Willow leaf sawfly gall - a bee-relative that grows inside a leaf.

Inside this gall is the larva of the willow apple sawfly.

"What gall!"  Or is that, What a gall!"  Or better yet, "What is a gall?"  

A gall is a small tubercle or raised bump that forms on a plant's leaf or stem.  It is created by any one of a variety of gall-producing insects as a means to produce offspring.  The range is astonishing, there being about 1500 species of gall producing invertebrates in North America alone.  Many of them belong to the Hymenoptera, the group of insects that includes wasps, bees, and sawflies.

The number of plants that play host to these gall-producers is also diverse, but typically specific to the particular species.  The above photo that I took is of one particular insect that uses willows in the subgenus Salix.  I photographed it in the southwest corner of Alberta near Waterton National Park.  It is likely the gall produced by the willow leaf gall sawfly (Euura pacifica, also known as Pontania pacifica sawfly).

The willow leaf gall sawfly is a small wasp-like insect (click here to see the adult) that lays its eggs inside the leaf of the Arroyo willow bush.  The egg incites the immediate area around it to begin growing and surrounds, protects, and nourishes it.  The leaf, in this case, forms a gall that enlarges over time and becomes the home for the growing larva until it matures.  What is interesting is that the male larva goes through five instars (moults) and the female goes through six.  

The mature larva creates a hole in the gall to escape and falls to the ground where it burrows and spins a cocoon.  If it is early in the year the pupating sawfly will emerge as an adult and lay eggs.  Later generations will remain in pupa form until the following spring where they emerge and begin the life cycle anew.

The gall itself does not seem to cause a problem for the plant and is not harmed.  There are certain wasp species that are parasitoids that will lay an egg inside the growing gall to feed on the gall tissue and/or sawfly larva ultimately resulting in its death.

Isn't nature fascinating?

Thanks for reading.

Eric Svendsen     www.ericspix.com