These growths, which resemble pine cones, are often found on willows.
In the summer a small fly called a gall gnat midge deposits an egg on the stem. The new larva secretes a substance on the stem which causes the willow to go into overdrive building a multi-layered chamber composed of hardened material that would have been leaves had stem growth not been arrested.
Here is a terminal bud without a gall:
Galls provide food and shelter for the organisms living within them.
The growth that we see on some willows is called “pine cone willow gall” and is the temporary home and food supply of the midge.
At certain times of the year, there are countless midges in the air at places like Col Sam Smith Park.
Salix eriocephala, which is commonly called Heart-leaved Willow or Missouri Willow, is always laden with galls and can be identified by this characteristic.
Here is the interior of a gall: