Oak galls, abnormal swellings on oak trees typically caused by insects, mites, or other pathogens, are a common concern for those caring for these majestic plants. In many cases, the various shapes and sizes of oak galls can help identify the specific insect or mite responsible for their formation. Luckily, there is valuable information available to effectively treat, control, and possibly prevent oak galls.
Treatment options vary depending on the extent of the gall infestation and the type of gall present. Some common oak galls include the horned oak gall, which has characteristic horns protruding from its surface, and the gouty oak gall, which can grow over 2 inches in diameter. Understanding the differences between various types of galls and their causes is crucial in addressing and managing these growths.
In this article, we will explore the causes, effects, and treatments for oak galls, delving into various treatment methods and preventive measures. Additionally, we will analyze the pros and cons of certain approaches, and even provide comparative tables when appropriate. By understanding oak gall treatment, you’ll have all the knowledge you need to maintain the beauty and health of your oak trees.
Oak Gall Basics
Definition and Causes
Oak galls are abnormal growths on oak trees caused by the presence of insect larvae, mostly wasps. These galls occur due to insects, like oak gall wasps, laying eggs in the tree’s tissue, which leads to chemical reactions that cause plant cells to swell and form the gall. In response to the foreign presence, the tree forms a barrier around the larvae, creating the gall’s unique appearance.
Oak Gall Wasps Life Cycle
The oak gall wasps’ life cycle typically involves alternating generations. In early spring, adult wasps emerge from existing galls and lay eggs in the swelling buds of the oak tree. These eggs later hatch into larvae, which feed on the plant tissue and induce the formation of new galls.
Types of Oak Galls
There are multiple types of oak galls, but the primary focus will be on horned oak galls and gouty oak galls.
Horned Oak Galls:
- Found mostly on pin, scrub, black, blackjack, and water oaks
- Caused by the wasp Callirhytis comigera
- Golf ball-sized woody growths with protruding horns
- May cause tree disfigurement and branch dieback
Gouty Oak Galls:
- Observed mostly on pin oak trees
- Caused by the wasp Callirhytis quercussenes
- Solid, woody masses up to 2 inches in diameter
- Can girdle branches, causing them to droop from weight
|Characteristic||Horned Oak Gall||Gouty Oak Gall|
|Tree Type||Pin, scrub, black, blackjack, water oaks||Pin oak trees|
|Gall Appearance||Golf ball-sized with horns||Solid, woody masses up to 2 inches|
|Wasp Species||Callirhytis comigera||Callirhytis quercussenes|
|Tree Impact||Disfigurement, branch dieback||Girdling branches, drooping|
In summary, oak galls are abnormal growths on oak trees, primarily caused by oak gall wasps. These wasps have a unique life cycle that helps them thrive. Horned and gouty oak galls are two common types, each with distinct appearances and impacts on their respective oak tree hosts.
Recognizing Oak Gall Infestation
Symptoms and Signs
- Leaves: Oak gall infestations usually start in spring with abnormal growths on leaves, causing them to appear yellow and distorted.
- Twigs: Affected twigs may exhibit woody, tan-brown galls, sometimes surrounded by fuzzy outgrowths.
These symptoms often indicate an infestation caused by insects, such as wasps or midges, leading to the formation of galls on oak trees1.
Abnormal Plant Growths
There are several types of galls on oak trees, such as:
- Horned Oak Galls: Caused by a tiny wasp, these galls can grow up to 2 inches in diameter and are found on branches of pin and willow oak trees2.
- Jumping Oak Galls: Resulting from a small wasp infestation, these round, seed-like galls usually appear on the leaves of valley oak and California white oak3.
Comparison of Gall Types:
|Gall Type||Appearance||Location on Tree||Causing Insect|
|Horned Oak Galls||Woody, up to 2 inches in diameter||Branches of pin and willow oaks||Tiny wasp|
|Jumping Oak Galls||Round, seed-like||Leaves of valley oak and California white oak||Small wasp|
Remember to examine your oak trees for abnormal plant growths, such as galls, to identify and treat infestations early for the best chance of maintaining healthy foliage.
Effects on Oak Trees
Oak Tree Damage
Oak trees can be severely affected by oak galls, which form through the activity of certain insects, such as gall wasps. These galls lead to small holes and damage on the tree leaves, twigs, and branches, often resulting in dieback.
