How Do Gall Wasps Make Galls: Uncovering Nature’s Tiny Architects

Gall wasps are fascinating insects known for their unique ability to induce galls on plants. These wasps are the largest group of gall-inducing insects and are responsible for producing various types of galls on plants such as oak and roses. Galls are abnormal plant growths that provide protection and sustenance for the developing wasp larvae.

These insects inject their eggs into the plant tissues, releasing chemicals that trigger the formation of galls. The specific shape and structure of the gall is often determined by the species of gall wasp involved. As the larvae develop and feed, the gall expands further, providing the young wasps with a secure and nutrient-rich environment.

Some examples of galls induced by gall wasps include woody, rounded galls on stems or leaves and woolly or mossy galls. Interestingly, certain gall formations can even be used to identify the species of gall wasp responsible for their creation. As such, the relationship between gall wasps and their host plants is an incredible example of the interconnectedness of nature.

Gall Wasp Basics

Life Cycle and Biology

Gall wasps are a family of insects known for inducing abnormal plant growth called galls. The gall wasp life cycle includes laying eggs on host plants, hatching into larvae within the gall, and eventually emerging as adult wasps. Some notable aspects of gall wasp biology are:

• Alternate generations: Gall wasps exhibit an alternation of sexual and asexual generations in their life cycle¹.
• Various gall types: Gall wasps create a wide range of galls, from woody to woolly or mossy².

Habitat and Distribution

Gall wasps are found in various habitats globally. They are particularly common on oak and rose plants². Specific habitat preferences can vary among the different gall wasp species.

Diversity and Classification

The gall wasp family is large and diverse³. Some key facts about their diversity and classification are:

• Over 1000 species: Gall wasps comprise more than 1000 species worldwide³.
• Species-specific galls: Each species of gall wasp produces a unique gall on a specific host plant³.

This table showcases an example of a gall wasp species, its host plant, and the type of gall it forms.

Pros of Gall Wasps:

• Ecosystem role: Gall wasps contribute to the ecosystem by providing food and shelter for various organisms.
• Biodiversity: The various species of gall wasps represent a unique aspect of insect diversity.

Cons of Gall Wasps:

• Damage to plants: The galls formed by gall wasps can cause damage to plants, particularly in cases of heavy infestation.

Gall Formation Process

Initiation of Gall Formation

Gall formation begins when a female gall wasp, belonging to the family Cynipidae within the order Hymenoptera, lays eggs inside the plant tissue of host plants like oaks or roses¹. The wasp injects a sting into the plant, depositing eggs typically in buds or young leaves during spring². The insect’s secretions and the larvae’s feeding stimulate the plant to produce abnormal tissue growth³, leading to the formation of galls.

Interactions with Host Plants

Gall wasps establish complex interactions with their host plants, often oak trees, as they induce the plant tissues to create galls, sheltering the wasp’s developing larva⁴. These galls provide:

• Protection from most natural enemies
• Food in terms of starch, sugars, proteins, lipids and minerals
• A microclimate with stable temperature and humidity

Inquilines like the oak gall wasp (Neuroterus sp.) and rose gall wasp (Diplolepis sp.) can exploit galls without playing a direct role in their formation⁵.

Gall Development and Maturation

The gall development process occurs in several stages:

1. Initiation: The wasp’s sting triggers the host plant to modify its tissues.
2. Growth: The host plant tissue proliferates, leading to a larger gall.
3. Maturation: The gall tissues develop and harden, protecting the developing larva.
4. Emergence: The wasp larva matures into a pupa, eventually emerging from the gall as an adult⁶.

Comparison of main life stages of gall wasp:

Gall formation represents an interesting area in the study of the natural history and biology of numerous gall wasp species worldwide.

