Last weekend I took a walk down Ashlock Hollow, a 3.3-mile fire road at the top of Bernheim’s Forest Hill Drive. I couldn’t help but notice the abundance of insect galls scattered among the beautiful fall leaves. A gall is an unusual growth on a plant caused by something in its environment. This can be caused by all kinds of things that might bring injury to the plant, but most often, when you see a gall, there is a tiny insect larva inside waiting to make its way into the world. There are around 1,500 species of insects that make their homes in plants in North America. Most of these insects are wasps or midges.
In late spring, the adult female deposits her eggs into the plant—this can be in the leaves, stems, bark, or even in buds. When the eggs hatch, the larvae feed on the plant’s tissues. As they feed, the plant releases an excess of growth hormone, cells quickly divide, and a gall forms around the larvae.
Once fall arrives, the growing larvae slow down as well all do, to prepare for overwintering. Like many insects, they produce concentrated levels of glycerol which protects the insects from freezing in the cold weather. Once temperatures warm up, the larvae begin to get active again and eat until they are ready to emerge in their adult form.
For some insects, this process can take years to occur. They stay, resting in the tiny gall, until they are fully developed. Galls can be found almost anywhere. I see them at Bernheim, certainly, but I also notice them around my Louisville neighborhood, and other places in the city. Anywhere there are plants, there are insects. You might think the insect is harming the plant by feeding on it and forcing the formation of the gall, but while it may not always look the nicest to some, most often it is not harmful to a healthy plant. Millions of years of evolution have offered insects this relationship with plants and some of them are quite beautiful or even cute, I think. Most of the insects that make galls rely on a single plant species to lay their eggs and are unable to live without that habitat provided. These specialists face increasing difficulties as native plants become less available to them. Many types of insects rely on specific plants like this, not just wasps and midges making galls. I think of the Spicebush Swallowtail Butterfly whose caterpillar can only feed on the spicebush plant or sassafras. If fall leaves are swept up and put out for the garbage truck, there goes with it many overwintering insects, including galls with larvae inside. As we become more estranged from the natural world and stop planting things that support diversity, there goes many species of all kinds of organisms.
Galls are also an important food source for other animals, like our beloved birds. They also provide a new home for other insects once the adult finally emerges. Many bird species rely on all sorts of insects swept up in “unsightly” fall debris or lost to the decline of native plant species and habitat availability. The decline of one thing always has an impact on another, that is the nature of ecology.
Though it may seem impossible to keep up with the changes the world is currently facing, there are lots of ways we as individuals can make big positive impact on the environment. It starts by first noticing the world around you and developing a relationship with the place you’re in. Ask questions, get curious. Then, by planting just a few native plants, you are improving the function of your local ecosystem. By leaving fallen leaves and dead stems you are allowing insects to stay safe in their homes over winter and mature to adulthood and providing food for many others. Just because we are not used to living this way, doesn’t mean we can’t. Don’t have any property? Volunteering at your local nature center (or places like Bernheim) or taking time to spread the word to a friend or neighbor and helping them with their yard is a good place to start.
Change is a natural part of life, the only constant, some say. Organisms all over the world are being forced to adapt to new ways of living due to human impact on the environment and have been for a long time. It’s time that we find new ways to adapt and develop a new understanding of what is beautiful. Not always just the lush green lawn or wide empty space. Sometimes a hidden gall on a plant we know, or the dead leaves that work all winder to provide newness to soil and safety to the organisms that live there (even salamanders use leaflitter).
Can we usher in a new understanding of the beauty of ecology, our relationship to nature, life cycles, insect galls, fallen leaves acting as homes for baby insects and more?
I always enjoy finding galls when on a walk or hiking on my favorite trails. Some have bursts of fall colors; others are deceptively hard to find. It can be a fun game and a true gift to be in on one of nature’s many secrets. Not so secret at all if you take the time to stop, notice, and ask it questions.
-Hanah Carter, Volunteer Naturalist
Doug Tallamy, PhD: discussion on how we can support ecology in our backyard/community
Article on insect gall diversity around the world
Fun personal blog on spicebush swallowtail, “If you build it they will come”
Information on insects’ “anti-freeze”
Article on salamanders and leaflitter
Figure 3: Ryan Hodnett via Finger Lakes Land Trust
Figure 4: Hilton Pond Center for Piedmont Natural History
Figure 5: Naturally Curious with Mary Holland