Galls and the Extended Phenotype

Or “galls I have known and loved.” Or “galls are just not balls, y’all”

JM Thomas, 2025

Why galls?

Complexity.

A mini-ecosystem or community.

Illustrate principles of evolution.

Just so dang cool.

DIVERSITY.

Judy Thomas BTC ’23, interested amateur.

What is a gall?

• All galls are abnormal growths of plant tissues not induced by the plant itself.
• Gall formers: insects, mites, nematodes, viruses, fungi, bacteria, or even another plant species (looking at you, mistletoe). Produce chemicals to redirect other organism’s (plant) growth.

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Https://www.gallformers.org/ (n.d.). Gallformers, What is a Gall?

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JM Thomas, 2025

Genotype v Phenotype v Extended Phenotype

• Genotype: the organism’s genes, coded in DNA.
• Phenotype: How the organism’s genes are expressed (many are not).
• Extended Phenotype:
• Beavers: environmental phenotype (extends into the environment).
• Cordiceps fungi: behavioral phenotype, on another organism.
• Galls: structural phenotype, on another organism.

Also most often an example of commensalism.

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JM Thomas, 2025

How do galls direct the plant?�

• Insect galls: egg/larva make a hormone-like chemical that redirects the growth of the plant part it is in (leaf, stem, trunk, root). A gall is plant tissue, but directed by the insect.

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• Fungal galls: spores form galls via irritation, fungal growth, release of chemicals. Plant tissue, directed by the fungi.*

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*“Several different fungi cause enlargements and thickening of leaves and shoots. Affected plant parts are usually many times larger than normal, and are often discolored and succulent.”*

*Source: Clemson University Cooperativfe Extension (2023). https://hgic.clemson.edu/factsheet/galls-outgrowths/#:~:text=Fungal%20Galls,rarely%20harmful%20to%20the%20plant.

JM Thomas, 2025

Insect Gall Facts:

• Rarely harm the host plant.
• Galls can take varied and interesting forms. Can be smaller, flatter, larger, bigger. At times one insect produces different gall structures.
• Often have complicated life stages (e.g. leaf gall to stem gall to root gall). Some alternate parthenogenic and sexual reproduction.
• Galls are named for the insect, RE: Oak Apple Gall Wasp, not Oak Apple Wasp Gall.
• Non-insect galls are called by scientific name.
• Gall formers depend on specific hosts.
• Tree burls likely not galls.

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JM Thomas, 2025

Note: Photos of the insects are difficult to come by, as they are often tiny, I have a few.

All the gall photos are mine, unless otherwise attributed. Gall insects are attributed images.

Insect galls…

• 30,000 gall-inducing arthropods worldwide (likely more)
• 2,000 species in US
• 800 of these are on oaks!
• 700 cynipid wasps
• 800 species gall midges
• Evolved independently in several evolutionary lineages, with several extinction events.
• Co-evolved with their host plants.
• Galls are noted in the fossil record 385mya, through the fossil record is of the gall (plant damage), as teeny insects don’t survive in the fossil record.

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JM Thomas, 2025

Gall-forming insects, cont.

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• Gall-forming wasps have characteristic flattened abdomens.
• Insect galls need meristem tissue (leaves, growth tips) during a time of rapid plant growth- so spring.
• The insect gall provides habitat, protection*, and food.
• About that food: galls have a higher concentration of nutrients than the plant tissue from whence they arise, another way the gall inducer redirects the chemical machinery of the plant.
• In the fall/winter you may find insect gall husks that are vacant.
• Insect galls size, color, hardness, shape can all change…making identification a CHALLENGE.
• *More on protection later…

JM Thomas, 2025

Gall Morphology

JM Thomas, 2025

https://www.researchgate.net/figure/Morphological-types-of-galls-based-on-position-of-galling-herbivores-and-gall-development_fig5_263010118

JM Thomas, 2025

Wool Sower Gall Wasp on White Oak

Alternating generations- leaf galls versus stem galls and the wasps look different!

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Callirhytis seminator

https://content.ces.ncsu.edu/wool-sower-gall-wasp, NC State Extension

Gall Gallery!

JM Thomas, 2025

Oak Apple Gall Wasp, always on white oak.

Amphibolips confluent

https://bugguide.net/node/view/783371. Creative Commons Licensing, Derek Hennen

See next slide

Family Cynipidae.

