The first two installments of this series (May 2015 and June 2015) taught us that galls, while mysterious, are not to be feared. Nevertheless, they are abnormal growths, and they can be managed in order to maintain the appearance of the plant affected.

The vast majority of plant galls cause little to no harm to the health of their host plant, which means that gall management strategies aimed at limiting the impact on plant health are usually not needed. This perspective is particularly true for galls produced by insects and mites. Also, for reasons not clearly understood, populations of arthropod gall-makers tend to rise and fall dramatically from year to year. It is not unusual for a season with heavy activity by a particular gall-maker to be followed by years with almost no evidence of the gall-maker’s handiwork.

It’s all about education

An effective gall management strategy should start with educating your clients. Although galls are abnormal plant structures, they are still fascinating plant structures. Plant galls are an outward result of a complicated physiological and chemical dance between the gall-maker and the plant host. If not viewed with a sense of wonder and fascination, at least insect and mite gall-makers should garner begrudging respect. So far, no human has managed to duplicate the work so handily done by a group of organisms that are often viewed with disdain. Imagine the plant secrets that would be unlocked if we could!

Of course, the fact that plant galls may be fascinating and most cause no harm may be cold comfort to some clients, since galls certainly affect the expected and desired appearance of the host. Couple this with the reality that gall-makers are difficult if not impossible to control, and it is small wonder that plant galls are often viewed with little wonder.

An immature horned oak gall.

Unfortunately, gall prevention is not an option for most arthropod galls based on our current knowledge and available tools. Little is known about the life cycles for many of our most noteworthy arthropod gall-makers. Almost nothing is known based on scientific research about pesticide efficacy against these gall-makers; much of what has been reported is based on anecdotal accounts. There is limited motivation for funding and conducting research aimed at adding arthropod gall-makers to pesticide labels, because the galls are seldom of economic significance and rarely cause harm to plant health.

Such is not the case, however, with black knot on Prunus and cedar-apple rust galls on juniper. Properly timed fungicidal applications may be helpful in preventing gall formation. There are also other management options. As noted previously, the black knot fungus will re-infect galled trees, but it takes around two years for infectious spores to be released from newly developing galls. Pruning and destroying new galls before they release spores can be helpful in managing the galls.

Planting other members of the Prunus genus near an infected tree could be a prescription for expanding a black knot problem. Knowing that the cedar-apple rust fungus requires two hosts, juniper and apple, to complete its development coupled with knowing that the galls on junipers arise from spores from apple means that keeping the two hosts apart is an important first step in gall management. It may not entirely prevent the problem, since spores may drift considerable distances; however, it would certainly be helpful.

As it begins to develop, an embedded horn can be seen protruding from the top of this immature horned oak gall.

Managing horned oak gall wasp

The complicated life cycle of the horned oak gall wasp presents a special gall management challenge. The wasp alternates between sexual and asexual reproduction between generations, a condition called heterogamy. The wasp also produces two entirely different types of galls between generations. One generation of wasps develops from insignificant leaf galls that appear as small bumps on leaf veins; the galls are difficult to detect with an untrained eye. Leaf gall growth starts in the spring and wasp development within the galls, from eggs to adults, takes about three months. This is the “sexual” generation, so both male and female wasps emerge from these leaf galls. Once they mate, the females fly to twigs and small branches to start the next generation of wasps that will develop in stem galls.

The females’ egg laying activity stimulates the growth of stem galls from cambial stem tissue. The sizes of the galls depend on the number of eggs laid; larger galls hold more eggs and resulting wasp larvae. The larvae spend 33 months in individual chambers within these very obvious gnarled, dark green, woody stem galls. The galls grow larger in size with each season. As the immature wasps near the completion of their development, the whitish-tan, cone-shaped “horns” that give this gall its common name begin to extend from the gall. Adult wasps emerge from the horns once they are fully extended. This is the “asexual” generation; all of the adults are females, there are no males in this generation. This form of asexual reproduction, where females do not require fertilization by males to produce fertile eggs, is called “parthenogenesis.” <cColor:Word\_R255\_G0\_B0>

Old horned oak galls have been abandoned by their wasp gall-makers.

