Green Balls That Grow on Oak Trees

Oak trees are prone to infestation from a variety of pests, many of whom are identified by the damage that they inflict on the tree. These pests include a number of tiny wasps that create small balls, referred to as galls, on oak trees. The exact identification of the wasp is usually established by the size and color of the gall. Certain wasp species cause the growth of green galls on oaks.

Pest Identification

The oak apple gall and the roly poly gall are among the green galls seen on oaks. The pests are common on all varieties of oaks in all oak growing regions. Though a number of other pests such as fungi, bacteria or mites may also cause the growth of galls on trees, wasps are the most frequently cause of this disorder, cites the University of Lincoln-Nebraska Extension.

Pest Description

There are about 1,400 different types of gall-creating wasps of which 80 percent infest only oak trees. The wasps are very small in size, measuring ¼ inch or less in length. The insects come in shades of red, yellow or black depending on species. The legless larvae and pupae are white. Every gall wasp is identified by the distinct characteristics of the resulting galls.

Gall Production

Galls are small, round balls that may be up to 2 inches in diameter or may be too small even to be noticed. The galls are actually abnormal plant growth resulting from wasp feeding that creates a disturbance in the normal growth process of plant tissues. As a reaction to pest feeding, the plant produces excessive amounts of growth regulating chemicals that in turn disrupts the normal cell growth. The galls contain the eggs and larvae of the wasp. The emerging larvae feed only the gall’s tissues and not on tree areas.

Control Strategies

Galls are primarily a cosmetic disorder of the affected tree and pose little threat to tree health. Foliage galls sometimes cause premature leaf drop or distortion by affecting the normal process of photosynthesis. There are no control options to reduce or eliminate galls once they have occurred. Using preventative insecticides may control populations of the adult wasps before they have a chance to lay eggs. Unless preventative pesticides are targeted precisely at wasp flight time, however, they will be of little use. Keep tree areas clear of debris and fallen foliage to remove overwintering pests in branches, twigs and leaves.

Oak trees have a lot of gall – ‘oak apple gall,’ that is

Q: My oak tree looks like it is making fruit the size of ping-pong balls. Now I know that they make acorns, so they couldn’t possibly make fruit, right? What are these round plum-looking fruits hanging from my oak tree? When you cut them in half they are hollow inside.

Q: My oak tree looks like it is making fruit the size of ping-pong balls. Now I know that they make acorns, so they couldn’t possibly make fruit, right? What are these round plum-looking fruits hanging from my oak tree? When you cut them in half they are hollow inside.
A: You are right; oaks make hard acorns not plum-type fruit. What you are seeing are galls. Galls occur on many woody plants. They can be caused by different organisms – fungi, nematodes or mites – but insects such as wasps are the primary cause. Your perfectly round ping-pong-sized galls were probably caused by a gall wasp or, as entomologists call them, the cynipid wasp. Most galls are small, but some can be as large as 2-inches in diameter. When the gall wasp lays an egg on developing plant tissue, it begins to change. The irritated plant tissue quickly surrounds the egg or immature insect and protects and provides food for the gall-maker until it matures. If you see a gall, inspect it closely for a pin-sized hole; this is the exit hole where the developed wasp left the gall.
The galls you are seeing on your tree are often called “oak apple galls,” which is a common name for galls shaped like small apples. Dr. Eileen Buss of the University of Florida entomology department says that “the galls probably occurred after the first spring leaf expansion on oak trees, but are not likely to be damaging to trees. Many gall-maker species are active (mating, laying eggs) from just before the trees put on new growth to early shoot and leaf expansion in late March to mid-April.” Since they are not damaging to the trees, there is no reason to treat the tree or trim away the galls.
There are many different types of galls. “If kids need to do a class biology project, collecting different galls on oaks can be fun and interesting,” Buss suggests.
– – –
Q: Can live oak leaves around my shrubs harm them? One neighbor advised me that these leaves were bad for shrubs. A landscaper I consulted recently suggested removing all the leaves from around my shrubs because some were dying. He suggested that they were preventing water from getting to my plants. I always thought leaves made good mulch.
A: Leaves make great mulch, but you can overdo it. If your leaves are piled too thickly over the roots of plants, they can prevent water from reaching the root zone. I am all for using your fallen leaves in your landscape. It helps to recycle nutrients on site instead of having them picked up by the yard waste truck or, worse, blowing them into the street where they can clog up the storm drains and add extra nutrients into creeks.
Whole leaves, especially oak leaves, can take years to break down. To use your oak leaves as mulch, it is best to grind them to a one-fourth- to one-half-inch pieces so they will break down quickly. Grind your leaves with a chipper, shredder machine or run over them with a lawn mower. Apply the mulch in a 2- to 3-inch layer. At this thickness, you will suppress weeds, help the soil hold moisture and add to the organic matter of your soil.
Wendy Wilber is an extension agent with UF/IFAS. E-mail her at [email protected]