Two-stage lifecycle: one sexually reproduced, the other parthenogenic (no boy wasps needed).

JM Thomas, 2025

Spongy Oak Apple Gall Wasp

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Biorhiza pallida

JM Thomas, 2025

Goldenrod gall fly,

Eurosta solidagnis

How I first found it

JM Thomas, 2025

Goldenrod Gall Fly, 

Eurosta solidagnis

https://soagithaca.org/secret-lives/

‘Integral’ gall , not detachable

JM Thomas, 2025

Spiny leaf gall wasp on rose

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Diplolepis polita

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Photo Credit John Bunch, Master Naturalist, Sedley VA

JM Thomas, 2025

Witch Hazel Leaf Gall Aphid

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Hormaphis cornu

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JM Thomas, 2025

“Pinecone gall wasp”

Each section contains one larva.

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Androclus quercusstrobilanus

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Oak-lobed stem gall wasp

EXIT ?

‘Detachable’

gall

JM Thomas, 2025

Ocellate gall midge,

“maple eyespot” gall midge.

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Acericecis ocellaris

JM Thomas, 2025

Cherry oak gall wasp on oak.

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Cynips quercusfolii.

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Primary source of commercial source of “nutgall” used in medicines, inks and dyes.

More on this later…

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Photo Credit: John Bunch

CC BY-SA 2.5, https://commons.wikimedia.org/w/index.php?curid=456846

JM Thomas, 2025

Blackberry seed gall wasp on brambles

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Diastrophus cuscutaeformis

https://www.gallformers.org/gall/1020

JM Thomas, 2025

Phylloxera caryaefoliae gall aphid on hickory

JM Thomas, 2025

Black cherry leaf gall mite

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Eriophyes cerasicrumena

JM Thomas, 2025

Beech erineum gall mite

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Aceria ferruginea

JM Thomas, 2025

Hackberry petiole gall psyllid

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Pachypsylla venusta

JM Thomas, 2025

Fusiform Oak Apple Gall Wasp

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Amphibolips 

Acuminata.

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How do you find out what the insect that created this gall looks like?

JM Thomas, 2025

JM Thomas, 2025

One way…

JM Thomas, 2025

“Annotations” available on website version or on Android devices. JT

JM Thomas, 2025

A gall aphid that is harmful: Grape phylloxera (Daktulosphaira vitifoliae)

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• Evolved in N. America. Native grapes have some defenses.
• The root stage of this insect almost destroyed the European wine industry.
• Nonative grapes are grafted onto N. American grape rootstock.

https://en.wikipedia.org/wiki/Phylloxera

Ohio State University, https://ohiograpeweb.cfaes.ohio-state.edu/ipm/insects/grape-phylloxera

8 life stages, 4 distinct developmental stages:

A sexual form, leaf form, root form, and winged form.

**An emerging harmful gall

• From a nonnative nematode: Beech Leaf Disease Nematode (roundworm)

JM Thomas, 2025

Litylenchus crenatae mccannii, a nematode

JM Thomas, 2025

And, to add some complexity…�Galls are certainly contain more nutrients, but they are also about PROTECTION from predators

Insect predators that lay their eggs in galls:

• Inquilines: insects that eat gall tissues-generally vegetarian.
• Parasites: that attack and consume the original gall larva.
• Hyperparasites: they attack and consume the parasite that is consuming the original gall-former larva!

-and-

• Some inquilines create their own galls called endogalls on the residue of the gall they consumed.

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JM Thomas, 2025

Gall-former defenses: everything wants to eat galls, these little bundles of nutritious goodness.

• Toxic chemicals-terpenes and tannins (nutgall).
• Thick, hard epidermis.
• Hairy or spiny epidermis.
• Far-ranging spacing (harder to find all of them).
• Empty decoys. Gall-formers that use this strategy create many empty galls.
• Mutualistic relationships with bees, wasps, and other insects which defend the gall. In these cases, the galls produce nectar or honeydew.

JM Thomas, 2025

Fungal Galls

• Less is known about fungal galls. They also produce chemicals that redirect cell growth.
• Many can only be identified only under a microscope.
• Rusts, smuts, and other fungal species

“Several different fungi cause enlargements and thickening of leaves and shoots. Affected plant parts are usually many times larger than normal, and are often discolored and succulent. Some leaf or stem galls turn brown and hard with age. The galls are unsightly but rarely harmful to the plant.”*

*Source: Clemson University Cooperative Extension (2023). https://hgic.clemson.edu/factsheet/galls-outgrowths/#:~:text=Fungal%20Galls,rarely%20harmful%20to%20the%20plant.

