Goldenrods can be quite abundant in fields along roads and in forest clearings. They are rather non-descript, weedy looking plants through the spring and early summer, but before the onset of fall, they explode in vibrant, luminous yellows that give them their name. Each flower is tiny but they are abundant and massed together at the tip of branches creating an impressive display.
Another season in which this plant may arouse the curiosity of passers bye is winter; quite often their tall, dry canes sport some odd looking round thickenings, half way along the stems. These balls are about an inch in diameter and they are very tough. You may puzzle about them; evidently they are not fruits, their placement is quite strange but they are most definitely part of the plant and not something that has been glued to it.
They are tumors called galls and they are provoked by some creatures, the gall makers. In addition to the highly visible round galls there are other types. Galls come in a wide array of shapes and sizes and location in the plant. Many different plants present galls of one sort or another; goldenrods and oaks seem to be more prone to this sort of attacks than most other plants.
How are these tumors produced? Do they serve any purpose? What causes them? Only recently have scientist began to crack some of these mysteries. There are many different agents that can cause these tumors; most of them are insects, but also mites, nematode worms, fungi and bacteria can produce them. Fortunately, in most cases, they don’t seem to have serious effects on the plants they use as hosts; other than being somewhat unsightly in some cases. The ones that affect your rose bushes are not going to make you very happy.
Galls are very specific kinds of tumors; unlike cancer they don’t grow in an uncontrolled way, instead they have a very definite structure. This structure serves the purposes of the gall maker not those of the plant. So, galls will be different depending on the organism that produces them. In fact one species of plants may carry several different types of galls, each one caused by a different organism and each one with a very characteristic shape, size and location in the plant.
The remarkable thing about gall makers is that they are capable of hijacking, taking control of, the genetic machinery of the plant and forcing it to grow tissues and an entire organ unlike anything else on that plant. This highly organized structure or organ, called a gall, provides shelter and food for the gall maker or its progeny.
Goldenrod round gall, early summer
Getting back to goldenrod’s round gall; it is caused by a small fly known by some as the peacock fly, although this name is confusing because it also applies to an unrelated kind of fly. Therefore, the best name for it is goldenrod gall fly. Early in the spring, when goldenrods are growing, female flies pick their target, the stems of growing goldenrods.
The fly pricks the stem of the growing plant with the egg laying organ, also called an ovipositor, depositing an egg inside the tender stem. Soon a tiny larva hatches and starts feeding on the tender tissues of the inside of the stem. When doing so a chemical in its saliva starts acting on the plant’s tissues altering the way they grow and forcing them to develop an unusual structure.
Goldenrod fly freshly emerged from a gall in early spring
In this manner the growing larva, or maggot, builds a home for itself and a source of food. The thick, tough walls of the gall provide an excellent shelter and the juicy tissues provide nourishment. The center of the gall is a round chamber where the insect fits snuggly. It can remain there feeling safe and contented; all its needs are taken care of until the next spring. It will grow through the summer, sleep through the winter and then emerge fully grown and transformed into a winged adult. The fully grown maggot is no bigger than a grain of rice; the adult is just like a winged grain of rice. Before going through metamorphosis the larva has one final task; it chews a tunnel all the way to the surface of the gall and stops just before breaking the skin. It has to do that because the adult fly does not have teeth and would be trapped inside otherwise.
Fly larva inside its gall in the middle of winter
Mysterious as this process is, it is not the whole story of goldenrods galls and their flies. A gall can be a universe within itself. Despite the protection provided by the sturdy walls of the gall, the larva isn’t as safe as it would seem. Several enemies have found the way to penetrate the defenses and make a meal out of them. Some raging battles take place inside the seemingly peaceful round balls. There are at least three kinds of parasites that can invade the sanctuary and take possession of the helpless maggot. Two of them are called parasitic wasps, black in color and even smaller than the fly. Another parasite is a beetle belonging to a kind called tumbling flower beetles which also attacks the galls. There are probably others, but with so much to learn, scientists are still studying the universe of the goldenrod round galls.
Often more than half of the galls that you see in a field of goldenrods contains, not a fly, but one of its parasites. Is this all? Oh, no! In winter black-capped chickadees and downy woodpeckers, knowing that goldenrod galls contain a small morsel of food, go after them when sufficiently hungry. Next time that you walk through a field of goldenrods in winter look for galls and you will notice the ones that have been cracked open by these birds. With a good eye you could even figure out which have been visited by one or the other of these two birds. Woodpeckers use their strong and sharp chiseled bills with precision, making a clean hole while chickadees, with their smaller bills, need to keep picking at the gall, chipping away here and there, until they finally reach the core; thus their holes are very sloppy by comparison. Both birds show a preference for larger galls; they seem to know that the smaller ones are likely to contain one of the smaller parasites rather than the fat and larger fly maggot.
Gall opened by a chickadee next to whole galls, collected in winter
This is the story of just one type of goldenrod galls, the one you are most likely to notice when seeing a goldenrod field. But as I said there is a fantastic gall fauna in goldenrods. There are at least fifty different kinds of gall makers, each one with its whole intricate story of chemical manipulation, delicate timing, enemies and intrigue. Several types of flies and midges and some small moths are responsible for these galls. Each builds a unique type of gall and has its unique life style, rhythm of life, assortment of enemies and location of the gall. In summary if you count all the types of gall makers and all the parasites or predators you realize that it is a real zoo, with a great variety of species.
One wonders how the goldenrod thrives despite the attacks from such a wide range of gall makers. Fortunately for the plant these gall makers have many enemies which don’t allow the populations to build up to dangerous levels. It is also fortunate for the downy woodpeckers and black-capped chickadees that these gall flies exist and provide them with much needed food in the dead of winter. And we are lucky too because we get to enjoy the beauty of blooming goldenrods in the fall and those of us who are bird watchers get to see the chickadees and woodpeckers year after year.
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