Snow Crawlers

Walking along a snow-covered stream bank in Camden State Park on a sunny February afternoon, I hear snow crunching under my feet and the gurgle of the cool, clear water as it travels over rocks and branches. It’s a picture-perfect winter day. In the distance, several deer bound up a snow-covered hillside, and woodpeckers call and drum in nearby trees. As I slowly traverse the bank, I spot several small, dark specks on the otherwise pure-white blanket of snow flanking the stream channel. One of the specks appears to move, then another. As a biologist at Southwest Minnesota State University who researches and teaches about aquatic invertebrates, I have a hunch I know what they are. Moving closer, I see a speck lift off the snow in flight, confirming that these are in fact insects that recently emerged, or left the stream, as active adult flies.

It may be surprising to learn that insects, both aquatic and terrestrial in origin, are an active and thriving component of certain winter ecosystems in Minnesota and elsewhere. And while research into these species has historically been sparse, we scientists who study them are learning fascinating things as we delve deeper into their lives. New research is turning up clues about their adaptations for life in chilly conditions, and a University of Minnesota–based research team called Bugs Below Zero is raising awareness about winter aquatic insects and their important role in stream food webs.

Like lakes, most surface-water-fed streams in Minnesota are covered by ice during winter, prohibiting insect emergence. However, spring-fed streams, fed by temperature-moderating groundwater, often remain largely ice-free throughout winter, allowing cold-adapted insect species to emerge and transition from the aquatic to the terrestrial world. The Driftless area of southeastern Minnesota is home to 700 miles of these streams. However, only a few such streams, like the one I visited in Camden State Park, exist in the southwestern part of the state. Similar winter insect communities frequent spring-fed streams in both regions.

Some insects, including species of non-biting midges, stoneflies, mayflies, crane flies, and others, complete their life cycle only during winter. These insects don’t just survive during winter, but they thrive, actively growing and developing in the stream as larvae or nymphs and eventually emerging as adults that walk on the snow, fly short distances, mate, and return to the stream to lay eggs. On sunny winter days with temperatures around freezing, hundreds of these insects can be observed on the snow along a stream bank in just a short period of time.

“People are largely unaware of winter insect activity and mistake them for debris on the snow,” says Southwest Minnesota State University biology major and student researcher Claudia Lensing. “However, these insects have developed incredible adaptations that allow them to survive in harsh Midwest winters.”

Winter active insects and other arthropods are specially adapted to the cold temperatures. This holds true for both those that are aquatic in origin, like the ones I study, as well as their terrestrially originating relatives that may be found far away from water sources, such as certain fly and spider species. The bodies of winter active insects often harbor antifreeze proteins, sugars, and other substances that act as cryoprotectants, allowing them to safely survive subfreezing temperatures.

Some species can tolerate freezing, while others can supercool, meaning they can depress the temperature at which they freeze. For example, the midge species Diamesa mendotae—one of the most common winter active insects in Minnesota—has a supercooling point of about -5 degrees Fahrenheit. This means its body won’t freeze, despite below-freezing temperatures, until the temperature drops to 5 below. Winter stoneflies in the genus Allocapnia can also supercool but are not as tolerant of the cold as Diamesa, freezing instead at around 10 degrees.

Recent research also shows that winter temperatures play a big role in the life history of winter active species—in short, colder is better, to an extent. Over the past few years, I’ve worked with several undergraduate student researchers at Southwest Minnesota State University who have collected winter active stoneflies and midges to better understand how temperature affects their longevity. We have found that both midges and stoneflies live significantly longer at or even below freezing as compared to slightly warmer temperatures.

“Studies of the winter invertebrate community can help us better understand impacts of climate change,” says Madi Foutz, an SMSU environmental science major and student researcher. If these species are unable to adapt to a warming environment and populations are not as abundant or are nonexistent, there could be cascading food chain implications.

Not only are winter active insects super cool (pun intended!), they are also super important in stream ecosystems. Research shows that winter active aquatic insects can be a key component of the diet of stream trout and other fish.

“Over 50 percent of insects drifting in a stream’s water column could be winter active midges,” says Bruce Vondracek, professor emeritus of the University of Minnesota’s Department of Fisheries, Wildlife and Conservation Biology. Young trout are more susceptible to starvation during winter and are especially reliant on these winter active species.

“Winter active insects provide an important food source to sustain small fish through this lean time,” says Minnesota Sea Grant extension program leader and fisheries and aquaculture extension educator Amy Schrank.

Bugs Below Zero, an interdisciplinary research team of scientists from around the state that is funded by Minnesota’s Environment and Natural Resources Trust Fund and housed at the University of Minnesota, is working to get the word out about the importance of winter active insects and to learn more about their emergence patterns and life histories, including how changing temperatures might affect both the insects and stream food webs.

“Bugs Below Zero’s events, educational resources, and hands-on data collection project are open to everyone,” says University of Minnesota associate professor Rebecca Swenson. “We hope that public science programs like this one encourage volunteers, outdoor recreationists, and classrooms to learn more about our streams, and the Bugs Below Zero project increases overall interest and engagement with Minnesota stewardship efforts.”

Along with an archive of educational information, videos, and activities, Bugs Below Zero also encourages citizen scientists to take to the streams during winter and spend time searching snow-covered stream banks for insects to help scientists better understand the world of winter active aquatic insects and what might happen to these species in a warmer world.

“We need everyone’s help to gather more data on distant streams,” says Corrie Nyquist, former Bugs Below Zero postdoctoral researcher. “Community scientists involved in Bugs Below Zero can contribute to new discoveries and help deepen our knowledge and understanding of winter active aquatic insects, which may be threatened by a warming climate.”

The next time you find yourself near a stream with open water in the winter, pay mind to those specks on the snow. They just might be telling us something.

Bugs Below Zero seeks volunteer community scientists to identify and submit information and photographs of aquatic insects on the snow, often near stream banks. For more information visit bugsbelowzero.com/get-involved-new.