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JM Thomas, 2025

JM Thomas, 2025

Taphrina polystichi ascomycete fungus

on Christmas fern, though a subspecies is

T. virginica

JM Thomas, 2025

Sweetleaf gall

Exobasidium symploci

Exobasidiums also infect rhododendrons, azaleas

JM Thomas, 2025

Rhododendron Leaf Gall.

Exobasidium rhododendri

JM Thomas, 2025

Black knot on wild cherry is a harmful fungal pest of all stone fruit/prunus, can kill trees.

Apiosporina morbosa

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JM Thomas, 2025

Pine Oak Rust Gall, a harmful fungus. Must be cut out of the tree (or tree removed) bagged and burned. This photo is the spore-releasing stage.

Alternating hosts between two- and three-needled pines and some oaks.

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Cronartium quercuum

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This gall and my neighbor went for a ride…

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How to be a gall hunter

• Walk in the woods
• Scan the plants. If you see anything unusual*, take a closer look.
• *Dead, discolored leaves; bumps, swellings or protrusions anywhere on the plant (though you can’t visually scan for root galls without digging up the plant).
• Take a photo. SEND IT TO ME.
• Identify the host plant- this is really important in identifying the gall.
• Go to: https://www.gallformers.org/ref/IDGuide

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JM Thomas, 2025

A couple galls from late January ‘25

Want to see a live gall insect?

• In spring, find a gall (best bet is to look on an oak tree leaf).
• Collect it.
• Put it in a clear glass jar with some cheesecloth on top.
• Check daily for emergence.
• Examine with with a hand lens.
• Release it near the host tree.

And now, lagniappes*

Cajun for “a little something extra.”

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JM Thomas, 2025

JM Thomas, 2025

Remember “nutgall?” Cynips tinctoriia or Cynips quercusfolii

Oak apple gall wasp galls

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Recipe for making iron gall ink. The National Archives of the UK, C 47/34/1/3, c. 1483*

*https://sarahpeverley.com/2014/01/29/iron-gall-ink-a-medieval-recipe/

Iron gall ink is very lightfast and durable, which is why we can still read many medieval manuscripts.

JM Thomas, 2025

NYBG Ecoquest

Received 10/2/23

Euthamia Leaf Gall Midge (Asteromyia euthamiae)

Goldenrod Brussels Sprout Gall Fly (Procecidochares atra)

Carbonifera Goldenrod Gall Midge (Asteromyia carbonifera)

JM Thomas, 2025

Quiz: Gall or Not?

Likely a chaga, a fungus that forms on the bark of a birch tree. Not a gall, as the fungus does not redirect plant tissue.

JM Thomas, 2025

Burl: burls have unclear causes, most causes unknown. Could be bacterial, viral, mechanical/insect damage. Not a gall.

JM Thomas, 2025

An evergreen bagworm moth larva. It coats itself in plant material, which provides camouflage and a protective surface.

Not a gall.

JM Thomas, 2025

YES! A Gall! Cedar-Apple rust, a fungal gall. Fungi has a complicated life history, involving alternate hosts and 4 different types of spores. “Rust” is a common name for any orange-colored fungal pathogen, mainly destructive agricultural pests.

On apple leaves

Dried out

Sources

• Gallformers.org
• Virginia Tech, “Galls made by wasps” at https://www.pubs.ext.vt.edu/ENTO/ENTO-145/ENTO-145.html
• iNaturalist
• Russo, R.A 2021, Plant Galls of the Western United States.Princeton University Press.
• “What are insect galls?,” Smithsonian Institution, Information Sheet 171, https://www.si.edu/spotlight/buginfo/galls
•  Labandeira CC (2021) Ecology and Evolution of Gall-Inducing Arthropods: The Pattern From the Terrestrial Fossil Record. Front. Ecol. Evol. 9:632449. doi: 10.3389/fevo.2021.632449
• A fun, short gall ink video: https://videos.files.wordpress.com/S4HTcdSd/oak-tree-natures-greatest-survivor_dvd.mp4
• Gall Wasp. Wikipedia. https://en.wikipedia.org/wiki/Gall_wasp

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JM Thomas, 2025