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The Garden Scoop

Hackberry Gall

As you might imagine, my family spends a considerable amount of time out observing the wonders of the natural world, and I am always fascinated by the way my kids view and interpret things in nature. Many times, their straightforward and simple perspective makes me feel like such a dummy. There is certainly wisdom in their innocent perspective.

A little earlier this summer, my five-year-old son and I were about to load up in the car and he was milling about in the driveway while I was loading things. He picked up a leaf and brought it over to me asking, “Daddy, what are all these little bumps on this leaf? Are they bug eggs?”

I replied that he was right that an insect caused the bumps, but those tiny things were actually plant structures called galls and that an insect larva was inside feeding before it would emerge later.

He matter-of-factly replied, “Oh, so it is an egg with a baby bug in it,” seeming satisfied with his deduction skills and looking a bit puzzled at me for not simply replying that it was, in fact, an egg. For which I replied, “Not exactly, part of the plant grew around the baby bug to protect it until it comes out.”

“Oh, so the plant made the egg then?” he replied. And I had to just agree with the little guy because in a way, he was kind of right. Although both he and his brother got a lengthy explanation of galls and the relationship between plants and insects as we drove to our Little League game that night. They were both surprisingly interested and engaged in the conversation.

The gall in question was actually hackberry nipple gall, which is quite common across the Midwest on our native hackberry (Celtis occidentalis) trees. I think most people have noticed this gall in their lifetime due to its widespread distribution and noticeable, bumpy appearance when present. I can remember picking this gall off of many leaves for further inspection when I was a kid without ever figuring out the weird bumps.

The insect responsible for this gall is the hackberry psyllid (Pachypsylla celtidismamma), and the gall does serve as an egg of sorts. Each spring, adult psyllids (pronounced “sill-ids”) lay their actual eggs on the emerging leaves of hackberry trees. After hatching, the young psyllids begin feeding on leaf tissue, sucking sap right from the leaf.

As a defensive response, the leaf initiates abnormal growth around the psyllid to contain the pest by producing the galls we see on the leaves. Over the rest of the summer, the psyllids comfortably feed on sap from inside their protective gall. So, in a way, the gall does work like an egg as my son had hypothesized since it is a protective structure that nourishes a developing organism until it has grown enough to emerge.

My son’s egg analogy made me think of the galls differently than I had before, and as we discussed the life cycle of these psyllids on our drive, I began to agree with him more.

I have always viewed the gall from the tree’s standpoint, as a negative thing or a plant response to an insect invader. My son was taking the psyllid’s perspective, viewing the gall as something that protects the insect. During our conversation in the car, he even made the comment, “But daddy, the bugs need a place to live, too” as I was explaining how the tree is attempting to protect itself from this “bad” bug.

The thing that really clicked for me from our conversation is the amazing relationship these two organisms share. From the psyllid’s standpoint, it is absolutely fascinating that this species uses the hackberry’s own defenses against itself. How could such a relationship come to be? The evolution involved is so interesting to think about. Until my son turned the focus onto the benefits for the psyllid, I had always viewed it from the tree’s standpoint, as an insect that was a problem.

In reality, the galls and their insect roommates have a negligible impact on tree health. These insects really are not that bad at all. In fact, the adults are so small that they can actually fit through window screens when they emerge in late fall to find an overwintering location. Any that do actually make it indoors will die because our homes are not suitable overwintering sites, leaving little or no negative impact to humans other than some unattractive looking leaves.

So, the next time you see bumpy leaves up in a hackberry canopy, think about it from the insect’s perspective. There is no need for any control of these native insects, they are simply part of our central Illinois ecosystem, and they need a home, too.

Ryan Pankau is a horticulture educator with the UI Extension, serving Champaign, Ford, Iroquois and Vermilion counties.