Help! I've Got Hackberry Leaf Galls!

Last week students in our outdoor classroom sent the picture below, wondering what it was.  It's a very logical question, since those...things are growing out of what is obviously a leaf, but no normal leaf has weird miniature mushroom-shapes growing out of it.

Hackberry Leaf Galls (photo: M. Sherman)

The leaf above comes from a hackberry tree, whose bark I think is fantastic, and which we will explore later in the winter.  We have a gigantic hackberry tree in the outdoor classroom, and it's at the end of the row of parking spaces near the road.  Here's our hackberry:

Hackberry tree with most leaves already gone for the winter.

If you search through the fallen leaves around the classroom, you can find lots of hackberry leaves right now.  They have toothed edges (lots of tiny points), and they narrow to a tip.  Also, the wider end of the leaf usually is lopsided with one side larger than the other.  Most of the hackberry leaves have one or more of those big lumps on the lower side of the leaf.  The lumps are called leaf galls, and they are scar tissue the tree has grown in self defense against a parasite.

Three hackberry leaves, two with galls, one without.

So, what exactly is a parasite, you ask?  A parasite is a small organism that lives on a larger organism and often uses the larger organism for food, harming the larger organism in the process.  The larger organism is called the host.  There are lots interesting types of parasites in this world.  Dogs and cats sometimes have fleas for parasites.  Deer often have ticks.  Humans can sometimes have lice.  And plants can have parasites too.  The parasite on our hackberry leaves can only live on hackberries, not humans.  It is a type of insect called a psyllid (SIL'-id).  Psyllids look just like tiny cicadas - smaller than a grain of rice.

I broke open this gall, but it was empty.  The adult has already emerged from it.

Hackberry leaf psyllids lay their eggs on the underside of hackberry leaves in the spring.  The eggs grow into immature psyllids that look like this.  The psyllids damage the leaves, which causes the leaves to grow a lump of scar tissue (a gall).  The psyllids eat hackberry sap and live inside the gall as they grow larger through the summer.  In the fall, the psyllids grow into adults and drill out of the gall.  They fly or crawl to find crevices in bark or buildings to overwinter safely.  When the weather warms up in the spring, they lay eggs and start the cycle again. 

Hackberries grow well in Middle Tennessee, yet they almost always have hackberry leaf galls damaging their leaves.  The trees don't seem overly harmed by the gall psyllids' damage.  This is normal parasite behavior.  Most parasites don't cause extensive harm to their hosts.  They take just a little food from them but not enough to kill them.  If a parasite ate too much of its host and killed it, the parasite would be out of food and would die too.  Parasites use their hosts in a sustainable manner so that their food source will be available in the future.