Goldenrod unfairly fingered as allergy culprit

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Though most wildflowers have faded by late September, goldenrod is just taking center stage. More than 100 species of goldenrod (genus Solidago) brighten North American meadows in late summer and early fall.

They are the bright yellow flowers that turn open fields into seas of gold. My hayfield came into full bloom about a week ago. Though beautiful as a cut flower, goldenrod is often blamed as the cause of hay fever. It is not.

Late summer hay fever allergies are triggered by wind blown pollen, usually from ragweed. Its flowers are small, dull and inconspicuous, so ragweed is often overlooked.

Not guilty

Goldenrod gets unfairly blamed for runny noses, sneezing fits, and weepy eyes because it’s so showy this time of year. But goldenrod is blameless. Its pollen is too large and sticky to be carried by the wind. It requires insects to move it from one flower to another.

Check a stand of goldenrod in full bloom, and you’ll find a variety of bees, wasps, flies, beetles and true bugs — all potential pollinators. In fact, it is an insect that makes the goldenrod story a surprisingly fascinating ecological tale.

Though blooming now, goldenrod stems began growing in the spring. That’s when tiny goldenrod gall flies mated, and females deposited eggs on the tips of growing goldenrod stems.

Though females may lay several eggs on each stem, usually only one survives. The egg hatches about 10 days later, and the tiny larva burrows down into the plant stem.

The combination of the larva’s chewing action and its saliva stimulates the growth of a gall on the goldenrod stem. A gall is a swelling of plant tissue around the gall fly larva.

It can ultimately grow almost as big as a golf ball, and it provides food and shelter for the developing larva. And the gall’s hard outer shell protects it from many predators.

Tunneling out

By late summer, the larva has molted several times and attained its full size. Its final act before going dormant for the winter is to dig an escape tunnel to just under the surface of the gall’s outer shell. From here the adult gall fly will emerge in the spring.

As fall and winter temperatures drop, gall fly larvae produce glycerol — a natural antifreeze that enables the larvae to survive frigid winter temperatures. When temperatures warm in the spring, the larvae transform into the pupal stage.

About two weeks later, the adult gall fly merges. It escapes through the exit tunnel it excavated the previous fall. It needs only to break through the thin layer of epidermal cells that remained on the mouth of the tunnel.

The adult gall flies then search for mates. They live as adults for only about two weeks.

Another aspect

The relationship between goldenrod and gall flies is fascinating, but it’s only part of the story. Tasty tidbits hiding inside goldenrod stems are just too tempting to be ignored.

Another player is this ecological drama is a small wasp. These female wasps drill through galls with a long ovipositor and lay eggs in the gall fly’s larval chamber.

When the wasp eggs hatch, they eat the larval gall flies. Even the galls that escape the attention of these wasps are not assured of surviving the winter. Dormant gall fly larvae are nutritious, tasty treats for downy woodpeckers and chickadees.

Field studies indicate that these small birds devour the contents of 50 to 60 percent of goldenrod galls in some places. For evidence of the gall fly’s life cycle, look closely at the goldenrod stems as you roam open fields this fall.

Evidence

First look for dry, dead stems from last year. They’re easy to spot among this year’s lush new growth. Snap off a stem just below the old gall and look for the small exit hole. That’s where the adult gall fly escaped last spring.

Then find a few fresh galls and collect the stems. Carefully slice lengthwise through the center of the gall with a pocket knife. Inside you’ll find a plump, cream colored larva, the holy grail of gall wasps, downy woodpeckers, and chickadees.

(Send questions and comments to Dr. Scott Shalaway, RD 5, Cameron, WV 26033 or via e-mail at his website, http://scottshalaway.googlepages.com.)

About the Author

Scott Shalaway, who holds a Ph.D. in wildlife ecology from Michigan State University, writes from his home in rural West Virginia. A former faculty member at Oklahoma State University and the University of Oklahoma Biological Station, he has been writing a weekly nature column for newspapers and freelancing for magazines since 1986. Send questions and comments to scottshalaway@gmail.com. You can also visit his Web site, http://scottshalaway.googlepages.com. More Stories by Scott Shalaway

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