I recently received some pictures of some weird looking growths in an Oak tree. They looked like a growth called a gall, which looks like a tumor growing on a branch or leaf. They are usually the result of the feeding or egg laying of certain insects that cause the plant’s cells to multiply at a dizzying rate. Immature stages of the insect or the larva feed on the inside of the gall and often use the gall as protection from predators.

The gall in this tree is a Horned Oak Gall. They grow to the size of a golf ball and have an interesting life cycle. The adults emerge from the gall in early summer and lay eggs on the leaves of the tree. The resulting larvae cause oblong blister-like galls to develop on the leaves.

About three months later, new adults emerge from those galls, they mate and the females lay eggs in the twigs of the tree. Small marble sized galls appear and grow together to form the larger gall. The cycle continues on and on, from leaf to twig, and this can lead to deformity of the tree if populations become too large.

Treatment is possible, but it requires specialized equipment that will inject insect control materials into the tree. This requires a tree care professional who is trained to properly inject these materials to do the job correctly. Sometimes, if the tree is not too large, the twigs or branches that have galls on them can be pruned out as soon as the galls appear during normal tree maintenance to reduce the ongoing life cycle.

What are gall wasps?

Popular knowledge about trophic relations established among vertebrates tends to eclipse the existence of little systems sometimes way more complex than those observed either on mammals, reptiles or birds. This is the case of gall wasps or gallflies, a family of micro-wasps able to induce a wide variety of tumours known as galls in different groups of plants. Despite most people have sometimes heard about these organisms, they probably don’t know that there exists a frantic fight between different groups of insects inside these tumours.

Do you want to know more about the mysterious world inside gall wasps’ galls? Keep reading!

Gall wasps or gallflies (Family Cyinipidae, Order Hymenoptera) are a family of plant parasitic micro-wasps that barely reach a few millimetres length. They belong to the Parasitica group inside Hymenoptera order, so females haven’t their ovipositor transformed into a sting like other wasps. In this case, this organ preserves its exclusively reproductive original function.

Periclistus brandtii female; ovopositor is marked in red (Image from the Catàleg de microhimenòpters de Ponent).

Female gall wasps use their long ovipositors for laying eggs inside host plant tissues (usually oaks and other species from the genus Quercus).

Gall wasps are phytophagous insects, that is, they feed on vegetable tissues only. So, this takes them away from most of wasps, which are usually carnivorous or parasitoids of other insects.

But the most distinctive trait of these organisms is, without a doubt, their capacity to induce tumour or gall development on plants.

The galls

What are galls?

The same way birds construct nests or beavers construct dikes, some gall wasps ‘construct’ their own galls. But unlike nests or dikes, galls aren’t actively constructed by gall wasps but induced as a consequence of their activity and interaction with plant tissues.

Despite there are more arthropods able to induce galls formation, gall wasps are the ones that induce the most diverse, complex and evolved typologies of galls known until the date, especially on Quercus (oaks and relatives).

There exists a wide variety of gall morphologies: 1, 2 & 3 – gall wasps’ galls on Quercus (Images by Irene Lobato); 4 – Neuroterus numismalis gall on Quercus (Public domain); 5 – Diplopedis rosae gallo on a Rosidae (Image by Lairich Rig, CC); 6 – Andricus quercuscalicis galls on Quercus robur (Image by Peter O’Connor en Flickr, CC).

