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Goldenrod Gall Fly

Bug-sicles in the Class Room:

Demonstrating freeze tolerance with larvae of the Goldenrod Gall Fly!

By Doug Collicutt
(Click links for more images.)
Goldenrod Gall FlyA recently emerged
Goldenrod Gall Fly.

Reading about animals that over-winter by freezing solid is one thing, and we hope you've already read "Frozen Alive" in our Winter Issue, but experiencing a "critter-sicle" hands-on is something else. In southern Manitoba there is one critter that is particularly suited for a class room demonstration of survival at sub-zero temperatures. The larva of the Goldenrod Gall Fly (Eurosta solidaginis) is both common and easy to get. This insect parasitizes the stems of goldenrod plants causing the plant to produce a large spherical gall on its stem. Here's a quick run down of the gall fly biology and some suggestions to help you bring this critter into the class room.

Goldenrod Gall Fly Life-Cycle

The goldenrod gall fly (Eurosta solidaginis) is a common and widely distributed insect found coast to coast in the central part of North America. The adult flies emerge from their galls in late spring. In Manitoba they emerge mid to late May, or even early June. Remember, the goldenrod stems have to have emerged first. Adult gall flies are small (about 5 mm), clumsy and are poor fliers. (They have difficulty righting themselves if they fall on their backs!) They do most of their traveling by walking.

Adult flies only live about 2 weeks, during which time they mate and the females lay their eggs. They deposit them at the tip of the emerging goldenrod stems, and they are pretty choosy about which goldenrod species they use. Of the many different kinds of goldenrods found in Manitoba, the flies are known to use only two, graceful goldenrod (Solidago canadensis) and late goldenrod (Solidago gigantea). The former of these two, graceful goldenrod, is by far the most common host for the gall flies in Manitoba.

The female may lay several eggs per stem, but each stem tends to end up with only one larva in one gall. In about 10 days the eggs hatch and the larva burrows down into the plant stem. The larva's chewing and the action of its saliva, which is thought to mimic plant hormones, results in the production of the galls which provide the larva with both food and protection. There they feed and grow, passing through 2 larval stages. The 3rd stage larva reaches its full size by late summer; this is the stage that will over-winter and is freeze tolerant. One of the last things the larva does is to excavate the exit tunnel that it will use to escape from the gall as an adult the following spring. The larva scrapes out a tunnel from its central chamber right to the edge of the outer wall of the gall, leaving only the plant epidermis (skin-like layer) remaining. It doesn't eat the material it scrapes out, which accounts for the debris usually found within the central chamber of the gall.

Graceful goldenrod usually grows in thick patches, so it's easy to find when its in flower in August and early September. Look for galls in autumn. You usually find lots of galls together. Gall flies don't fly very far from where they emerged.

Late autumn's cold temperatures stimulates the fly larva to produce large concentrations of glycerol, as an intra-cellular anti-freeze. The larva allows most of its body tissues to freeze, but keeps the inside of its cells liquid. And there it remains, inside the gall, throughout winter, which can mean experiencing temperatures of -40 C in Manitoba. For 9 months it remains as an inactive grub. Then, with the warmer temperatures of spring the larva is stimulated to transform into a pupa, where the final transformation to winged adult takes place. (I've found pupae in galls as early as late April.) The pupal stage lasts about 2 weeks, then the adult fly emerges and crawls out to the end of the previously excavated exit tunnel. Here it anchors itself and pumps body fluids into a special portion of its head. This swelling "balloon" bursts the outer "skin" of the gall and the fly pulls itself out. It will rest and allow its wings to inflate and dry for some time before heading off to find a mate and start the cycle over again.

Perils to Gall Flies

Even inside their tough galls the larvae of gall flies are not completely safe. In nature, where there's a resource to be had, some critter will find a way. Gall fly larvae are subjected to attack from several insect species, including some small species of wasp. The female wasps are able to penetrate the galls with a long ovipositor (egg-laying tube) and they lay eggs right in the chamber or on the gall fly larva. When the wasp larvae hatch they proceed to devour the gall fly larva. Even larvae that avoid insect predators and make it into the cold of winter are not completely safe. Small birds such as chickadees and downy woodpeckers can peck into the galls to extract the tasty and energy rich tid-bit the larva represents.

