If you enjoy hiking or walking in natural areas in the winter, you may have seen weed stems with dried out ball-shaped growths. These growths are called stem galls.

Goldenrod is the most common weed, or should I say native flowering plant, that you are likely to encounter with stem galls.

During our walk this week we noticed a number of galls on goldenrod stems on our property. We have a fairly large population of goldenrods and encourage them in some areas, mainly because they provide pollen and nectar for bees and other pollinating insects. Plus, in early fall, they add a nice pop of bright yellow color to the landscape.

Goldenrods also provide food for birds in winter, but not in the way we normally think. It is the galls that feed the birds, or to be more exact, it is the insect living inside the gall that provides the food.

Goldenrod galls are produced by a colorful fly called – you guessed it – the goldenrod fly. These flies look like an undersized but colorful house fly.

In the spring, the female lays several eggs on the stems of young, fast-growing goldenrod plants. Once hatched, the tiny fly larvae burrow into the tender goldenrod shoots and begin feeding inside the stem.

The larva’s saliva contains a chemical that mimics certain plant hormones, causing cells in the feeding area to multiply much faster than normal. This over-stimulated growth eventually turns into a gall. Often only one gall is produced per plant, although several eggs are laid along the stem. But on occasion more than one can be found.

Inside the gall, the plant tissue continues to develop into an edible and nutritious mass. The fly larva, or maggot as all fly larvae are called, gnaws at the center of the growth, forming a hollow area where it lives. The exterior of the gall becomes hard with a shell-like covering that provides protection.

As winter approaches, the maggot goes dormant and overwinters inside the gall. Later, in early spring, they pupate and eventually emerge from the gall as an adult fly.

Before sleeping for the winter, however, the maggot creates a small escape tunnel inside the gall. He tunnels to the outer shell and then stops, making sure the tunnel opening is still sealed.

The adult fly has no teeth or any other means of chewing its way through, so the preformed tunnel is essential when the fly finally emerges from the gall.

During the winter, birds, especially downy woodpeckers and chickadees, eat the hibernating larvae.

Lesser Downy Woodpeckers will find the fly tunnel on the gall and carefully open the tunnel exit. It then uses its long tongue to extract the tasty morsel. Since the tits do not have such a long tongue, they are not so delicate, they just tear the gall, tearing off pieces until they reach the maggot.

Another element comes into play. Sometimes the goldenrod fly larvae are parasitized by wasps that lay their eggs inside the unsuspecting larva that is feeding on the gall. When this happens, the maggot fly is killed as it is consumed by the wasp larva, so no exit tunnel is formed and the wasp larva overwinters there instead.

Downy woodpeckers prefer the much larger, tastier fly and sometimes turn their noses up at galls harboring wasps.

Woodpeckers tap on galls with their beaks to determine which galls are worth opening. How they are able to determine which is a fly and which is a wasp is still under debate.

We know woodpeckers have very good hearing and can hear insects moving around under bark or inside wood. Goldenrod maggots, however, do not move during the winter.

I have a theory that woodpeckers, tapping on the gall, can hear the difference in resonance between a gall containing a juicy goldenfly larva and one containing a smaller wasp.

Some humans use goldenrod larvae for food, although indirectly. Ice fishermen harvest the galls and cut them open to use the larvae as bait for fish.

When you come across a gall look closely at the holes, if there is a neat little hole you know a woodpecker has been there before you.

There is so much going on in nature while we are not looking; we only see the remaining evidence.

Happy hiking!