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The Gall Of Aspens: Poplar Twig Gall Flies

Poplar twig galls
Eli Sagor (CC BY-NC 2.0)
Poplar twig galls

"Is not disease the rule of existence? There is not a lily pad floating on the river but has been riddled by insects. Almost every shrub and tree has its gall, oftentimes esteemed its chief ornament and hardly to be distinguished from the fruit. If misery loves company, misery has company enough. Now, at midsummer, find me a perfect leaf or fruit." - Henry David Thoreau, journal entry, September 1851

Before moving our growing family to a larger house last spring, I discovered another creature on our property investing in local real estate. One day, while working in the backyard, I noticed something in the bare boughs of one of our aspens.

Not on them, but inside.

Near the end of the branch, the chalk-white bark of last year's new growth was swollen to the size of a marble. Looking further, I saw that the short twig was inflated in three of four other places. That season in my botanizing I had started looking for galls--the strange growths many organisms cause in host plants--so my eye caught the telltale shape previously unseen.

I did what anyone would do after seeing something curious and wonderful. I googled “aspen gall” and found Euhexomyza schineri, the poplar twig gall fly.

The images of the mature poplar twig gall fly, to my untrained eye, looked like any other fly I’ve swatted away on a summer afternoon. I wondered how often I’ve seen one. Their black bodies are coarsely hairy, red peripheral eyes are set in the sides of their heads, and transparent wings sprout tangentially out of their thorax.

The females deposit their eggs into the spring buds of several species of poplar trees, while the twig is rapidly growing. Two or three tiny green larvae hatch inside of the twig, which swells out from the irritation to form their cozy winter cabin. In the spring, they pupate, tunnel their way out of the gall, and fly off to begin the cycle afresh.

I inspected the tree more closely, and discovered that more than one twig was providing hospitality to these homesteaders. Our aspens were gall fly nurseries! Nearly every branch in the tree had signs of occupation.

I snapped off a few infested twigs for closer observation. My kids peered past my elbows as I operated with pocket knife and kitchen cutting board. We carefully peeled the gall layer by layer, through the thin bark, phloem, and cambium, through the woody interior, bisecting the gall and opening each half. Inside, surrounding a thin strand of pith, were two small chambers, lined with a pliable foam-like material. The children leaned in to see the tiny dormitory as, with the point of the knife, I gently pushed away foam until we saw a new texture: a tiny, glistening, green blob, unresponsive to our intrusion. I carefully tried to extract it from the cell, but its skin was so thin that the gentlest touch smashed it into green paste.

I left another gall in a jar in the warmth of our home, and checked in on it every day. In a few weeks, an exit hole appeared in the gall, and I found the miniscule grub desiccated and black at the bottom of the jar. Many of the sleeping larvae fared no better outside than under my care. The aspen turned out to be the reserve pantry for overwintering black-capped chickadees, who crowded the branches and pecked at the galls through the end of the season. Many of the blemishes on the tree were the residue of their snack raids.

Since then, no aspen escapes my inspection, and nearly every one I encounter is full of galls. Studies have found the flies do little harm. Within a few years, the galls often diminish into the growth of the branch. Occasionally, they swell and warp the branch into shapes out of a Dr. Seuss book.

Our old home was larger and more comfortable than a gall, but in the spring, we too emerged in search of another home. We found something a little bigger, and in place of an aspen, we now have an old cottonwood and willow on our lot, each with a host of their own residents to discover.

"Field Notes" is produced by the Montana Natural History Center.

(Broadcast: "Field Notes,"  4/14/20 and 4/17/20. Listen weekly on the radio Tuesdays and Fridays at 4:54 p.m., or via podcast.)


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