September in Lake County, Illinois is a month of big sky punctuated by tips of tall prairie plants in an array of autumnal colors. Before the trees really get going with their own colorful show, sparks of bright yellow from the many varieties of goldenrod (Solidago spp.) dominate the open spaces. Most of the summer these plants go unnoticed, adding merely another green hue to the lush surroundings, but September is their time to shine. What may also go unnoticed, even now as goldenrod demands our attention, is the hidden world inside each plant in the form of a gall.

Look closely at a single goldenrod plant and you will likely notice lumps along the stem or bunches of leaves near the top. While these features are seemingly part of the plant, they are actually abnormal growths created by tiny insects that are providing shelter and food for their offspring over the winter. Known collectively as galls, these bumps and bunches are found on a variety of plants, from large oak trees to small grasses. Scientists have found fossil galls dating back to the Cretaceous Period, an era when flowering plants were first appearing. Galls come in myriad forms, colors and sizes—each caused by a distinctive insect, mite, fungus, bacteria or virus.

The irritation or injection of chemicals by an insect as it lays eggs in a plant is typically the cause of a gall. Once the the eggs hatch, the larvae burrow into the plant’s tissues. Saliva secreted by the larvae as they feed can also cause swelling in the plant and formation of a gall. The gall is not a fruit, although, it does contain abundant nutrients and provides a safe abode as the insects within undergo metamorphosis. While the presence of galls may appear harmful, the problems associated with them are mostly aesthetic. Galls rarely cause long term damage to plants.

The two types of galls pictured here are commonly found on goldenrod throughout Lake County.

What seems to be a green flower at the top of the plant is actually a bunch gall created by a goldenrod bunch gall midge (Rhopalamyia solidaginis). This tiny fly lays its eggs at the tip of the goldenrod, thwarting any future growth of the plant’s stem. The leaves, however, continue to sprout and bunch together, making a whorled rosette. Scientists call these midges “ecosystem engineers” due to the fact that bunch galls can increase biological diversity of insects and spiders by creating a niche habitat within the tightly bunched leaves.

Further down the same stem in the above photo are two spherical bumps created by a goldenrod gall fly (Eurosta solidaginis). These ball galls began forming last spring when the female fly deposited eggs on the tender shoot of the plant. Over the summer, as the tiny larvae within ate and grew, the galls grew as well, swelling to about an inch in diameter. This fall, each plump larva will chew a tunnel up to the gall wall, leaving a thin skin of plant tissue as a “door.” This tunnel will act as an escape route come spring when the fly has pupated and emerges as an adult. That is, if it makes it that long.

Opportunistic animals, such as downy woodpeckers and chickadees, looking for food during winter (and ice fishermen looking for bait) have figured out that there is a sleeping snack inside each gall.

This autumn, take part in our Hike Lake County Challenge or join a naturalist for a Walk on the Wildflower Side and search the seas of wildflowers for a glimpse at the hidden life inside a goldenrod gall at your Lake County Forest Preserves.