The mysteries of goldenrod

Can you imagine what it would be like to emit a chemical that would make a house grow around you?  Well, insects can do just that!  The houses the insects build are called galls. Galls are growths on leaves, stems, and the twigs of many different plants that are caused by irritation, damage or a chemical secretion that comes from insects while they are laying their eggs.

Aphids and sawflies are the main insects that cause galls, but some moths, beetles, nematodes, psyllids, and true bugs also cause them to form.  This wonderful world of nature our Creator has made never ceases to amaze me.

Since there are so many gall types, I will write about the gall caused by the goldenrod gall fly (Eurosta solidaginis).  It is the one you can easily find here at Akwesasne. The goldenrod gall fly is common and distributed coast to coast in North America. First, let me tell you a little bit about its host, the goldenrod. Goldenrod plants are very graceful looking plants that can be found in un-mown grassland areas.  They are also found along the edges of forests.

 This is the flower that many people wrongly associate with hay fever. Goldenrod does not cause allergies.  It blooms in late summer when ragweed and other grasses that cause hay fever allergies also appear. It is often recognized and called ragweed, but the ragweed has green flowers.  It never has beautiful deep yellow flowers.  While people do not grow goldenrod in their gardens, it might be a good idea to do just that.  The goldenrod is a very environmentally important plant.

The numerous flowers produce large quantities of nectar that are an especially important nourishment for Monarch butterflies who are migrating many miles south to their winter habitat. The pollen of goldenrod is eaten by soldier beetles whose larvae are predators of insects that are harmful to many of our important plants.  Ambush bugs, assassin beetles, praying mantises, crab spiders, as well as daddy longlegs seek out a meal on goldenrod plants by preying upon insects that visit it for nectar. They also end up preying on each other.  It is a cycle of nature - a natural balance.

In the spring and early summer the newly formed goldenrod leaves are food for different animals, the deer being one of them.  In the winter, American Goldfinches and Juncos eat the goldenrod seeds.  The Downy Woodpecker pecks a hole into the galls on the stem and pulls out a wonderful winter food insect, a larva. Perhaps, in late summer before winter sets in you might want to gather some of these seeds and throw them into a cleared spot in a corner. If you do this, you will be helping a large diversity of wildlife.

Now let us get back to discussing the goldenrod gall.  In the summer the goldenrod plants are attacked by goldenrod gall flies (Eurosta soligdaginis).  The female lays her eggs on the terminal buds of the goldenrod plants.  The terminal bud is the primary growing point at the tip of the stem.  After the eggs hatch the gall fly larvae burrow into the stem where their saliva (which is thought to contain plant hormones) starts to produce the galls. A structure develops that has a protective coat and inner tissue that contains starch and protein providing food for the growing larva.  The female lays several eggs on each stem, but for some reason, not yet understood, each stem ends up with only one larva in one gall.

The larvae go through three stages.  The first two it eats and grows. In its last stage, in late summer, it reaches its full size.  In this stage the larvae inside the gall can withstand the freezing weather as it waits to emerge the next spring.  As fall temperatures drop, the larvae are stimulated to produce glycerol (a type of antifreeze). The last thing the larvae do in the third stage is to excavate its exit tunnel.  It remains as a grub in this (house) the gall for nine months.

When the warm temperatures of spring arrive the larvae is transformed into a pupa.  The pupa’s final transformation is to turn into an adult. This winged adult crawls to the end of its tunnel where it anchors itself so it can pump body fluids into a special portion of its head.  The fluid swells and bursts the outer skin of the gall allowing the goldenrod gall fly to pull itself out of the gall.  It rests and allows its wings to dry before it looks for a mate to start the cycle again.  Keep in mind that only a few larvae make it to the adult fly stage.

Even though the galls are tough, the larvae are not completely safe.  Gall fly larvae can be attacked by several insect species of parasitic wasps and beetles.  For instance, one small wasp penetrates the thick walls of the gall with her ovipositor (egg-laying tube) and lays her eggs in one of the gall’s chambers.  When these wasp larvae hatch, they eat the gall fly larvae. Even the gall fly larvae that manage to avoid insect predators and make it to winter are still not safe. Many of the larvae in the galls provide important food in the winter for small birds like chickadees and woodpeckers.

The next time you see a gall on the stem of a plant just think about the little larvae in there.  Think about how great it is that a tiny larvae can build its own winter home right around its tiny body and survive until the following spring when it will turn into a fly and start the cycle all again.  There are so many wonderful things in nature for each of us to learn about and to enjoy.