Ecological Significance and Relationships

Parasitoids and Inquilines

Gall wasps are part of the family Cynipidae and play a unique ecological role, inducing galls on host plants to protect their offspring. Many other insects also live within these galls as inquilines (non-gall-forming species) or parasitoids that attack gall-making insects. For example, some parasitoid species, like Eulophidae and Torymidae, lay their eggs inside the galls, where their larvae then feed on the gall-forming wasps’ larvae, controlling the gall wasp population.

Natural Control Methods

Several natural control methods can help manage gall wasp infestations:

• Pruning: Removing affected branches can limit gall development and prevent wasps from spreading to new parts of the tree.
• Burning: Affected plant parts can be burned to destroy galls and the wasps within.
• Predators and parasitoids: Insects like birds, other wasps, or even fungi can serve as natural enemies, reducing the gall wasp population.

Impact on Host Plants

Despite the swelling and distortions caused by insect galls, plant tissues remain mostly functional, and the overall health of the host plant is rarely affected. However, galls can still have some negative impacts on certain plant species, such as:

• Reduced aesthetic appeal, especially in ornamental plants
• Stunted growth, particularly in young trees or small plants

Overall, understanding the ecological significance of gall wasps and their interactions with host plants, parasitoids, and inquilines aids our understanding of the complex relationships within ecosystems. This knowledge can be utilized to develop better ecological management strategies and promote healthier plant communities.

Reader Emails

Over the years, our website, whatsthatbug.com has received hundreds of letters and some interesting images asking us about these insects. Scroll down to have a look at some of them.

Letter 1 – Unknown Galls on Elm Leaves

Subject: Eggs
Location: Arlington, Texas
May 24, 2014 9:56 am
Hello,
I took these pic at a nature preserve in Arlington, Texas on May 23rd. My daughter REALLY wants to know what they are – it was her sharp eagle eye that spotted them. Hope the pics are clear enough. We plan or returning frequently to watch what happens with these eggs. Thank you!
Signature: Mary Sarabia

Hi Mary,
These are not eggs, but Galls.  Galls are growths on plants, and they may occur on leaves, stems, buds, roots and many places on plants.  Galls may be caused by Gall producing insects including wasps, flies and moths, or they may be caused by other arthropods like Mites, or they may be caused by viruses or injuries.  According to BugGuide:  “Gall insects (and mites) are usually highly specific about what kind of plants they use, and even what part of the plant. To maximize your chances of getting a gall identified, record the plant species (include photos of the leaves, flowers, fruits, etc. if you’re not sure), and if it’s a leaf gall, note the position on the leaf (if it’s not obvious from the photo): upper side or underside; midrib, side vein, or somewhere else. Also note whether or not the gall is detachable, the size of the gall, and anything else distinctive about it that may not be clear in the photo. With oaks in particular, which are hosts for hundreds of kinds of galls, every little detail can help to narrow down the options.”  It appears that the affected plant in your image is an Elm, and we tried to research Elm Galls, but we could not find an exact match to your Galls.  The University of Minnesota has some examples of Elm Galls, but none look like your example.  Generally, the Galls do not harm the plant.  When Galls are the result of Insects, and Insect Galls do tend to be the most common Galls, it is generally produced in the larval form.  When the egg hatches, the larva releases a substance that causes the Gall to form, and then the larva feeds off the developing Gall.  Oak Galls are the most common and Wasps in the family Cynipidae are the most common Gall producers.

Wow!   Daniel!   Thank you so much for all of the information!   I had,  in fact,  decided that they were galls.   I found photos that appear to be exactly the same,  but they are all on oak leaves.   I plan to hike back out there today.   I will take more pictures with the provided guidelines.   The look-alike galls on the oak leaves were identified as “cynipid wasp” and “callirhytis”.   Would you like me to send further photos?
Thank you so much for your help!
Mary Sarabia

Hi Mary,
The closest visual match we were able to find were also Oak Leaf Galls and the Spiny Rose Gall, and we eliminated them because Gall Wasps are so specific about the host plants.  Any follow up photos that are significantly different than the ones you have submitted can be added to the posting.  Please submit any images of different Galls using a new submission form.

