Hickory Leaf Stem Gall

Holly Welch

We are getting into gall season and Holly Welch sent the picture on the right for identification just as I was writing on one of my favorites, the hickory leaf-stem gall.  They start as smooth swellings along the leaf petiole and occasionally on new shoots.  Initially pale, they later may develop some redness as seen here.  Aside from homeowner concerns, they don't cause any significant damage to the tree.  I find them commonly on young trees where the affected leaves are at eye level.

They are caused by aphids called Phylloxera caryaecaulis and they have a very interesting lifecycle.

Cut open with tiny aphids

"Eggs hatch as new buds open and grow in the early spring. Young aphids crawl to newly expanding foliage where they feed on the new growth by piercing the epidermis and sucking cell sap. The feeding causes the tissue to grow galls which enclose the aphids. Several generations of aphids develop within the galls during May, June, and into July.  If the thought of a hollow gall filled with wiggling little aphid-like creatures grosses you out, you may want to skip this closeup video of the cut open gall above.

Escape opening on the underside of the leaf.

In late July, the galls split open, turn black and jagged in appearance, and the aphids emerge. At this time most of the affected leaves fall prematurely. There are several generations of aphids per year. Eggs are laid in late summer or fall in cracks and crevices of the bark and in old stem galls. Aphids overwinter as eggs in the gall and bark crevices" https://wiki.bugwood.org/Phylloxera_caryaecaulis

Freshly opened gall

I found this newly opened gall that was full of winged aphids.  After filming the inside of the gall I put it in a bug box and returned home.   When I checked the clear plastic box an hour later there were 50+ crawling all over the walls.  When I lifted the lid they all climbed to the top edge, began warming up their wings and then took flight as seen in this video.

Hickory gall "aphid"

You can file this under TMI but although they look like an aphid and walk like an aphid, they technically are not aphids although that term was used in most references.  Aphids are in the superfamily Aphidoidea while our "aphids" are in the Phylloxeroidea superfamily.  Looking at them under magnification they lack the twin tail pipes (cornicles) of an aphid.

Finally, there is an interesting Missouri connection to a closely related species, Phylloxera vitifolia.  It damages European grape vines and came close to wiping out the French wine industry in the 1860's, only to be saved by Missouri grape vine root stock.  Hermann Jaeger was one hero in the story and he influenced a 10 year old named George Washington Carver.