Moreover, there exists a close relation between galls wasps and plants, so almost every species or genus induces a specific gall typology. Because of this, in the same way as nests and dikes galls are considered an extended phenotype of gall wasps (that is, a characteristic trait of an organism that manifests outside its body but that allows its identification).

How are they formed and what is their function?

Galls are result of a totally or partially deformation and an extreme growth of different plant tissues, such as leafs, leaf nerves, stem or fruits.

Usually, gall formation doesn’t necessarily affect plant production and growing, except when they undergo a massive developing on plants surface that causes serious damages on their tissues. In these cases, gall wasps can become terrible pests (e.g. the chestnut gall wasp, Dryocosmus kuriphilus, a species native of Asia that has become a pest on chestnuts from Europe).

Chestnut gall wasp female (Image by Gyorgy Csoka, CC) and its galls, which causes deformation and drying of leafs (Foto de Irene Lobato).

Molecular basis driving gall formation are currently unknown. However, it’s known that this process begins at the time females inoculate their eggs inside plant tissues.

Female of a gall wasp laying eggs inside plant tissue (Public domain).

From this moment on, galls begin to grow more and more around the eggs until they get enclosed inside one or different chambers. Inside these chambers, larvae feed on nutritious tissues from the gall while being protected from external damages. The movement and activity of larvae promote gall growth.

Larval chambers and chestnut gall wasps larvae (Dryocosmus kuriphilus), at left (Image by Irene Lobato); the inside of a gall with a single larval chamber on Quercus, at right (Image by chickeninthewoods, CC).

Once adults are completely developed, they travel through the gall tissue to reach its surface. This process can take them a lot of time and a big waste of energy. Usually, adults don’t feed and dedicate their short life to mate.

Gall with emergency holes, through which adults reach the surface of the gall (Image by Irene Lobato).

A miniature and complex trophic net

Usually, galls harbour a rich variety of arthropods besides the wasps that have induced their development. Some of them feed on nutritious tissues induced by other gall wasps to complete their life cycle; others live as parasitoids of different species of gall wasps and cause their death; besides, there are some of them that develop only during the late states of galls life.

So, the inside of galls is the scenario of a miniature and complex trophic net and that of a survival battle between different groups of arthropods:

True gall wasps

These group of gall wasps are able to induce galls formation de novo. They usually have a strong body, the radial cell of the anterior wings opened in its upper margin and the abdomen with well differentiated segments (typical characters of the tribu Cynipini, one of the most abundant groups of true gall wasps).

Andricus kollari female: 1 – detail of the opened radial cell; 2 – segmented abdomen (original image by TristramBrelstaff, CC).

Inquiline gall wasps

Some gall wasps have lost their ability to induce the formation of galls. These are known as inquilines, and their larvae develop inside other gall wasps’ galls while feeding on their tissues. So, inquiline females lay eggs inside galls in formation. Despite inquilines aren’t able to induce galls formation de novo, they can alter its development.

Synergus equihuai female, a new species recorded from Mexico by Irene Lobato & Juli Pujade during Master’s degree final project: 1 – closed radial cell (it could either be opened); 2 – a big tergite covers the rest of the segments (Image taken by Marcos Roca-Cusachs).

Relation between inquilines and true gall wasps is a specific type of cleptoparasitism known as agastoparasitism, because inquilines larvae “steals” the nutritious tissues from the gall they occupy. Inquilinism of gall wasps is an obligatory relationship for inquilines, because they need other gall wasps’ galls to complete their life cycle.

Usually, this relationship doesn’t affect neither negatively nor positively true gall wasps, except when larval chambers of both groups are near from each other. In this case, the faster development of inquilines and their competition for food could finish with true gall wasps’ life. If this happens, the only adults that would emerge from galls would be that of inquilines (lethal inquilines).


Parasitoids form one of the main groups of arthropods that develop inside galls. The most of them belongs to the family Chalcidoidea (order Hymenoptera), that is totally made up of parasitoid wasps.

Torymus aceris female (Image from the Natural History Museum_ Hymenoptera Section on Flickr, CC).