Letter 2 – Unknown Galls on Sagebrush

SAGEBRUSH GALL
March 9, 2010
FOUND THIS INTERESTING GALL ON SAGEBRUSH (ARTEMESIA TRIDENTATA) IS THIS THE WOOL SOWER CALLIRHYTIS SEMINATOR OR RHOPALOMYIA?
ERNIE
OKANOGAN, WASH.

Hi Ernie,
Galls, unusual growths on plants, are often caused by insects, but there might be other reasons that the plant tissue becomes distorted and produces the odd growths.  There is a nice online piece on Gall Making Insects by John A. Byers that has good information.  This is neither a Wool Sower Gall and we are not certain if this growth on sage is caused by a Midge in the genus Rhopalomyia without doing additional research.  We did find a paper online that was published by the Journal of the Kansas Entomological Society that mentions, but doesn’t picture, a Fruit Fly, Eutreta diana, that is called a Sage Stem Galling Fly.  We found the fly pictured on the Diptera Site, but again, a photo of the actual Gall produced by the fly has eluded us.

Letter 3 – Unknown Gall from Panama

tropical gall
Hi,
Your gall page is great! Any idea what insect might be producing this gall? I’m pretty sure the tree is in the Myrtaceae family, but beyond that, I couldn’t say. The tree is in western Panama at an altitude of about 800 m. Thanks for any help.
Mary

Hi Mary,
We don’t know anything about your Gall from Panama. It is unusual in that it is a twig gall and not a leaf gall. We will post your photo to see if any of our readers knows the answer, but you shouldn’t get your hopes up too high for an identification. Tropical insects are even difficult to identify when they are large, spectacular looking, or highly unusual, unlike this relatively innocuous Gall.

Letter 4 – Gall on California Black Walnut Tree: Velvet Gall Mite???

Yesterday we noticed this gall on our largest protected California Black Walnut Tree, and did some internet research. We can’t locate a convincing photo, but believe it might be the result of the Velvet Gall Mite, Eriophyes caulis. We will check with local California Black Walnut experts Clare Marter-Kenyon and Julian Donahue to see if they know of this mite on Los Angeles trees. According to the information we can locate: “Little is known about the mites that occur on black walnut, but the velvet gall mite is common in some areas. The mite itself is so small that it cannot be seen with the unaided eye. Injury The velvet gall mite causes a conspicuous velvety red growth up to an inch long on the leaf stem, often causing the leaf to curl or twist over on itself. Galls may be numerous on individual trees but they are considered to be harmless to the tree. Control No control is recommended. ”

Letter 5 – Two Species of Leaf Gall

Hi – I wondered if you could identify the above images? The first one was on an elm tree – the whole tree was coveredin them. The second one (Oak ‘eggs’) was on the leaf until it dropped off in the autumn – what happened then I don’t know. They started out bright red but by autumn were a very pale pink. Many thanks!
Jo

Hi Jo,
Both of your photos illustrate Galls, growths that occur on various parts of plants that are usually caused by an insect or mite. Wasps, Moths, Aphids and Flies can all produce Galls. According to the University of Kentucky College of Agriculture website: “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.” Galls are not harmful to the plant. Regarding your images, we are not entirely conviced that the tree you indicate is an elm is really an elm. Also of interest with the color change in the Oak Galls is this citation from Lutz’ Field Book of Insects: “Of the Galls caused by insects, Oak Galls have been used in dyeing, tanning, and the manufacture of ink.”

Hi Daniel – thank you very much for your reply. I thought they may be galls but wasn’t sure. The tree was identified by a wildlife trust (I work for them so quite a few people saw the leaves and said they were elm). However no one was sure what the ‘lumps’ were. Have a good Christmas Best wishes
Jo

Letter 6 – Gall on Grape: Grape Tube Gallmaker

Eggs or Gall?
Thank you very much for taking the time to create a site like What’s That Bug. My wife and I check it each day to see what new creatures have been identified. My question for you is, are the red objects in this picture insect eggs or just gall of some kind? The picture was taken in June at Burdette Park in Evansville, IN. I appreciate any help you can offer since I know you are very busy. Thank you again for all your time. Sincerly,
Sean and Emily Kemp

Hi Sean and Emily
Our old edition of Lutz’s Field Book Of Insects identifies these green or red galls on grape as Cecidomyia viticola. The insect that produces the gall is a midge. Outdoor Decor has some images.

