What Are Those Funny Looking Tumours Growing on Goldenrod Plants?

Newly formed gall on Goldenrod Plant

Have you ever noticed those strange tumour-looking bulges on some wildflowers?  especially Goldenrod plants?  I didn't know what these were, and your students probably don't either. 

Living inside these round spheres are tiny insects!  Called "galls" or "cecidia"   these bulges are actually hibernation dens!

A parasitic fly called the "Goldenrod Gall Fly" [Eurosta solidaginis], lays its eggs on the stem of a Goldenrod plant.  The female fly injects her egg-laying tube called an "ovipositor" into the Goldenrod stem.  In about 10 days, the larvae hatch and begin to eat the inside of the plant's stem.  The swelling and creation of the round-shaped gall is the plant's response to the insect living inside.  The larva is laid in spring and will live inside the plant for the entire summer.  Sometimes galls become very big - about the size of a Ping-Pong ball!

Dried gall showing exit hole.

As winter approaches, the larvae produce a chemical that prevents them from dying in the cold weather.  This chemical is very much like anti-freeze.  The stems of the Goldenrod plant, along with the gall, dry and become brown.  Before winter, the larva creates a spring exit tunnel by eating almost to the surface of the gall.  It then returns to the center where it lays dormant during the winter.

Unless eaten by predators, such as other insects or birds, the larvae turn into grubs and then adult insects that will emerge next spring.  Many birds have learned to hunt for large galls in order to get a tasty meal.  Some fishermen are known to carry a few galls in their pocket and use large grubs as bait.

Two grubs living in a dormant gall.

There are two types of parasitic wasps Eurytoma gigantean and Eurystoma obtusiventris that steal the galls made by the Goldenrod Gall Flies.  These two wasps hunt for galls into which they lay their own eggs.  The Eurytoma obtusiventris lays its eggs inside the Gall Fly larva while the Eurytoma gigantean eats the whole gall fly and takes its place in the gall.  

Round galls are created by the Goldenrod Gall FLY, while elliptical galls are created by the Goldenrod Gall MOTH.  The moth's life cycle is much the same as that of the fly except that theie galls are more camouflaged and elongated. 

It should be noted that there are other causes of galls as well which include other parasites such as fungi, bacteria, insects and mites.


Fun Classroom Activity

Take your class for a walk to your local park, woodland or even hunt in the grasses alongside parking lots and shopping malls.  Goldenrod grow just about everywhere.  Have students search for and collect galls at various times of the year.

Look for mature, brown galls that do not have any exit holes.  This will ensure that the grub is still in the gall.  Cut open the galls and try to identify the insect species inhabiting its home.  

This is a great introduction or companion to teaching students about the various types of animal homes.  Photographs in this category can be found at Animal Homes.  Complement your lesson plan with a slide show on this same topic at Free Slide Presentations