch-hazel cone gall maker Hormaphis hamamelidis, and the elm cockscomb gall maker, Colpha ulmicola. They feed on the leaf surface which later folds and grows inward to produce a pocket more or less enclosing the insects. Reproduction occurs within the galls and the young leave through a small opening on the lower surface of leaf. The life cycle of aphids of open galls of witch-hazel cone gall is like that of other aphids with alternation of generations between parthenogenetic wingless ones and sexual winged adults. However, the aphids are modified in form so they hardly resemble free living forms one usually see on plants. The witch-hazel cone gall maker produces sexual winged adults when the gall becomes crowded and these fly to birch, where they have several generations. These aphids are all females and have white wax oozing from wax pores along the margin of the insect. After several generations on birch, winged females return to witch hazel where they produce wingless males and females that mate and lay eggs to overwinter on witch hazel.

example of closed galls on ponderosa pine caused by cecidomyiid midges in Colorado

Closed galls are made by larvae with mandibles such as Coleoptera, Lepidoptera, Diptera, and Hymenoptera. None mate and produce young in the galls. Most galls are simple (monothalamous) and contain a single larva. Fewer galls are compound (polythalamous) and have several larvae in separate chambers. Galls can also be classified according to their form and texture. Blister or spot galls are shallow and produced by Cecidomyiidae. Spangle galls have more thickness and resemble shallow plates on lower surfaces of leaves (Cecidomyiidae, sometimes Cynipidae). Button galls are made by Cynipidae. Tube galls on grape and hickory are made by Cecidomyiidae. Oak bullet galls and oak "apples" are produced by Cynipidae. Irregular woody galls on rose, oak, and blackberry are Cynipidae. Galls on flower heads are made by Cecidomyiidae, sometimes by mites or Cynipidae. Egg scars from tree crickets and leaf hoppers can become calloused and resemble galls.

The gall-making habit has been evolved independently in the different insect and mite groups. The gall insect lays its egg upon the host plant or within its tissues. The egg hatches to a small larva which must contact the meristematic cells of the cambium or growing parts. At this point the gall begins to grow. Galls are the result of cell multiplication (hyperplasia) and quite often also the cells are enlarged (hypertrophy).
cross-section of young normal pinyon pine needle

At left is a cross section of a normal young pinyon pine needle stained with dyes to show the various cells. The large hole on the right is a resin canal. The vascular bundle is surrounded by the thick-walled endodermis in the center of the needle. The red colored cells are the mesophyll parenchyma cells containing chloroplasts (red).
cross-section of young normal pinyon pine needle at 170 x

At left is another cross section of a normal needle of pinyon pine but at 170 x magnification. Now compare this to the same magnification for a "round gall" prepared in the same way below.
cross-section of red gall of pinyon needles at 170 x A cross section of a "round gall" of pinyon pine needles stained with dyes as above. Note the enlargement of parenchyma cells and the change in chemical properties since no dye is taken up (probably less chloroplasts).

Thus galls are first seen developing during the spring and early summer when plants are rapidly growing. The cells of many galls also are richer in starch and sugars than normal tissues and the insect uses this rich food source to grow in a confined place.
cross-section of round gall of pinyon needles at 100 x in polarized light showing starch grains
Microscope view of cross section of "round gall" from pinyon pine needles caused by cecidomyiid Janetiella near coloradensis under polarized light at 100 magnification. The cellulose and starch grain show brightly in polarized light due to alignment of molecules. Thin section of a normal needle did not show the bright spots of starch granules.

cross-section of round gall of pinyon needles at 100 x in polarized light showing starch grains
Enlargement of cross section of "round gall" from pinyon pine needles caused by cecidomyiid Janetiella near coloradensis under polarized light at 100 magnification. The starch grain show brightly with an "x" interference pattern (see top left granule) in polarized light.

Some reports have found higher levels of plant growth hormones, namely auxin (indoleacetic acid) and gibberellins (at least 13 natural compounds of similar structure), in the galled tissues compared to corresponding normal tissues. The question is always, what is the cause and what is the effect? It could be that the insect induces higher levels of plant growth hormones, or produces these directly, and that these then are the cause of gall formation. On the other hand, the plant growth hormones might be an effect, i.e. that the galling causes a byproduct of plant growth hormone synthesis.

