Hackberry gall psyllids

Posted by Lauren Lenz. Lauren is a junior at UVM studying Environmental Studies. 

What: While collecting some walnuts from a black walnut tree located behind my house, I noticed that many of the leaves covering my driveway had some curious raised growths on them that resembled blisters. The blisters ranged in color, from a dried up dark brown, to a bright and healthy looking green. I’ve seen this before on different kinds of leaves, but have never quite understood exactly what they are and what causes them. Are the growths some sort of fungus? Do they occur because of an insect? Are they harmful to
the tree or plant?

I did some initial research and concluded that perhaps insects caused these blisters, also known as galls. I contacted Scott Stokoe, Dartmouth College farm manager/educator, and avid naturalist, to get some more information on galls. First, he told me that many people see galls and imagine them to be tumor-like growths that are detrimental to the health of the tree. However, that's not exactly what galls are. He had me look at a goldenrod gall, and instructed me to see if I could see or feel any noticeable differences between the stem of the goldenrod and the gall. I looked and felt, and found no differences between the texture and color of the stem and the swollen gall growing within it. The most amazing thing about galls is that they mimic the growth of the stem, and because of this, do not harm or kill the plant.

Scott explained that a female insect (in the case of the gall on the stem of the goldenrod, the goldenrod gall fly) creates the gall when she lays her eggs in the stem. The laying insect or sometimes the larva itself produce chemicals to create an abnormal growth in the plant which results in a place for the larva to live and grow. We cut into the quarter-sized goldenrod gall and found a small larva inside. We also noticed a pathway to the edge of the gall that did not break the outer wall of the plant. I looked up a bit more information that explained that when fall approaches, the larva burrows a path to the outside edge of the gall, but will not use the “escape tunnel” until it leaves the following spring. Therefore, the goldenrod gall fly larvae will winter inside the gall until the promise of spring arrives.

I went back out to my driveway to get a closer look at the galls that appeared to be growing on the fallen leaves. I identified the leaves as being from a Hackberry (Celtis occidentalis) tree, whose most common leaf galls are created by a type of midge fly called the hackberry gall psyllid, or hackberry nipple gall maker (Pachypsilla sp.). I cut many of them open, hoping to find insect larvae inside. However, while all the galls showed signs that larvae were the previous inhabitants, none of them actually contained a larva. Upon closer
observation, the underside of all the galls had tiny holes in them, where it appears the larva inside may have taken their “escape tunnel” out. When did they leave the gall? When the tree dropped its leaves? What and where are they now?

Ecological Notes: Many plants have various types of galls, in all sorts of different places (petioles, leaves, lead shoots). However, each species of gall-making insect only creates galls in one specific species of plant. Every insect species creates their galls in a particular place on their host plant, and each species creates uniquely shaped galls. Gall making is not limited to insects; galls can also be created by fungi and bacteria!

Where: Burlington, and all over!