Parasitoids from galls inoculate their eggs inside larvae of different gall wasps using their long ovipositors. So, it’s expected that the only adults that will emerge from a gall attacked by parasitoids will be the parasitoids themselves.

Nowadays, there exist programs for managing gall wasps’ pests which contemplate the use of parasitoids, such as Torymus sinensis to fight against the chestnut gall wasp.

Secondary entomofauna

This category includes a wide variety of different arthropods that develop inside galls at the late stages of its lifespan, acting as secondary successors: beetles, butterflies, flies, thrips, etc. These organisms usually develop once all gall wasps have emerged from galls.

. . .

Sometimes, nature can be more complex than we could imagine, and the case of galls is only an example. From now on, when you go out for a walk, remember that even in the tiniest spaces, there exist very complex and developed systems plenty of rich and diverse relations.

Most of the information has been extracted from my Master’s degree final project (University of Barcelona, 2015-2016), titled “Separation and identification of inquilines from the genus Synergus (Fam. Cynipidae, Hymenoptera) from galls developed on Mexican species of Quercus”.

Some of the most remarkable studies consulted were the ones that follow:

Main image property of Beentree (Wikimedia Commons).

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M’agrada S’està carregant…

What Gall! The Crazy Cribs of Parasitic Wasps

Plenty of animals build their homes in oak trees. But some very teeny, tricky wasps make the tree do all the work. “What nerve!” you might say. What… gall! And you’d be right. The wasps are called gall-inducers. And each miniature mansion that the trees build for the wasps’ larvae is weirder and more flamboyant than the next.

If you’ve ever spent a Summer or Fall around oak trees – such as the stalwart Valley Oak – Quercus lobata, or the stately Blue Oak, Quercus douglasii – you may be familiar with the large, vaguely fruity-looking objects clinging to the branches and leaves. Commonly called oak apples, these growths are the last thing you’d want to put in your mouth. They are intensely bitter, loaded with tannin compounds – the same compounds that in modest amounts give red wine its pleasant dryness, and tea its refreshing earthy tang.

Oak apple gall from the California Oak Gall Wasp (Andricus quercuscalifornicus).

That said, the oak apple’s powerful astringency has been prized for millennia. Tanning leather, making ink or dye, and cleaning wounds have been but a few of the gall’s historical uses.

But on closer inspection of these oaks – and many other plants and trees such as willows, alders, manzanitas, or pines – you can find a rogue’s gallery of smaller galls. Carefully peeking under leaves, along the stems and branches, or around the flower buds and acorns will likely lead you to unexpected finds. Smooth ones. Spiky ones. Long skinny ones, flat ones, lumpy, boxy ones. From the size of a golf ball down to that of a poppy seed. These structures wear shades of yellow, green, brown, purple, pink and red – and sometimes all of the above. A single tree may be host to dozens of types of gall, each one caused by a specific organism. And their shapes range from the sublime to the downright creepy. One tree may be encrusted with them, like a Christmas tree laden with ornaments and tinsel; and the next tree over may be almost completely free of galls. Why? It’s a mystery, like many other aspects of Cecidology, the study of plant galls and their inducers.

A cluster of galls by the Crystalline Gall Wasp (Andricus crystallinus) under the leaf of a Blue Oak (Quercus douglasii).

Who causes galls? Lots of creatures; midges, mites, aphids, flies, even bacteria and viruses. But the undisputed champs are a big family of little wasps called Cynipids- rarely exceeding the size of a mosquito, a quarter of an inch in length.

What Are These Bumps on My Oak Tree Leaves?

From the moment they sprout in spring to the final drop in fall, leaves on our trees captivate us.

That’s why anytime leaves don’t look their best, we want to spring into action and help.

Our blog reader, Gwen, recently asked, “I believe insects have burrowed into the leaves of my burr oak tree, now they’re dying and turning orange. Is there a cure?”

If you also see bumpy, blistered oak leaves with a funky color, read on to learn why this is happening and what you can do.

Why You See Bumps or Round Balls on Oak Tree Leaves

Oak leaf blister disease and oak leaf gall are the top two culprits when it comes to problems with oak tree leaves. There are a few ways to tell them apart.

