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Otto: The wonder of the goldenrod gall fly

Published: Saturday, March 23, 2013 5:30 a.m. CDT • Updated: Friday, March 29, 2013 12:37 p.m. CDT
Caption
(Photo provided)
The next time you’re tempted to dismiss a growth on a plant stem as “just another goldenrod gall,” remember that it’s the work of E. solidaginis – a fruitful fly indeed.

Let’s face it, fruit flies have an image problem.

Some are serious agricultural pests (remember California’s infamous medfly infestation?) and others, namely the Drosophilidae, are primarily known for the way they hover around overripe fruit. (Who hasn’t swatted away these tiny creatures as they’ve swarmed over a bowl of browning bananas? People raising tiny toads, that’s who. But that’s another story for another day.)

However, there is another type of fruit fly living in our midst, one that goes about its business quietly, aiding several other beneficial species along the way: the goldenrod gall fly.

If you’ve ever walked past a field of goldenrod, you’re probably familiar with this industrious creature. Or at least the products of its industry.

In late spring and early summer, females of the species Eurosta solidaginis lay individual eggs in the terminal buds of goldenrod, or Solidago spp., plants. The eggs hatch in a week or so, and the wee larvae burrow their way down into the pith, or spongy middle part of the stem.

Once ensconced, they go about their business of feeding on plant juices and growing. As they expand, so too does the plant tissue around them, probably in response to chemicals emitted by the baby bug.

This time of year, the immature flies, for the most part, are safely insulated within the plant’s stem, nestled in their homes that by this point have swollen to roughly the size and shape of a gumball.

I say “for the most part” because not every goldenrod gall fly lives this life of luxury. Some are parasitized by beetles and wasps, while others are consumed by chickadees and downy woodpeckers.

Those that do survive will go on to wake from diapause in spring, then pupate and emerge as adults that will begin the cycle all over again. (Here’s a fun fact: Adult gall flies do not have the proper appendages to engineer an escape and hence would be helplessly trapped inside their former safehouses, were it not for one amazing feat. Driven by instinct, and a pair of mouth hooks, the gall fly larva excavates an exit tunnel in fall, stopping just short of the thin epidermal layer of the plant. Post pupation, all the adult fly needs to do is ram through the fragile skin, like a football player breaking through a banner on game day, and it’s free to take wing. Go team Gall Fly!)

At this point, you may think we’ve come to the end of the Tale of the Gall. Successful emergence means the gall’s usefulness is over and done with, right?

Well, not quite.

Like so many other fascinating facets of nature, the end of this story actually is the beginning of several others.

Provided they weren’t excavated by bird beaks, or chewed upon by rodents, those galls have plenty of utility left.

The tough, corky interior still can protect inhabitants from intruders, and the chambers that once housed gall fly larvae can provide roomy, even spacious, accommodations for a new round of occupants.

Scientists investigating secondary uses of goldenrod galls have discovered all sorts of creatures holed up inside these sturdy, ready-made structures. Small species of wasps, bees, spiders and beetles – many that are important parasites or predators of pest species – take advantage of the space inside the galls’ nearly impenetrable walls.

Wasps will cover the chamber’s exit hole with a tiny bit of mud; bees will fill it with a parchment-like plug; and spiders may add a dollop of silken web as a door.

The next time you’re tempted to dismiss a growth on a plant stem as “just another goldenrod gall,” remember that it’s the work of E. solidaginis – a fruitful fly indeed.

• Pam Otto is the manager of nature programs and interpretive services at the Hickory Knolls Discovery Center, a facility of the St. Charles Park District. She can be reached at 630-513-4346 or potto@stcparks.org.

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