For example, horned oak galls caused by tiny gall wasps can produce woody galls up to 2 inches in diameter around the stems of oak trees, sometimes leading to significant branch dieback by girdling the branches1. Similarly, jumping oak galls grow on leaves, looking like small seeds or BBs, and fall off carrying the larva within them2.
In order to prevent oak tree dieback and minimize damage, some of the management strategies include:
- Timely detection and removal of infected trees and galls
- Maintaining good tree health by providing adequate care and nutrients
- Reduces the risk of further spreading
- Preserves the health and lifespan of the oak tree
- Can be time-consuming and require regular monitoring of the trees
- The removal process may be costly or ineffective in some cases
|Galls Removal||Reduces the spread of gall wasps and minimizes oak tree damage||Time-consuming and costly; might not be effective for large infestations|
|Tree Care and Monitoring||Improves overall tree health, making them more resistant to infections||Requires regular monitoring and maintenance|
Oak Gall Treatment and Management
When to Contact an Arborist
Oak galls are caused by tiny wasp species, such as horned oak gall wasps, that lay their eggs in tree buds, leading to abnormal growths on oak trees. When infestations become severe, it is advisable to consult a specialist, like a local arborist. They can help identify the type of gall, provide treatment suggestions, and even recommend a management service to protect the tree’s health.
Pruning and Pesticides
Pruning infested branches can provide relief from oak galls. However, avoid pruning during the active life cycle of the wasps (spring). Prune in winter, when the wasps are dormant, to prevent further infestation. Pesticides are another option, especially for targeting eggs and larvae. However, use caution when applying pesticides, as they can harm beneficial insects, such as parasitic wasps that help keep gall wasp populations in check.
|Pruning||Removes infested branches||Timing is crucial to avoid further infestation|
|Pesticides||Targets eggs and larvae||Can harm beneficial insects|
Alternative Treatment Methods
Green repellents: Some people use eco-friendly, non-chemical repellents to deter wasps from laying eggs on their trees. Examples include sticky traps or natural oils applied to the tree’s surface.
Encouraging predators: Encouraging parasitic wasps and other natural predators can help to control gall wasp populations. Planting flower beds or installing bug hotels can provide habitats for these beneficial insects.
Managing tree stress: Oak galls can cause stress to the tree, particularly in pin oaks, which often suffer from branch dieback. Regularly watering and fertilizing, as well as conducting routine insect and disease inspections, can help promote overall tree health.
By combining various treatment methods and understanding the life cycle of gall wasps, we can more effectively manage and control oak gall infestations and promote healthy tree growth.
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 Gall: Perhaps a species of Oak Gall Wasp
I looked through your egg photos and saw nothing that looked like my discovery. I know you are swamped so I’ll be patient. I found this "egg" on an oak (possibly Chestnut) leaf in East Central Indiana on July 25, 2007. The colors are beautiful and staturated with the little spikes surrounding the sphere. Hoping for a finding. Thank you,
This is not an egg but rather some type of Gall. We do not know enough about botany to be able to identify what tree your Gall is on, but that would be a great assistance. It is not the usual oak leaf we can identify, but there are many different species of oaks. Just assuming it is an oak, our Field Book of Insects written by Lutz and revised in 1948 states: “Oak. — More than three hundred different galls have been listed.” The University of Kentucky department of Entomology website states on its Common Oak Galls page: “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. ” Interestingly, The Kinsey Institute for Research in Sex, Gender and Reproduction, which is celebrating its 60th Anniversary this year, is in Indiana. The Institute is named for Alfred Kinsey, who shocked the world when his best seller, Sexual Behavior in the Human Male was published in 1948. Kinsey’s attention to detail regarding research can be traced to his earlier, though far less provocative, studies on Gall Wasps. We would like to believe that your gall has been produced by one of the Gall Wasps in the family Cynipidae that so fascinated Alfred Kinsey. We cannot find an exact match to your Gall on BugGuide.
Letter 2 – Oak Leaf Gall: Amphibolips coelebs
???eggs on bur oak leaf?????
I just took a photo of these bug eggs???? on the leaves of our bur oak tree today (Sept 22, 2008) . The eggs are hard (so my husband say’s I couldn’t get up the nerve to touch them). So it’s the beginning of fall here in Iowa. I was wondering if you could identify the egg for us. Our son who’s 4 spotted them on the tree. There are only a few leaves with the clusters. After trying to identify these in a few books and the help of your web site our 4 year old said why don’t you ask that cool web site we go to….so here I am asking for help.
