NatureZen: Galls Galore!
by Dr. Malle Carrasco-Harris
When you think of animals in Overton Park, what comes to mind? Maybe you imagine the busybody squirrels or birds with snacks. Perhaps it’s my old friend, the slithering copperhead! You may actually think of an insect in perpetual motion: a butterfly, bee, or infamous mosquito.
Within the last four months or so, a new group of animals has stolen the show for me: gallformers! We can discover tiny ecosystems in our own backyards, neighborhoods, and parks by exploring unique plant structures called galls. It’s a topic truly full of wonder, and exploring these little known worlds can help bring us a fresh sense of discovery and awe in places we are already familiar with.
So, what is a gall? A gall is a plant structure induced by another organism. For those who enjoy sci-fi: a gall inducer hijacks the genetic machinery of its plant host, coercing the plant to grow the inducer a “build-to-suit” shelter that also provides nutritious food. The inducer, also known as a gallformer, is manipulating the plant to grow a completely novel structure in a specific place. IT’S WILD!!
The structures made by an oak gall wasp (Amphibolips sp.)
Gallformers include insects such as wasps, midges (a type of fly), and aphids to name a few, but species of mites, fungi, nematodes, and bacteria can also induce galls. The inducers are generally incredibly small – we’re talking in the millimeter range – so oftentimes the species isn’t identified by its own appearance but by the appearance of the gall it induced. I think of it like recognizing a painter by their brushstrokes rather than by their portrait or signature.
The structures made by Iatrophobia brasiliensis, a cassava leaf gall midge
The gall structure can be large and obvious, like the gouty oak galls you’ve likely seen on an oak in your neighborhood, or the particularly gouty red oak tree behind Hole 1 on the golf course. It also may be something unassuming and incredibly ephemeral, like this Hackberry Winged Gall Midge. This small, oddly shaped gall was gone mere days after I observed it. Whatever form it comes in, the gall’s design and timing makes it a reliable indicator for the species of inducer. This is called a secondary or extended phenotype. Absolute biological wizardry if you ask me!
Because I have been keeping an eye out on the Hackberry Petiole Galls, I happened to observe this maroon gall that reminded me of a weird-shaped berry. The mosquitoes were hungry that day, so I didn’t linger and snagged the best pictures I could with a cellphone. I went to my trusty site to identify it as a Hackberry Winged Gall Midge. Another reviewer on iNaturalist agreed with my suggestion and also said I reported the first one in Tennessee! There are only 55 observations total on iNaturalist, and I can understand why! I went by two days later and I could not find a single one.
Gall formation involves tinkering with the plant’s genes, so many leaf galls are seen in the spring, when we get that flush of new growth and electric green leaves. Some slower growing galls, such as those on the stem or petiole, continue to grow during the summer, with their inducers exiting later in the season.
This woolly catkin gall wasp, which makes a fuzzy pink-and-white home, emerged this spring and took a rest in the Veterans Plaza pollinator garden.
I spent good parts of this spring looking for galls and other “signs of life” on plants. I wrote in my nature journal that various of these treasures reminded me of green apples or the hull of a snap pea. Insects and mites cause galls by entering plant tissue while it is still forming. They stimulate the tissue not just to modify its growth, but also to produce more nutrients. As a result, galls serve as both shelter and food source for the developing larvae or nymphs. That may explain why they reminded me of tasty snacks.
The dried brown version of these crispy green galls was the first gall I identified last winter when I became hooked! You can find these all over the place in Overton right now, and it helps me identify hackberry trees even before noticing their funky-textured bark. They’re always on the petiole of the leaf, the little stem that connects the leaf to the twig.
Since I kept returning to the same galls to check on their status, I was able to see gall structures whose original inhabitants had grown up and out, but had new tenants who came in behind them to eat the abandoned homes. Nature likes to recycle nutrients!
Galls aren’t the perfect nursery, though. They are targeted by predators, parasitoids, and inquilines that live and share space within the gall (some of which don’t actually harm the gall-inducer but live rent free in the ready-made home). There are even hyperparasitoids that kill the parasitoids–I can’t make this up.
This wasp in the Torymidae family is a parasite of gall-forming insects.
At this point you’re probably wondering, do they harm the host plant? Often the answer is no. A gallformer needs a home to belong to. Where would the next generation live if they wiped out the hosts? So, galls and their inducers, inquilines, predators, and even hyperparasites are part of a tiny, balanced ecosystem.
However, you may be familiar with the idea of non-natives or invasive species, such as Kudzu. Invasives can be problematic in the environment if there’s nothing around to keep their numbers in check. So a gallformer in a non-native habitat can be problematic if it doesn’t have its natural checks and balances, as has been seen with the Chestnut Gall Wasp.
I could go on for another 800 words about galls, we’ve just scratched the surface! Gall inducers are often small, difficult to track down, and live on plants hard to study in a lab setting. There is still so much mystery surrounding these ultimate hijackers! Google puts the answers to many of our questions at our fingertips, but galls and their inducers remind us there are still new things to behold in this world.
This ~3mm red-eyed wasp was found in a grocery sack I was storing galls in. At first I was really excited that it may be a gall former, but some digging has led me to believe he’s in a genus that actually parasitizes gall wasps (Sycophila). Drama!
When you come to the forest or to another natural area, I hope you find the inner child in you that is filled with wonder and exploration. I wonder if you can look beyond the blue jays and butterflies to something new, tiny ecosystems right under our nose. Just look for signs of life on imperfect plants!