Management strategies for the horned oak gall wasp must take into account the two locations where the gall-maker resides. Adding to the complexity is that nothing is synchronized. Leaf galls occur every year providing a constant stream of wasps producing new stem galls. The annual reservoir of wasps dedicated to producing stem galls makes managing horned oak gall by pruning out infested stems a never-ending process. In the late 1990s, horned oak gall was the target of one of the most complete insecticide efficacy trials conducted against any insect gall-maker. The trials were performed at the University of Kentucky by Dr. Eileen Eliason ((Buss); now at the University of Florida) and Dr. Dan Potter. The researchers reasoned that the leaf-galling generation may be the “weak link” in the wasp’s life cycle relative to insecticide suppression owing to the annual initiation of new galls and the location of the wasp larvae in small, vulnerable leaf galls. The researchers made foliar applications of contact insecticides targeting females as they laid eggs. Wasp larvae inside leaf galls were targeted with foliar applications of translaminar systemic insecticides and trunk injections of concentrated solutions of systemic insecticides.

Their results revealed an insecticide conundrum related to an unintended consequence that is not unique to controlling insect gall-makers. All of the application methods and most of the insecticides used in the trial provided a significant suppression of the horned oak gall-maker. However, the insecticide treatments also had a severe impact on beneficial parasitoids, which accounted for about a 70 percent mortality rate of the leaf-galling generation in the untreated “control” trees. In other words, while insecticides were proved to be effective, the unintended consequence was to kill the very beneficial insects that could account for a natural reduction of the gall-maker almost equal to the insecticides!

As immature wasps near the completion of their development, the whitish-tan, cone-shaped “horns” that give this gall its common name begin to extend from the gall. Adult wasps emerge from the horns once they are fully extended.

A final galling note

Ephraim Porter Felt best expressed the wonder of plant galls in his pivotal 1917 New York State Museum publication (No. 200), “Key to American Insect Galls.” Felt stated: “Insect galls are obvious and frequently excite surprise because of the strange form or the wonderful coloring and delicacy of structure.” Such strong appreciation for plant galls has influenced many scientific careers.

Alfred Kinsey (yes, that Alfred Kinsey) investigated gall wasps to earn his Doctor of Science degree from Harvard University in 1919. His fertile research reports and fruitful collecting spawned a greater understanding of the evolutionary relationships among gall wasp species (phylogenetics). Of the 18 million insects in the New York Museum of Natural History’s collection, 5 million are gall wasps collected by Kinsey. There is no doubt that his stimulating work aroused others to pursue scientific research on plant galls. However, the allure of gall wasps did not hold Kinsey’s attention for long; he later went on to pursue other thought-provoking research at Indiana University.

Kinsey’s long scientific career started with plant galls. It just shows that you never know where a healthy interest in plant galls will take you!

All photos courtesy of Joe Boggs

The post MANAGING GALLS appeared first on AmeriNursery.com.

In the first installment of this series on plant galls (May 2015), we talked about the difference between gall-like structures and true galls, including bacterial crown galls, fungal galls, leaf/petiole galls, flower/fruit galls, bud galls and stem galls. Galls galore! But there’s more.

Let’s dig deeper into insect and mite (arthropods) galls. Unlike bacterial crown galls, which are a mass of plant cells that have been modified by bacterial DNA, or fungal galls, which are an assemblage of fungal cells intertwined with plant cells, galls produced by insects and mites are constructed entirely of plant cells.

Dominant arthropod gall-makers include insects belonging to three orders: Hymenoptera (wasps, sawflies); Hemiptera (aphids/adelgids, phylloxerans, psyllids); and Diptera (midge flies). The only mites capable of inducing galls are eriophyids (order Trombidiformes, family Eriophyidae).

There are more than 2,000 species of insect gall-makers in the U.S.; however, three quarters belong to only two families: Cynipidae (“gall wasps”) and Cecidomyiidae (“gall midges”). Note the “cecido-” in the name of the gall midge family; Cecidology is the scientific study of plant galls. Of the over 800 different gall-makers on oaks, more than 700 are gall wasps. In other words, when trying to identify an insect gall, keep in mind that there is a high probability it was produced by a gall wasp or gall midge.

Although insect and mite gall formation is not entirely understood, researchers theorize there are two possible pathways. Some gall researchers believe certain types of plant gall growth are directed by the feeding activity of the gall-maker. The galls are produced by a combination of constant but subtle feeding irritation, perhaps coupled with the release of chemical inducers by the gall-maker.

Mossy rose gall is a good example of a plurilocular, unilarval gall (multiple chambers but only one larva per chamber), while hickory phylloxera petiole gall is a typical unilocular, multilarval gall (one chamber hosting several larvae).