Thanks so much for your time.
The Sims Family
Des Moines, Iowa
Dear Sims Family,
Though they might look like eggs, they are actually Galls. Galls are growths on plants caused by insects, mites or other creatures. We believe your Galls are Amphibolips coelebs and they are caused by a tiny Gall Wasp. We identified them thanks to a wonderful old research book written by Frank E. Lutz, but we couldn’t find an image online. Galls don’t generally cause the plants any harm. In the case of the Gall Wasps, the growth creates a food source for the developing larva.
Letter 3 – Oak Galls
I found cocoon-like brown hard masses on the front side of a leaf.What are they???
Wed, Oct 8, 2008 at 6:13 PM
I was sweeping off the porch at the campground that I stayed at during the summer/fall.We had a big wind storm come thru and there were a bunch of leaves on the porch.On a lot of the leaves,I noticed these little hard brown cocoon-like circles on the front side of a leaf.I was thinking that cocoons were laid on the under part of the leaf,and that they were soft to touch,but these are hard and round.So I decided to cut one open and inside are these tiny red-orange color worm-like things inside.They moved a little bit,so I just thought that they were a worm of some kind.I’ve looked all over the internet and came up with nothing.Finally I posted my question and someone told me to contact you.I have pictures of the “cocoons” and the “worms” inside.The worms are very small and hard to see on the picture.Please help me identify w hat I’ve found!
Curious Nature Lover
Galls are growths on plants that may be caused by insects, mites, bacteria or fungus. The Galls may occur on the leaves, stems, roots or flowers of the plants. Most often, the Galls are plant specific. We located a drawing in a very old copy of a text by Frank E. Lutz that we own. The drawing is of a Oak Leaf Gall known as Dryophanta polita. Since the text is a field guide, there was no additional information beyond the identification. When we tried a web search of that name, be were led to several online texts that we could not access entirely. One such text is Insects Affecting Park and Woodland Trees and the google teaser is “Oak leaf bullet gall Dryophanta polita Bass. A small, globular gall occurs in
numbers in August … ” Another reference led to the common name Polished Oak Gall. At this point, we can only speculate that Dryophanta polita is a Gall Wasp in the family Cynipidae, but curiously, it is not listed in BugGuide’s taxonomy for the family. Another interesting side note is that Alfred Kinsey, most widely known for his studies of human sexuality and his best selling books in the 1950s, was first and foremost an entomologist who specialized in Gall Wasps.
Letter 4 – Oak Gall
Bug eggs on Oak leaves
Location: Southeastern Iowa
August 16, 2010 2:35 pm
The attached photos show a multitude of bug eggs on the leaves of the oak tree in my backyard.
I cannot identify what bug these eggs are associated with. I would like to know if there is any danger to the tree’s health or to my home from these bugs.
Hi Jay D,
This is not an egg. It is a Gall. A Gall is a growth on a plant caused by an wasp, midge, mite, or occasionally another type of insect. According to BugGuide: “There are more than 2,000 gall-producing insects in the United States; 1,500 are either gall gnats or gall wasps.” The insect produces an enzyme that causes the plant tissue to grow in a deformed manner, and this growth serves as food for the developing gall larvae. It is generally believed that the Gall does not harm the plant. We believe your Gall is a Gall Wasp in the family Cynipidae, but we could not find a conclusive match on BugGuide. There is one image of an unidentified Gall Wasp Gall on BugGuide that looks similar to your Gall. BugGuide gives this advice for Gall identification: “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.” An interesting side note is that Alfred Kinsey who shot to notoriety in the mid twentieth century with his ground breaking studies on human sexuality began his professional career as an entomologist who specialized in the study of Gall Wasps. He approached his studies on human sexuality with the same rigor that he used in collection over 1 million specimens of Gall Wasps.
Letter 5 – Oak Gall
Subject: Black Oak Visitors
Location: Maumee, OH
September 18, 2013 9:21 am
The black oak next door is hosting two creatures that I am asking you to identify. One is probably some kind of wasp gall. It is a small white ball with bright red spots. The other is most likely a moth. Both are nestled on the back side of the leaf. The attached photos were taken on Sept. 17, 2013.
The gall might be caused by a Cynipid Wasp and it looks like a smaller version of the Loxaulus maculipennis we found pictured on both the Oklahoma Biological Survey and the Henderson State University Plant Gall page.
