Hackberry Nipple Gall (the name alone describes it best)

Posted by: Alison O'Connor, Larimer County Extension

When I first learned about hackberry nipple gall during my undergraduate degree at Iowa State University, I snickered. How could you not? We were in our woodies class (ha!) [woody plant materials for those of you who are more mature than me] and I'm fairly certain it was at the point in the semester where we were all looking for a good laugh.

Hackberry nipple gall in all its glory.

But good ol' hackberry. And its nipple gall. This year, I have never received so many calls and emails about, "This weird thing plaguing the leaves on my tree" and "What are the warts on my tree leaves?" It often seems that we have a "great year" for certain things...and it's the Year of the Hackberry Nipple Gall. We do see it every year, but this year it seems to be more common or more obvious.

Look at those galls!

Psyllids (sill-ids) are the cause. This insect is teeny-tiny and so small it can easily fit through the screen grids on your windows. Fortunately, the hackberry psyllid is only interested in hackberry. If you could zoom in on the insect, it would look like a miniature locust/cicada.

Adult hackberry psyllid on the tip of a pencil. Photo by B. Ogg.

Adult psyllids tend to overwinter in cracks of trees to survive cold winter temperatures, but they also snuggle in other crevices, like around windows and siding. If they wander indoors, they meet their fate, since our homes are not very accommodating to the insects.

In spring, the adults will lay eggs on emerging hackberry leaves and the young nymphs will feed on the leaf tissues. The tree responds by creating a gall, which is simply abnormal growth. The nymphs spend the rest of the summer sucking plant juices in the comfort of the gall until they pupate in the fall as an adult and emerge to overwinter and start the cycle again.

The gall is a comfortable cocoon for the hackberry psyllids to spend their summer.

I can see why people are concerned...not only by the leaf appearance, but also by premature shedding of the leaves on the ground. Fortunately, hackberry is one of the toughest trees we have in the landscape and it seems to be unaffected by the galls and early leaf drop--so there is no need to worry, apply insecticides or cut the tree down. (To me, what's even more fun is walking on the gall-ridden leaves--they "pop" under your feet!)

A hackberry tree that has shed many of the afflicted leaves in late August.

Interestingly, if you can prevent raking/destroying the leaves in the fall, a beneficial wasp overwinters in the galls that will parasitize the new nymphs the next spring. Nature does its best to help control pest populations.

Nipple gall would not be a reason to stop planting hackberry--this tree has so many benefits. As my co-worker Eric Hammond described: it's reliable...like your friend with a truck. It's drought tolerant, has good yellow fall color and is tolerant of many soil types and urban conditions. The nipple gall just provides a great conversation starter when you have friends for dinner. "Hey Joe! Let me show you the nipple gall on my tree!"