Oak Leaf Blister Symptoms

Trees suffering from oak leaf blister will develop raised spots scattered on the leaves. Underneath, leaves will feel slightly sunken or indented. That’s what was plaguing Gwen’s oak trees!

The blisters are first a pale green or orange and then eventually turn brown or black. Severely infected leaves will curl or drop early.

Galls on Oak Leaves

Galls are abnormal growths that crop up on trees after insects or mites settled in. They’re clusters of brown or orange balls that vary in size and cover tree leaves and branches.

Depending on the what insect is causing it, some galls are hairy while others are perfectly round and smooth.

What to Do About Bumps on Oak Leaves

Good news and bad news. Oak leaf blister and oak leaf gall cause little harm to trees. But both are ugly to look at–especially on your once-beautiful oak tree!

Luckily, infected trees typically bounce back with proper plant health care. Most of the time there’s no need for a chemical treatment.

If your tree has oak leaf blister:

  • Keep the tree well-watered in hot, drought-like weather.
  • Cover the tree with a ring of mulch in spring.
  • Apply fertilizer right before winter to help boost health.

If your tree has oak leaf gall:

  • When galls are in their early stage in spring, prune infected twigs.
  • Rake and dispose of fallen leaves in autumn. This may help get rid of resting insects and mites that brought on the infestation.

Q: I noticed little fuzzy balls attached to some fallen oak leaves. Also, I keep picking up brown marble-size balls in the yard. They are not acorns. Do you know what they are?

Ted Morris, Houston

A: Odds are what’s happening in my garden is what’s happening in yours. After reading your letter I discovered fuzzy, blond, tan and rusty-colored tufts stuck to the backsides of leaves. Other people also have asked about the curious, puffy things on live oak and post oak foliage.

They are wool-bearing galls, aka woolly leaf galls, says Mike Merchant, a Texas A&M AgriLife Extension Service urban entomologist. They are no cause for alarm.

“These galls generally cause no real harm to the plant,” he says.

Several types of galls appear on leaves, stems and other plant parts. These abnormal growths develop around irritations caused by several insects, mites, fungi and bacteria. The common woolly leaf gall is the plant’s response to irritations caused by tiny, plant-feeding wool-bearing gall wasps, or Andricus quercuslanigera, Merchant says. After a wasp lays eggs on a leaf, the tree encases the eggs in galls, which in turn shelter the developing wasps.

Gall-inducing insects typically are specific to particular tree species. It’s possible the marble-size balls you’ve found are mealy oak galls. These woody galls form on live oak twigs and stems and are the work of the mealy oak gall wasp, or Disholcaspis cinerosa.

When it first forms, the mealy oak gall is pinkish-brown on the outside and yellowish, soft and moist on the inside, acording to a Texas A&M extension publications report. By late fall, the sphere is brown and dry.

Beneficial insects may live in galls with the mother wasp or after her family has left the micro habitat. Pretty nifty, I think.

The oak apple wasp, Amphibolips confluenta, is responsible for spherical, spongy galls on red oaks. Thin-shelled oak apple galls are green, then brown and about the size of a golf ball.

It’s not necessary to treat insect-triggered galls.

Affected foliage may drop early. Insecticides that target gall-producing insects must be applied when the insects are laying eggs. But that’s difficult, and the chemicals may harm beneficial insects that offer biological control. Prune and discard heavily infested parts if you like.

The woolly leaf gall is soft to the touch, but don’t confuse it with the stinging puss moth caterpillar (Megalopyge opercularis), named for its pussy willowlike fine fir. Many Texans call this creature an asp.

The puss moth caterpillar/asp is one of several stinging caterpillars in Texas, including the saddleback caterpillar, buck moth and the hag moth. These stingers belong to the insect family known as flannel moths and are covered with fine hairs and venomous spines.

The puss moth caterpillar/asp does little harm to trees, but when touched can cause a severe sting, throbbing pain, rash, headache, nausea and sometimes shock. If stung, apply an ice pack and a baking soda poultice, and take an antihistamine or see a doctor if necessary.

Puss moth caterpillar populations vary each year, and their numbers are largely determined by the presence of natural predators – flies and wasps.