Letter 7 – Unknown Gall

egg?
Cool website! My boys found this in Sedona, Arizona. It is attached to the leaf. Is it an egg?
Thanks,
Pinkie
Charlotte, NC

Hi Pinkie,
This is some type of Gall, but we are not sure what. Galls are growths produced on plant leaves, stems and other parts by usually wasps, flies and moths. They are not harmful to the plant. Inside the Gall, young insects are developing, so it is akin to an egg. The gall is usually very plant host specific. A quick internet search did not identify your particular Gall, but in time, we might have a more conclusive answer.

Letter 8 – Gall on Artemesia

Galls on Artemesia tridentata
March 13, 2010
Here is a photo of some different galls then I sent before. These are more common and are less rounded and in multiples joined together. I’m not sure if its a fly, beetle or mite. One guy said those other galls might be cecidomyiid fly. Both are on Artemesia tridentata.
Ernie
Okanogan, Wash.

Hi again Ernie,
Thanks for sending in more photos of Galls.  Perhaps an expert in Galls will be able to do a conclusive identification.

Letter 9 – Unknown Galls

bug larvae on Spanish Stopper
Location:  Florida Keys
August 11, 2010 11:30 pm
Hi!
I’m just curious as to what these things are on the underside of Eugenia foetida.
Susan

Hi Susan,
These appear to be Galls.  Galls are growths that appear on plants and they are often caused by insects, but not always.  We have not had any success determining what has caused these Galls.  When they are caused by insects, they are usually very plant specific.  Perhaps one of our readers will be able to assist with the proper identification of your galls.

Thanks Daniel!
Everyone from the park (volunteer for Dagny Johnson Bot. SP) tells me they are galls but that seems to be enough of an answer. I would like to know more. I would guess that they are caused by insects as they are on so many Spanish Stopper especially this year.
Susan

Letter 10 – Dormant Cedar Apple Rust Gall

Subject: Egg Case?
Location: Gainesville, Georgia
March 4, 2014 9:12 am
One of my students found this on an evergreen tree in his yard. It is approx. 1 inch long. Is this an egg case or a part of the tree?
Signature: K. Baer

Dear K. Baer,
This is a Gall, which is a generic name for a growth on a plant.  Many are caused by insects, mites and other “bugs” but this is a Cedar Apple Rust Gall in its dormant state and it is a fungus.  Here is what the Missouri Botanical Garden website states:  “Symptoms on juniper: Brown, perennial galls form on twigs. When mature (usually in two years), the galls swell and repeatedly produce orange, gelatinous telial horns during rainy spring weather. The galls of cedar-apple rust are often over 2 inches in diameter, while cedar-hawthorn rust galls are rarely over 2 inches in diameter. Occasionally the twig beyond the gall dies, but usually no significant damage occurs on the juniper host.”  The University of Missouri Extension website has more images and information.

Letter 11 – Galls we believe

Subject: What eggs are these? Or are they not even bug eggs?
Location: North Andover, ma
June 5, 2015 5:33 pm
Hi,
Went on a hike with my 2 kids today and came across two plants with these long green vertical eggs* I was curious to see what bugs laid these eggs or if they were even eggs at all.
Thanks!
Signature: Maggie

Dear Maggie,
We believe these are Galls, and though they are theoretically not eggs, many Galls are produced when insects, like Gall Wasps, lay eggs and the developing larva causes a growth on a plant leaf, stem, root, or other plant part.  The growth acts as food for the larva, and the Gall does not harm the plant.  Other Galls can be caused by mites, viruses or injuries.  Knowing the plant species is often helpful in the identification of the insect that produces the Gall.  Though your Galls resemble those on the maple leaf on the Little Nature Museum site, your plant is not a maple and Galls are often very plant specific.  We are postdating your submission to go live on our site next week while we are away from the office.

