Curious About Galls

It is a beautiful winter day.  Our hike takes us right through a fallow field that just two months earlier was bordered by blazing yellow goldenrod.  Now browned leaves droop from stems that sport an inflorescence of tiny fluffy seedheads on top.  Something else catches the interest of my hiking companion.  We look closer to find a ball-shaped swelling on the stem.  “What is that?” she asks.  Before I could answer, she points out another on a nearby plant.

               “This is a really interesting thing,” I begin.  “It’s a gall.”  Now I’m not sure how much she wants to hear about galls, but this manipulation of plants by animals just fascinates me, and I’ve studied and journaled and read about them over the years.

               There are over 2,000 kinds of galls found on North American plants alone.  Each is made by a particular species of insect or mite or even nematode. Some are caused by bacteria or fungus.  Some galls are good – like the bacteria within the root gall of leguminous plants that assist with the uptake of nitrogen.  Some cause agricultural headaches.  Most are benign.  The gall itself is plant material that the gall maker has caused to grow providing food and shelter for the occupant.

               “This is a goldenrod ball gall,” I say. “It’s made by the picture-winged fly.” This is Eurostes solidagnis, also called goldenrod gall fly.  Gall makers are very specific as to the type of plant they parasitize.  In this case, the female goldenrod gall fly lays an egg in the stem of Solidago altissima in the spring.  The egg hatches in ten days.  The larvae bore down into the stem, giving off hormones from its saliva (auxin and cytokinins) which stimulate the plant into hyper-growth. This extra tissue forms the gall surrounding the larva in its chamber where it stays for a full year.  The larva can also counteract the plant’s natural defenses.  Jasmonic acid - another hormone - is often produced by plants in response to an herbivore’s attack.  It can trigger the release of compounds that prevent the offending insect from digesting the protein it seeks from the plant, making the feeding on that plant moot.  But the larva turns off this defensive mechanism.  After a year, the larva pupates within the gall, and a few weeks later the adult emerges. They live only two weeks “on the outside” - enough time to mate and allow the females to lay eggs and begin the cycle all over again.

               “Here’s another one,” my little girl said, “and another one!”  “This one has a big hole in it,” she noted as she plucked the stem.

               “Gee,” I said, taking it from her to examine, “Seems this larva got dug out of his gall by an enterprising bird – maybe a chickadee or woodpecker.” Getting an idea, I asked my young companion to collect a complete gall, which she promptly did.  “Do you want to see what’s inside?” There was no doubt that this interested her.  I took out my Swiss army knife, knelt down to use a thigh as a table, and carefully sliced through the gall tissue right down the middle.  Prying the two pieces apart we found an inner chamber, inside of which was a tubby whitish little larva.

               “Ewww,” she said.  First reaction.  Wait for it…  I put the larva in my hand. In a couple seconds she said, “Let me see.”  There we go - curiosity trumping revulsion.  We examined the little fellow for a minute before continuing on our hike.  We soon found ourselves in a forest setting.  Here she found another natural thing that interested her.  “Look what I found,” she presented me with an oak twig with a brown golf-ball-sized growth.  “What is this?” she asked. 

               I’m pleased that she is finding things that I actually know.  “That’s an oak apple gall.”

               “Another gall?”

               “Yup – this one is made by a wasp that laid her eggs at the base of a bud this past spring.”

               “Look, it has a hole in it.”

               “That’s where the adult came out of its protective gall this summer.”

 “Let’s break it open and see what’s in there.”  She’s getting the hang of these galls now.   The brown, crispy gall broke open quite easily to reveal an almost hollow cavity with radiating fibers.  These filaments are the remnants of vessels that transported nutrients and water to and from the larvae that once nestled in a chamber in the center. 

               “Look,” she exclaimed as she examined the interior of the gall more closely, “there’s a bug in here.”  Sure enough, another little critter found the gall a convenient place to spend the winter, but its plans were foiled by our curiosity.

               On your hikes this winter, be on the lookout for galls of all sorts.  They’ll be on leaves, stems, buds, and roots of woody plants, herbaceous plants, garden and agricultural plants.  Hopefully your curiosity will overtake your revulsion and take a peek inside to discover the inhabitant.