Letter 6 – Oak Leaf Galls
Subject: What are these spots?
Location: Red Creek, NY
September 19, 2013 12:34 pm
Every fall we see these funny looking things on our oak tree leaves. Our class would love to know what these things are. Would you be able to help us figure this out? We are not sure if it is an insects eggs , fungus or what they are? Thank you Red Creek ABCD Preschool class.
Signature: ABCD Preschool
Dear ABCD Preschool,
These are Galls, which the University of Minnesota Extension website defines as: “abnormal plant growths caused by various organisms (insects, mites, nematodes, fungi, bacteria, and viruses). ” There is great diversity among Oak Leaf Galls which you may see on the Missouri Botanical Garden website. Many Oak Leaf Galls are caused by tiny Wasps in the family Cynipidae, and the images you supplied look somewhat like the photos posted to BugGuide of Galls produced by the Gall Wasp Callirhytis furva. The Gall is produced on the leaf and the larva remains inside the Gall, feeding on the growth which does not harm the tree. While we can’t say for certain we have identified the species correctly, we are relatively certain the Galls in your photo were produced by a Gall Wasp in the family Cynipidae, a group of insects studied extensively by Alfred Kinsey before he turned his research to Human Sexuality in the mid twentieth century, however that might be too much information to provide for your preschool children.
Thank you so much for your help!! My one little student has asked me to check for your reply all morning! As soon as he is awakes from his nap I will share this with him 🙂
Hello I am sorry to bother you again. My student, Emanuel keeps asking what you look like. Would you be so kind as to send him a picture? He is over joyed that you answered his question!
Thanks ABCD Preschool
Daniel, who responds to all the questions posted on What’s That Bug? has a photo posted on The Bugman of Mount Washington link.
Ok thank you so much! I will show Emanuel Monday morning!
Letter 7 – Oak Leaf Gall
Subject: What is this egg sack from?
Geographic location of the bug: Decatur, IN
Time: 06:32 AM EDT
My grandson and I found this at the bus stop this morning and would like to know what it is and what we should do with it.
How you want your letter signed: Adventurous G-ma
Dear Adventurous G-ma,
This is not an egg sack. It is an Oak Apple Gall, and it was formed when a Gall Wasp lays an egg on an oak leaf, causing a growth that serves as food for the developing Gall Wasp larva. Galls do not harm the tree. Similar images can be found on Buckeye Yard & Garden Online and Missouri Botanical Garden. Interestingly, Alfred Kinsey, the famous entomologist, studied Gall Wasps before he turned his scientific methods to humans which resulted in the famous (and infamous) Kinsey Reports. Kinsey founded the Institute for Sex Research at Indiana University and his reports resulted in the publication of the groundbreaking “Sexual Behavior in the Human Male” in 1948 and “Sexual Behavior in the Human Female” in 1953. They were best selling books that changed the way America and the world think about human sexual behavior.
So…are there wasp larvae in it or is it empty?
We suspect it is empty.
Letter 8 – Oak Gall
Subject: Oak Gall
Geographic location of the bug: Mount Washington, Los Angeles, California
Time: 4:30 PM EDT
While we are waiting to clear up our technical difficulties receiving images, Daniel is spending some extra time in the yard. He noticed this Oak Gall several days ago. It is growing on a California Live Oak tree Daniel started from an acorn in 2000 that is now over 20 feet high and it has begun producing acorns. We have always been intrigued that Alfred Kinsey began his career as an entomologist who studied Cynipid Gall Wasps, and that he collected well over a million specimens, and that he transferred that obsession to the data collecting methods that eventually produced the Kinsey studies on human sexuality.
Letter 9 – Oak Gall
Subject: Eggs on oak leaf
Geographic location of the bug: Maine
Time: 09:07 AM EDT
Your letter to the bugman: I found this egg mass on an oak leaf in my backyard in September. There are just six eggs laid together on the leaf. I believe they are some kind of insect eggs but I do not know what!
How you want your letter signed: Hannah
These are Galls, and they are theoretically not eggs. Galls are growths on plants (leaves, stems, roots, etc.) that are caused by a variety of reasons, including insects. Gall Wasps on oaks are quite common and quite diverse. The Gall is a growth caused by the Wasp larva that then provides a food source for the growing larva. So the larva does not eat the plant directly, but it does feed on the growth that its presence has caused. We will attempt to provide you with a less general identification.