Getting and Storing Goldenrod Galls

You can find graceful goldenrod plants wherever there are unmown grassland areas or forest edges. They are among the most aggressive of our native plants and are quick to colonize disturbed or neglected areas. And just about anywhere you find graceful goldenrod plants you will find galls. If you're up for an adventure and want to really bring home the reality of what these little insects face, you could arrange a field trip to collect some galls in the middle of winter! Stake out a site in the fall or scout one out in winter where you know there are some galls and go collect them on a brisk, sub-zero day. (You could combine this with a "tracking" field trip! See "Makin' Tracks", this issue.) However, if you want to be assured of getting some goldenrod galls, then it's best to try and find them, and even to collect them, in fall before the snow flies. As the galls are usually only 30-50 cm above the ground, they may be covered by snow in late winter.

Collect a good number of the galls to ensure that you get a number of healthy larvae, some may be parasitized or damaged. But, for conservation, don't take more than 10% of the galls you find in one area. Store the galls outdoors in a cloth bag or some other container that will prevent them getting moist, to prevent fungal damage. And be sure to put the bag in something that will keep the galls safe from mice or squirrels, like a garbage can. Or you can store them indoors in a freezer, provided they've already been exposed to sub-zero temperatures in the wild.

Late February is the earliest you should bring the galls into the class room, if you want to have the larvae pupate and transform to adults. The larvae need a prolonged exposure to cold before they will proceed to the pupal stage. If you want to release the adult flies you should wait until they would normally pupate and emerge, probably in late May. You could then release any adult flies back to where you found them. If you just want to demonstrate the sub-zero survival of the larvae, then you can extract them from the galls anytime during winter. If you're not going to keep the larvae to allow them to transform to adults, consider putting them to good use, perhaps as food for aquarium fish or put them on a bird feeder as a tasty snack.

Extracting the Larvae

To extract a larva from a gall:

  1. Hold the gall firmly in your hand.

  2. Push the flat blade of a knife into the gall along the stem axis (parallel to the stem), inserting it no more than 1 cm.

  3. Twist the knife blade sharply until the gall splits open.

  4. Examine the cavity for a plump, cream-coloured larva. Carefully pluck the larva from the gall, using the tip of the knife to gently pry it out, if necessary.

  5. If the gall splits unevenly and you can't find the cavity or a larva, try splitting it again or try another gall, not every gall will contain a healthy larva, many die or get parasitized by other insects.

Demonstrating Freezing and Thawing

There are a couple of ways that you could demonstrate the remarkable tolerance of the gall fly larvae to sub-zero cold. The first would be to bring in a gall on a particularly cold late winter day, either from your collection kept outside or directly on arriving back from a collecting field trip, and immediately crack it open to reveal the cold hard larva within. I've been told they bounce well when they are frozen! Then watch as it warms and begins to squirm around in just a few minutes!

The alternative, or as an extra step, is to pop out some larvae at room temperature, watch them squirming around for a bit, then put them into a freezer for a couple of hours. Remove them when they are "frozen" again and watch them warm up and start squirming all over. Either way, it's amazing to watch something "come alive" out of a deep freeze.

For you chemistry types out there, you may want to draw comparisons to car radiator anti-freeze and it's effects on the freezing point of water. The most common chemical in anti-freeze, ethylene glycol, is a close "relative" to glycerol, the gall fly larva's cryoprotectant (anti-freeze). Consider setting up an experiment to determine the concentration of car anti-freeze needed to maintain water as liquid down to varying temperatures. You could place a series of test tubes with varying concentrations of anti-freeze in an outdoor setting and observe which concentration protects against freezing at various temperatures.

Thanks for learning about the Goldenrod Gall Fly! Bye for now!

Some other bug features you might enjoy:

Silk Moths | Giant Water Bug | Dragonflies of Manitoba


Jan Storey, author of "Frozen Alive" (in this issue) provided a lot of the background information and some images for this article.

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