Reader Emails

4275

Letter 1 – Crystaline Gall Wasp

Lovely red stranger on oak leaves
Can you identify these red beauties attached to the base of some of the oak leaves in our backyard in Pleasant Hill, California? I included a picture of the oak leaves to assist you in identifying the type of oak – I’m not sure what type it is. Thank you!
Mabel Neys
Pleasant Hill, California

Hi Mabel,
We have been scouring the web for about an hour trying to properly identify your Oak Gall, probably to the chagrin of our other readers with questions sitting in our “in box”. Sadly, we will have to leave this identification at Unknown Oak Gall, probably one of the Gall Wasps in the family Cynipidae, and possibly in the genus Andricus based on the closest image we could find on BugGuide, Andricus fullawayi. We decided to give it one more try and located Andricus crystallinus, the Crystalline Gall Wasp. Looks like a perfect match. Galls do not harm the oak trees.

Letter 2 – Grape Tube Gallmakers from Italy

What is This?
July 8, 2010
Dear Bugman,
Recently, I went hiking to an old monastery in Turin, Italy. The trail was punctuated by large cement crosses that had something to do with the myth of the Stations of the Cross. Worried about having a Jesus overdose, we sought an alternate route. Along the way, we came across this deformed tree. Thinking it was perhaps the work of Satan, we went back to the JC trail.
I am not convinced that these odd thorny growths are from the devil’s hand. Seems he could do a lot better than this. I suspect it’s a bug!
Any ideas?
Regards,
Godless Hiker
Turin Italy

Dear Godless Hiker,
These odd thorny growths appear have been produced by the Grape Tube Gallmaker, Cecidomyia viticola, a species of Midge.  The plant appears to be a wild grape which would support our identification.  We ran a similar photo from Alabama in 2008.  Galls are generally considered to be growths on plants that may be caused by insects including wasps and flies, or by mites or other arthropods.  BugGuide indicates: “Gall insects (and mites) are usually highly specific about what kind of plants they use, and even what part of the plant. To maximize your chances of getting a gall identified, record the plant species (include photos of the leaves, flowers, fruits, etc. if you’re not sure), and if it’s a leaf gall, note the position on the leaf (if it’s not obvious from the photo): upper side or underside; midrib, side vein, or somewhere else. Also note whether or not the gall is detachable, the size of the gall, and anything else distinctive about it that may not be clear in the photo. With oaks in particular, which are hosts for hundreds of kinds of galls, every little detail can help to narrow down the options.”

Reader Emails

Over the years, our website, whatsthatbug.com has received hundreds of letters and some interesting images asking us about these insects. Scroll down to have a look at some of them.

Letter 1 – Grape Tube Gallmaker

Eggs on Scuppernong
Hi, I stumbled upon your great website while trying to find out what these apparent eggs are on a Scuppernong (wild) leaf in our front yard woods here in Northeastern Alabama. Got any ideas? Many thanks for any help.
Mike O’Brien

Hi Mike,
Your letter attracted our attention since we had no idea what a Scuppernong was. Thanks to Wikipedia, now we know a Scuppernong is a Wild Grape. These are Grape Tube Gallmakers formed by a Midge, Cecidomyia viticola. Dave’s Garden website has photos. Galls are growths on plants caused by insects (like wasps, flies or aphids), mites, fungus, bacteria, viruses and other sources. Galls can be found on leaves, stems, roots, buds and other plant parts. Most Galls are harmless, though unsightly, and a few are destructive. The Grape Tube Gallmaker is an example of a harmless Gall. The larval Midge forms the Gall and the Gall acts as the food source for the insect. The are