If lately you’ve been noticing odd, fuzzy puffs on the ground beneath an oak tree, you’re probably witnessing a mysterious, fascinating outbreak of leaf galls that seems to be under way in our region, thanks to parasitic wasps.

I first noticed ours about a month ago, on the windshield of the car after returning from Minneapolis. Just three or four on the driver’s side windshield, t above the wipers – pea-sized bits of pinkish fluff I took for chunks of foam from someone’s discarded sofa. A flick of the lever swept them aside.

Later, in the morning after a good rain moved across Skunk Hollow, I found a dozen scattered on the car hood. A sweep of my hand failed to dislodge them – they were stuck fast to a wet surface. Huh.

That evening I collected several from the driveway gravel, added a few from the concrete apron, a couple of dozen from spaces between the deck boards … and now I had a handful of mystery:

Were they some kind of seed, perhaps? Wind-borne portions of an exotic bloom? Something made by spiders?

The puffs seemed to have collected beneath the oaks, so – maybe acorns at an early stage? Well, no, it was already September and fully formed acorns were plentiful on the ground.

I cut one open with a penknife – it took a little pressure – and found a hard, rice-shaped pellet, cream-colored on the inside and darker where the hairs emerged.

As often, a Google image search soon pointed toward a tentative answer – I had a lot of galls. Specifically, the woolly galls that form on oak leaves after certain wasps implant their eggs.

Back outside, I began turning over the season’s first fallen oak leaves until I had not just one confirmation but dozens, and eventually hundreds upon hundreds:

Wasps of the family Cynipidae had colonized our oaks last spring, drilling into new buds and inserting eggs. As leaves sprouted from the buds, they formed woolly scar tissue around these wounds. And now the autumn phase had begun, with the leaves shedding the little puffs at first and then dropping with the galls still attached – usually with the gall-weighted side down, the better to shelter wasp larvae from the ravages of winter until the return of warm weather produced vast swarms of new wasps.


MinnPost photo by Ron Meador I had a lot of galls.

Tiny, harmless wasps

Sallie and I already wage a running summertime battle with paper wasps that build nests under the eaves, throughout the woodpiles, in the kayak cockpits and sailboat bilge and dark corners of the garage. … Keeping up with them can be an annoyance, not one we’d care to multiply.

Happily, a bit more inquiry established that gall wasps are small and harmless to humans (also, for that matter, to the trees that host them in such large numbers). But that barely scratched the surface of what we wanted to know.

In time I learned that gall wasps, whose species number well over 1,200, were the first research passion of Dr. Alfred Kinsey before he moved on to the sexual habits of humans. (Also, that Sallie once worked at a hospital in Bloomington, Indiana, with a woman who happened to be Dr. Kinsey’s daughter, and preferred to talk about something else.)

And I found that sudden, massive outbreaks of wasp galls have been noted elsewhere in the metro this fall. On the of the Cooperative Extension system, a Hennepin County resident asked on Oct. 1, “What is this seed?” and included a photo very much like mine, reporting that “there are over 1,000 in my yard, but I cannot find it on any plant or tree.”

Two weeks earlier had come a question from East Bethel in Anoka County: “I have an oak tree that has fuzzy growths on the leaves. They look like cotton. I thought they were galls but I just see a little seed-like thing inside. What could they be?”

And in recent years, similar posts queries have come from Arizona, Ohio, Maryland, Iowa ….

Minnesota DNR’s observations

So on Tuesday I emailed some of my questions to Val Cervenka, an entomologist who manages the forest health program at the Minnesota Department of Natural Resources. She replied:

Good questions! We’ve been seeing lots of galls as well this year. We saw quite a few woolly oak galls on bur oak. …

We sent these photos to a gall expert in Ohio because we were curious, and he was sure of the genus but could only guess at the species lanata. If you weren’t seeing these on bur oaks, it’s probably another species, since galls can be fairly species-specific. There are many, many kinds of galls on oaks.

To my knowledge these are not cyclic insects. I chalk it up to weather, as we can with so many insect phenomena we see every year. Each year is unique weather-wise, and this must have been the perfect one for these gall insects. You may see the same thing next year, or you may not, depending on winter weather and their survival.

