Those weird bumps on Minnesota’s trees have stories to tell

Strip off a tree’s leaves, and you can discover all sorts of new things. Wasp’s nests, bird nests, and a lot of smaller and sometimes bizarre discoveries.

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Case in point: What some naturalists call “poop on a stick,” otherwise known as black knot fungus.

Aptly named appearance-wise, it wraps around branches of shrubs and small trees often in the Prunus family of cherry and plums. There’s a similar twisty, blobby green to reddish swelling that shows up on thimble berry bushes that’s caused when a gall wasp lays eggs on the plant.

Abnormal growths or swellings on plants and trees usually fall into the category of “galls,” growths most commonly caused by the larvae of gall wasps, gall midges, fruit flies, aphids and mites or triggered by fungi, bacteria or viruses.

“Now’s a great time to look for galls,” said Sam Guida, who lives between Duluth and Two Harbors and started noticing the diversity of galls while doing an iNaturalist challenge during the winter. When he found what looked like a pine cone on a willow tree, he went down a rabbit hole learning about galls. “It’s turned into a fascination ever since,” he said.

He’s documented and identified enough species for a self-published guide, “Galls, Burls and Brooms of the Northwoods,” and is working on a larger field guide for this part of nature that has largely been overlooked. (Brooms are parasitic growths on branches, while burls are knobby growths on the wood.)

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Handiwork of the goldenrod gall fly at Schaefer Prairie near Glencoe, Minn.

Dml - Star Tribune/The Minnesota Star Tribune)

Some of the easiest galls to spot, if you live near any open fields or restored stretches of prairie, are created by the goldenrod gall fly.

“They look like someone shoved a ping-pong ball into the plant,” he said. “You can pop that open and see a little larva in there.” Those larvae also produce sugars to help their bodies survive the cold, making them — if you dare — surprisingly sweet to eat.