Certain eriophyid mites provide an example. These unusual mites are much smaller than spider mites (you need at least 40x magnification to see them); however, both types of mites use their sharp, piercing mouthparts (chelicerae) to rupture plant cells so they can feed on the contents. Only the feeding activity of some species of eriophyid mites induces gall growth; there are no spider mite gall-makers. This gall-growth pathway may explain how simple felt-like erineum patches (a.k.a. “erineum galls”) develop under the direction of a number eriophyid mite species. However, it does not explain how highly organized plant gall structures develop. <cColor:Word\_R255\_G0\_B0>

Some types of insect and mite galls are composed of complex plant structures and may include functional plant organs such as nectaries. These types of plant galls require a different gall-growth theory; one that includes the ability of the gall-maker to turn plant genes on and off as they direct plant cells to form highly organized plant structures.

Research has revealed that some gall-making insects and mites produce chemical replicas of plant hormones, or “plant hormone analogs” meaning the molecules are nothing like plant hormones but the plant’s response is the same as with plant hormones. The gall-forming process is usually initiated by the female when she injects gall-inducing chemicals into the plant along with her eggs. The eggs themselves may ooze gall-inducing chemicals and once the eggs hatch, the interaction continues with the immature gall-makers continuing to exude chemicals to direct plant growth to suit their needs.

The resulting galls provide both a protective home and nourishment for the next generation of gall-maker. The continual direction of gall growth by the gall-maker using chemicals to turn plant genes on and off speaks to why some find insect and mite plant galls so fascinating.

The oak anthracnose fungus, Apiognomonia quercina, is seen infecting the plant tissue of the small oak apple gall..

The chemicals exuded by gall-makers can only act upon meristematic plant cells, such as the cambial cells mentioned in Part 1 of this series or the meristematic cells in leaf buds; the precursors to leaf cells. Under the influence of chemicals exuded by a gall-maker, the meristematic cells that were originally destined to become leaf cells begin marching to a different drummer. Once the errant leaf cells fall under the chemical spell of a gall- maker, there is no turning back – they will become gall tissue. This means that gall formation cannot occur once meristematic leaf bud cells are committed to becoming leaf tissue; it’s one reason the leaf-gall season begins in the spring! However, once the galls start growing they will continue to grow, even after the leaves fully expand.

Stem galls that arise from cambial tissue present a different scenario. Since cambial cells remain “free agents” throughout the growing season, galls can be formed from these cells anytime during the growing season, although most stem galls also start growing early in the season to provide ample time for the gall-maker to complete its development.

The large burl affecting this walnut is not a true gall.

Insect and mite gall identification

Identifying insect and mite galls is challenging because of the limited number of accurate and updated gall identification resources, both online and printed. This is particularly true for galls found in North America; the European gall literature is more robust.

The existing gall identification resources tend to follow the same general outline. Most begin by separating galls based on the gall-maker, such as wasp galls, midge galls, and so on. While it is ultimately important to know the gall-maker, gall identification usually starts with the gall itself.

We’ll provide a quick recap from Part I: The first step is to consider where the gall is found on the plant. Most identification resources use the following locations: leaf/petiole galls; flower/fruit galls; bud galls; stems galls; and root galls.

Next, galls are described based on their structure such as general appearance (for example, ball-like, hairy, etc.), number of chambers, and the number of gall-makers housed in the chambers. Recall that unilocular galls have only one chamber; plurilocular galls have multiple chambers. Unilarval galls only have one gall-maker per chamber; multilarval galls have more than one gall-maker per chamber. The aforementioned ball-like larger oak apple gall is a unilocular, unilarval gall. The hairy-looking mossy rose gall produced under the direction of the gall wasp, Diplolepis rosae; a species first described by Carl Linnaeus, is a plurilocular, unilarval gall with many chambers but only one larva per chamber.

Looking like a green apple, a large oak apple gall contains a wasp larva; once the larva pupates, the mature gall turns tannish brown and appears empty.

The ball-like hickory petiole galls produced by several phylloxeran species (Phylloxera spp.) are usually unilocular, multilarval galls; there is a single chamber housing many of these aphid relatives.

Insect and mite gall laws

While the highly organized plant galls produced under the direction of many insect and mite gall-makers represent a wide range of forms and locations, there are certain consistencies that can be summarized as “gall laws.”