Cervenka kindly shared an email address for her gall expert, professor Joe Boggs at Ohio State’s extension service, and meanwhile, with her tentative ID in hand I looked up the gall wasp Callirhytis lanata.

The lanata images I found online looked exactly like the galls I’d found in Skunk Hollow, but ours were on northern pin oaks, Quercus ellipsoidalis. The single gall I’ve found on a bur oak leaf looked different from the rest but a lot like Cervenka’s specimens – elongated and marked with red bands, caterpillar-like.

Specialized wasp-oak relationships

MinnPost photo by Ron Meador The wasp eggs exude chemicals that turn plant genes on and off to get the tree to produce the structure – the gall – that’s needed to protect the eggs and then later the larvae.

I telephoned Boggs and he explained that it’s difficult to pinpoint a wasp species from a gall photo because the sum of what science knows about gall wasps is greatly exceeded by sum of what it has yet to learn. But what’s known is pretty interesting:

These are very specialized relationships between one species of wasp and one species of tree. A tree can host more than one species of gall wasp, but each wasp species is a parasite on just one kind of oak.

And, generally, the wasp species is identified by looking at the gall rather than examining the actual insect. As far as I know, no scientist has ever observed or grown a wasp through the entire cycle from parent, to egg, to larva, to pupa, to a new adult wasp.

In gall formation, he explained,

What’s happening is the wasp inserts its eggs into the bud of the tree – not the leaf, which emerges later – and only into the bud, because this is meristematic tissue. It’s undifferentiated at this point – the type of tissue it becomes depends on what influences it’s exposed to.

The wasp eggs exude chemicals that turn plant genes on and off to get the tree to produce the structure – the gall – that’s needed to protect the eggs and then later the larvae.

And think about this: These are not we’re talking about, these oak trees. But we’re talking about a natural process in which one organism controls the genes of another to produce something it needs for survival.

Doesn’t that sound like something we might find useful to understand better? That might lead to something we could maybe even market?

A story of co-evolution, perhaps

I asked Boggs if he felt oaks as a group were distinctive in terms of their insect-driven gall production and he said he thought they were – “you just don’t see this in maples or elms or other tree groups.” And he feels that’s not just coincidence:

I tend to think that oaks and gall wasps have co-evolved, and for wasps the oaks’ importance is about tannins, which are acids, and which put up a strong defense against other insects and diseases. So the wasp is choosing a host that’s already well-defended, and usurping those defenses for its own purpose. Nothing eats oak-leaf galls that we know of.

There are even galls that produce nectar, which attracts carpenter ants and certain hornets, though not honeybees – recruiting another line of defense for the larvae inside.

As for our plentiful woolly galls this fall, he said large outbreaks are common, typically regional in scale, but infrequent in any one area.

There does seem to be a cycle, though nobody knows what it is or what drives it. Certainly it’s not a periodical cycle like cicadas, because there’s no benefit for wasps in the way there is for the 13- or 17-year cicada – a large, slow-growing insect that’s feeding in root tissues that are fairly nutrient-poor.

For the wasp, feeding on the most nutrient-rich portions of the tree, the advantage is in a big population surge that overwhelms any predators.

What I have learned over the years is you’ll have an outbreak of one kind of gall, then years without them, and it’s especially so with woollies, which are produced by many wasp species. Three years ago we were finding them everywhere — in Cincinnati first, then Dayton, then Columbus and so on — but we didn’t see them before that and we haven’t seen them since. Now it sounds like it’s Minnesota’s turn.

Heck, I have photographs from many years ago, when I was first studying galls and my camera skills weren’t so good, of certain galls that I still haven’t identified for sure. I’m waiting for another outbreak of the same type so I can find out what species I was looking at in the first place!

Common Oak Galls

ENTFACT-408: Common Oak Galls | Download PDF

by Lee Townsend, Extension Entomologist, and Eileen Eliason
University of Kentucky College of Agriculture

Galls are irregular plant growths which are stimulated by the reaction between plant hormones and powerful growth regulating chemicals produced by some insects or mites. Galls may occur on leaves, bark, flowers, buds, acorns, or roots. Leaf and twig galls are most noticeable. The inhabitant gains its nutrients from the inner gall tissue. Galls also provide some protection from natural enemies and insecticide sprays. Important details of the life cycles of many gall-makers are not known so specific recommendations to time control measures most effectively are not available.

