Of Golden Value is the Canadian Goldenrod Plant

by Jessica Lehr, Director, Pollinate Collingwood

During many of my autumn and winter rambles across fields, I come across goldenrod, Solidago canadensis, with a ball-shaped growth in the plant stem.  I often wonder about the insect that overwinters inside.  Yes, those ball-shaped growths, known as galls, are a valuable overwintering habitat to flies, wasps, moths, and beetles, as well as a winter snack shack to black-capped chickadees and downy woodpeckers!

These Canada goldenrod are the plants that do it all!  Considered by pollination ecologists to be a plant that attracts a large number of native bees, this species of plant likes sun to part shade areas, a variety of soil types, dry to moist soil conditions, and blooms yellow flowers in late August through October).  The plant is also considered valuable as it supports conservation biological control, meaning that it attracts predatory or parasitoid insects that prey upon pest insects!  This is a mouthful, so I will explain how this works below.  

First though, I’d like to note that this 3-5 foot tall native plant is of the aster family.  Many people falsely accuse this plant of causing allergies and name it incorrectly as ragweed.  Ragweed (seen in the image to the right) is the species of plant that many people are allergic to. 

Common Ragweed (Ambrosia artemisiifolia)

Goldenrod can spread via seed or underground rhizomes (shoots) and is often considered an aggressive growing native plant.  There are more than 25 native goldenrod species in Ontario, not all of which are as aggressive in growing.  I am enjoying watching my shorter zig-zag goldenrod, with its white flowers, growing in shady locations around my home.  

In addition to providing pollen to native bees late into the fall, this plant provides habitat (food and shelter) for animals at many trophic levels.  A trophic level is the position an organism plays in the food web.  Goldenrod is in the primary level, as a producer (growing with energy from the sun – photosynthesis).  Native bees are in the second level, being a primary consumer (meaning it requires food from a primary source for survival).  Another primary consumer is the goldenrod gall fly, (Eurosta solidago)!  This native species of fly is specially adapted to survival in goldenrod galls, meaning that goldenrod is the insect’s host plant.  Without Canada goldenrod, this fly would not exist and with that the support it provides to organisms further up the food chain.  

Female E. solidago lay their eggs in one of three types of goldenrod in the spring, right below goldenrod buds.  After 4-6 days the eggs hatch and the larvae begin to eat the inside of the plant stem.  As a larva eats, a chemical is released in their saliva which causes the plant to grow abnormally by increasing cell productivity at the site.  The gall, or growth, is noticeable about 3 weeks after the eggs are laid.  As the gall grows, the larva eats at the centre, creating a cavity to live in.  The plant continues to grow and bloom as if the gall and fly larva weren’t there.  

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Goldenrod with gall, Photo by Jessica Lehr

The larva moults twice over the summer and by the autumn they are about ¼ inch in length.  At this point, they eat a tunnel to right near the outer edge of the goldenrod gall.  This is in preparation for their departure in the spring, which happens once they become an adult.  As an adult, they do not have any mouthparts and so this tunnel is essential to their survival.  The thin layer of outer gall is thin enough to allow the flies to emerge by pushing through with a ‘bubble type’ growth on their head.

As a bonus cool nature fact, the goldenrod gall fly produces cytoprotective chemicals in its body to prevent freezing over the winter inside of the gall.  This ‘antifreeze’ can protect the larvae through -40C!

Overwintering in a gall is risky as the galls are identifiable to birds and some beetles as a food source and to two types of parasitic wasps as a place to deposit their eggs and food source for their own larvae.  As far as humans, some ice fishers will cut open the galls to remove the larvae for fishing.  I have even heard that we can eat the larvae as survival food, though I personally wouldn’t suggest it!

Rounding out the benefits of goldenrod is the fact that it supports predatory insects, including crab spiders and assassin bugs, that will help with pest control.   Assassin bugs will eat Japanese beetles, which are many times larger than themselves!

And finally, the seeds of the Canada goldenrod will support several seed-eating birds, including American Goldfinch and Dark-eyed Juncos; two of my favourite winter birds to watch. 

Next time you pass a gall, be sure to think of the insect inside and be grateful for Solidago canadensis.  The plants and the wildlife that interact with it are a valued part of our local environmental diversity.  Perhaps even consider adding some Canada goldenrod along the sides of your garden!

Interested in reading more???

Habitat Planning for Beneficial Insects: Guidelines for Conservation Biological Control

Glorious Goldenrod

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