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Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

Galls form when insect larvae burrow into goldenrod stems in springtime, chewing out a small chamber where they overwinter and pupate. © Don Blegen

February 2005

The gall of that fly!

Burrowed in a swollen plant stem, a tiny fly larva waits out winter.

Anita Carpenter


Galls form when insect larvae burrow into goldenrod stems in spring. The larvae chew out a small chamber where they overwinter and pupate. © Don Blegen

Finding insects on a cold, white winterscape is improbable, but not impossible. Minute snow fleas appear as black specks jumping on or hovering above freshly fallen snow. A mourning cloak butterfly floats about on an unseasonably warm day, seeking shelter as the temperature drops. Cecropia moth caterpillars are wrapped in slender brown cocoons attached to twigs.

One insect that is easy to find overwinters as a dormant larva, the fruit fly, Eurosta solidaginis. Its home is inside a brown, marble-sized swelling midway up a Canada goldenrod stem. The bug's relationship with Canada goldenrod, Solidago canadensis, began last spring when a female fly deposited a single egg sometime in late May to early June on the young leaves growing at the tip of a goldenrod stem. After three to four days, the egg hatched and the young larva bored its way through and into the growing stem. The plant responded by increasing cell production at the injured site and formed a gall. The young gall was stem-colored (green) or may have been reddish-green like a young apple.

As the gall grew, the larva ate out a pocket in the gall's center, forming a chamber in which to live, overwinter, and pupate. Throughout summer, the barrel-shaped white larva fed on the inside of the gall, molted twice and continued to grow, reaching a "gigantic" size of 4 to 6 mm by early October. Since the adult fly has no chewing mouth parts, the larva prepared for its spring exit by chewing a tunnel out to the gall's surface, but leaving a thin "epidermis" over the outer opening. On a brown winter gall, you'll see the exit tunnel marked by a pinhead-sized dark spot on the gall's equator.

To survive winter, the mature larva entered a resting stage or diapause as a third instar larva. To prepare for the sub-freezing conditions, colder temperatures induced the larva to convert glycogen into glycerol and sorbitol, effective antifreezes that reduced the water content in its body so ice crystals could not form and cause cell injury.

If ice anglers cruising for bait don't collect the gall, the larva will awaken from its dormancy in spring and pupate inside the gall. The adult fruit fly will crawl along the tunnel then inflate and deflate a balloon-like structure between its eyes to force open the thin covering and escape.

Newly emerged flies are about 6.5 mm long, slightly smaller than houseflies. Males are smaller than females. Their translucent wings are mottled with brown splotches. Adults do not feed and typically live about ten days. During that short time the flies mate, females lay eggs only on young Canada goldenrod stems, and the cycle begins anew.

Wander the snowy fields and collect a few goldenrod ball galls. Carefully cut one open and find its dormant occupant. Place other galls in a jar and wait for the inhabitants to emerge. Although the hardened gall seems adequate to protect the larva inside, the larva can be parasitized when the gall is young, tender and growing. Two species of small parasitic wasps in the chalcid family can deposit their eggs through the growing gall tissue right into the fly larvae. The following spring, a tiny black wasp may emerge rather than a fruit fly.

Some Canada goldenrod stems have two ball galls instead of the usual single ball gall. This occurs when two different female fruit flies each lay one egg on the stem tip or the same female lays two eggs at different times. Usually the upper gall is smaller and is more often parasitized.

In your search, you may notice galls with different shapes on Canada goldenrod. A moth larva, Gnorimoschema gallesolidaginis, likely formed the elliptical stem gall. This insect does not overwinter in the gall. Look for its tiny exit hole. A closely bunched cluster of leaves formed in the terminal bud was caused by another type of fly or midge, Rhopalomyia solidaginis; it's called the goldenrod bunch gall.

Ubiquitous Canada goldenrods support many insects with diverse life cycles, but none is more interesting than a tiny fruit fly that lives most of its life encased within its stem. For only ten days, the fruit fly experiences the freedom of flight and the warmth of a Wisconsin sun before its dependents burrow back into the Canada goldenrod's inner sanctum.

Anita Carpenter watches the goldenrod stems rustle on winter walks near her Oshkosh home.