Crown gall is found in many woody and herbaceous plants in over 60 families, especially the pome and stone fruit trees and raspberries and grapes. Crown gall is produced by the bacterium Agrobacterium tumefaciens, and it has been discovered that the organism modifies the genetic material of the host plant cells by transferring part of the bacterium's DNA (T-DNA) into the plant's DNA so that this is expressed during RNA-protein synthesis in the plant. In this case several genes might work to produce the gall, some of which might code for synthesis of enzymes regulating the plant growth hormones indoleacetic acid (IAA) and Gibberellins. This is a general theory that could just as well be used by insect gall-makers although no evidence has as yet been found of insect DNA incorporation by plants.

The species of insects producing galls are many, e.g. over 1,440 in North America (representative of Europe as well):

Diptera: This order (flies) has the largest number of gall-making insects. Felt (1917) listed 701 species of which 682 are in the Cecidomyiidae. The hessian fly, chrysanthemum midge, pea midge, and clover leaf midge are the best known of the Cecidomyiidae. Trupaneidae have some gall makers. The Oscinidae make small galls in grasses. The Agromyzidae are mostly leaf-miners, some cambium and pith miners, and a few gall makers (Agromyza) on linden, lime, populus, wisteria, and iris.

Hymenoptera: This order is also important. The majority of species are in the Cynipidae. Of 488 hymenopteran gall makers, 444 were in Cynipidae (Felt, 1917). About 80-90 percent of these cynipid species occur on oaks. These galls vary in form, some are bullet like, some fruit like, some woody, mossy, or woolly. Both simple and compound galls are made by Cynipidae. The only other family of importance is the Tenthredinidae (sawflies) which a few members are gall makers or leaf-miners with some gall-like properties. Aside from the seed feeders (Eurytomidae, Torymidae, and Chalcididae), that live in a gall-like environment, the Cephidae bore in stems of plants and dwarf their growth, sometimes causing galls to form.

Coleoptera: The beetles have few gall makers, only about 12 species in USA (in Curculionidae, Buprestidae, and Cerambycidae). The Curculionidae include several economic species: the pine gall weevil Podapion gallicola, the grape cane gall maker Ampeloglypter sesostris, and the virginia creeper stem gall, A. ater. The Buprestidae contain the red-necked cane borer Agrilus ruficollis, the raspberry cane borer Oberea bimaculata and the bronzed birch borer Agrilus anxius. The Cerambycidae have the gall-making maple borer Xylothrechus aceris and Saperda concolor and S. populnea produce galls on poplar and willow.

Lepidoptera: Only 17 species in USA make galls. Most gall-making moths belong to the genus Gnorimoschema (family: ?). The Tortricidae and Elachistidae include some gall makers while a few species are found also in the Aegeriidae, Tineidae, Olethreutidae, Lavernidae, and Pyralididae. The lima bean vine borer Monoptilota pergratialis is the best known.

Homoptera: About 60 species in USA make galls, of these there are 47 species of Aphididae, 11 Psyllidae, and 2 Coccidae. All produce open galls, usually of the pocket or cone type. The galls are small and thickly placed on leaves (e.g., grape phylloxera, Phylloxeridae).

Norway spruce gall caused by a species of Adelgidae (Chermidae).

Galls like above cut to reveal aphids in the cavities.

my finger

Sources of Information

Borror, D.J. and DeLong, D.M. 1964. An Introduction to the Study of Insects. Holt, Rinehart and Winston.

Agrios, G.N. 1988. Plant Pathology. Academic Press, Inc., New York.

Borror, D.J. and White, R.E. 1970. A Field Guide to Insects. Houghton Mifflin Co., Boston.

Byers, J.A., Brewer, J.W., and Denna, D.W. 1976. Plant growth hormones in pinyon insect galls. Marcellia 39:125-134.

Craighead, F.C. 1950. Insect Enemies of Eastern Forests. U.S.D.A. Misc. Pub. No. 657.

Felt, E.P. 1917. Key to American insect galls. N.Y. State Mus. Bull. 200.

Felt, E.P. 1940. Plant galls and gall makers. Comstock Publishing Co., Inc. Ithaca, N.Y.

Frost, S.W. 1959. Insect Life and Insect Natural History. Dover Publications Inc., New York.

Smith, R.C., Kelly, E.G., Dean, G.A., Bryson, H.R., and Parker, R.L. 1962. Insects in Kansas. Extension Division, Kansas State Univ., Manhattan, Kansas.

Thompson, M.T. 1916. An illustrated catalogue of American insect galls. Supplemental list of American gall insects, Nassau, N.Y.
and © by John A. Byers