The First Gall Law: Galls are abnormal plant growths produced under the direction of a living gall-maker; they do not arise spontaneously, nor are they in response to plant wounding that does not involve a gall-maker. This law removes certain tree growth such as “burls” from the gall arena. It is believed these abnormal plant growths are the result of runaway plant hormone production; they are not considered “true galls.”

The Second Gall Law: Insect and mite galls are abnormal plant structures that are composed entirely of plant tissue; they’re not part of the gall-maker. Infections by fungal plant pathogens can illustrate that insect and mite plant galls are indeed plant structures. The oak anthracnose fungus, Apiognomonia quercina, normally infects the leaf cells of its namesake host producing blackened, necrotic leaf tissue. The fungus can also infect the plant tissue of the small oak apple gall produced under the direction of the gall wasp, Cynips clivorum, because the gall is constructed from hijacked leaf cells. The resulting tissue necrosis makes the gall appear to slowly dissolve away.

The Third Gall Law: Galls can only be formed from meristematic plant tissue and once plant tissue stops differentiating, galls cannot be formed by a gall-maker. This explains the seasonality of leaf and bud galls, as well as the ability for stem galls arising from cambial tissue anytime during the growing season.

The Fourth Gall Law: Gall structures and locations on the plant are so species-specific that the species of the gall-maker can be identified by the gall structure alone without the need to see the gall-maker itself. Although galls may change color and texture as gall-makers develop, and are said to be “mature” once the gall-maker completes its development, the changes are predictable.

There are more than 20 different types of “oak apple” galls, so named because of their resemblance to varying sized apples. However, only one gall wasp species, Amphibolips quercusinanis (syn. A. inanis), produces the so-called larger empty oak-apple gall. The 1 to 1½-inch-diameter galls arise from leaf buds. The galls hold a single wasp larva and as the larva develops, the galls bear a striking resemblance to green apples with the apple-ruse made complete by reddish speckles that resemble insect damage to the “apples.” The galls are filled with soft fiber radiating spoke-like from a center kernel that houses the resident wasp larva. Once the larva pupates, the mature galls turn tannish brown and the fiber degrades, causing the galls to appear empty, thus the common name.

It is common to find the gnarled, woody, horned oak galls produced by the cynipid wasp, Callirhytis cornigera, affecting only a few trees in a whole row of seemingly related pin oaks (Quercus palustris).

The Fifth Gall Law: Gall-makers are specific to certain hosts. Their activity may be confined to a plant species, or certain varieties, cultivars or provenances within a species. It is common to find the gnarled, woody, horned oak galls produced by the cynipid wasp, Callirhytis cornigera, affecting only a few trees in a whole row of seemingly related pin oaks (Quercus palustris). This observation speaks to the impact of only slight genetic differences in the gall-makers’ host. Galling activity may also be confined to a group of related species within a genus. It is uncommon for gall-makers on white oaks to also reside on red oaks. What this means for gall management is that making slight changes in tree selection may eliminate a recurring gall problem in a landscape.

A weevil has colonized a cedar-apple rust gall.

Sixth Gall Law: Insect and mite galls may house other occupants that have nothing to do with gall formation; however, these interlopers still rely on the gall for both their lodging and food. Biologists call these gall-guests “inquilines,” a term derived from the Latin inquilinus, which means “tenant” or “lodger.” Inquilines are divided into cecidophages, arthropods that feed on the gall tissue while the gall-maker is developing, and successori, arthropods that feed on the galls after the gall-maker moves out.

More than 20 different arthropods may wholly or partially depend upon the gnarled, woody, horned oak gall produced by the cynipid wasp, Callirhytis cornigera, for their livelihood.

Studies have shown that more than 20 different arthropods may wholly or partially depend upon the gnarled, woody, horned oak gall produced by the cynipid wasp, Callirhytis cornigera, for their livelihood. Arthropods may also colonize fungal galls, such as the aforementioned fungal black knot and cedar-apple rust galls, which may introduce some confusion as to the identity of the true gall-maker! Indeed, with the wide array of possible inquilines in some galls coupled with the predators and parasitoids that feed on the gall-maker as well as the inquilines, biologists often describe galls a mini-ecosystems; it’s a jungle in there!

Next up: Gall management.

The post INSECT AND MITE GALLS: MYTHS AND MISCONCEPTIONS appeared first on AmeriNursery.com.