Gall makers must attack at a particular time in the year to be successful. Otherwise, they may not be able to stimulate the plant to produce the tissue which forms the gall. Generally, initiation of leaf galls occurs around “bud break” or as new leaves begin to unfold in the spring.

Twig and Stem Galls

Twig and stem galls, such as the gouty oak gall and horned oak gall, are solid, woody masses that can girdle branches or make them droop from the sheer weight of the heavy growths. The galls can grow to more than 2 inches in diameter. Horned oak galls can be found on pin, scrub, black, blackjack, and water oaks while gouty oak galls occur on scarlet, red, pin or black oak.

These galls have a long and complex development that takes two or more years to develop. The first stage is a blister-like leaf gall that occurs along larger leaf veins. The second stage is a knotty twig gall that is started in mid-summer and becomes fully mature in 1 to 2 years. Adults emerge in the spring. Gouty oak twig galls are smooth; hormed oak galls have horn-like projections. One female wasp can emerge from each horn.

Generally, insecticidal control is not satisfactory because the wasps are physically protected within the galls. Correctly timing applications to provide effective preventive control is difficult. Where practical, pruning of infested twigs may help to reduce the problem on lightly-infested trees. However, pruning is impractical if large trees are heavily infested. A commercial arborist may be able to provide assistance with valuable plantings.

Leaf Galls

Leaf galls rarely affect tree health so control is rarely justified. However, an application of carbaryl (Sevin) at bud break may reduce infestations of some galls. It is difficult to spray moderate to large trees without special equipment and the necessary protective clothing to protect the applicator from spray drift.

Oak Apple Galls

These are large (1- to 2-inch diameter) rounded growths that are filled with a spongy mass. A single wasp larva is located in a hard seed-like cell in the center. Galls are usually found on the petioles or midribs of leaves. They will dry to a brown, paper thin wall. Removing and destroying galls before they dry and wasps emerge from a hole may help to reduce the infestation. While large and spectacular, they cause no measurable harm.

Roly Poly Galls

This is a group that are similar in size and appearance to hollow green grapes. Inside, in a small, loose “seed like” structure, is the larva of a tiny wasp. These galls seem to appear in place of leaves or reduce the size of leaves, but they do not affect tree health.

Wool Sower Galls

These are wasp galls that appear on white oak in early summer and resemble toasted marshmallows. One gall is actually a group of small hairy galls joined at a common spot on a twig. They can be pulled apart to see seed-like structures that contain the developing wasps.

Vein Pocket Gall and Leaf Pocket Gall

These galls are caused by the larval (maggot) stages of very small flies called midges. Vein pocket galls are elongate swellings that occur along mid- and lateral leaf veins of scrub and pin oaks. The process begins when the unfolding leaves begin to flatten out. At this time, the small midge lays its eggs. The tiny maggots move to the veins and begin to feed. In a few days they are covered by gall tissue and complete development by mid-spring. The mature larvae drop to the ground and remain there over the winter. A related midge species causes the marginal folds or leaf pockets that can be found on red oak or pin oak. Several small larvae may be found inside the swollen folds of the leaf.

Jumping Oak Gall

Caused by a small wasp, they typically appear on the leaves of valley oak and California white oak. These round, seed-like galls fall off leaves when mature. One wasp lives inside each gall; the wasp’s activiy may make the gall “jump” several inches off the ground. Female wasps emerge in the spring from galls on the ground and lay eggs in opening buds. After several weeks, small blister-like galls form on the young leaves. During the summer, males and females mate and females lay eggs in the mature leaves.

Issued: 4/93
Revised: 1/98

CAUTION! Pesticide recommendations in this publication are registered for use in Kentucky, USA ONLY! The use of some products may not be legal in your state or country. Please check with your local county agent or regulatory official before using any pesticide mentioned in this publication.


Images: University of Kentucky Entomology.

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