by Alisha Davidson, Ph.D.
ML&SA Research and Development Coordinator
Most winter stoneflies are about 1 cm long. They emerge in the winter and are found walking around in the snow. Photo credit: Dysmorodrepanis, Wikimedia Commons.
While riparians are used to seeing the emergence of various insects during the spring and summer months, the winter months are known for their stillness and quiet. Life takes a break, and apart from the occasional indoor ant, few insects are seen during this time. One important exception, however, is the winter stonefly. During the coldest winter months, winter stoneflies’ aquatic immature stage (known as larvae or nymphs) hatch from rocky stream bottoms and crawl up through openings in the snow or ice that covers the water. By the time they have emerged from the water, they are fully formed adults about 1 cm long. Although they have four wings, they stay close to the snow and ice, and walk to find mates. Winter stoneflies should not be confused with snow fleas. Snow fleas (which are not fleas, but small insects that munch on decomposing organic matter like leaves) are only about 1 mm long and can seemingly cover the snow, hopping around as they do. Winter stoneflies are much less common.
Little is known about how winter stoneflies survive both the freezing water and air temperatures. However, one hypothesis is that overall, insects are either freeze-tolerant (i.e., the liquid in their body can partially freeze without killing them) or freeze-avoidant (i.e., they produce anti-freeze compounds that allow the liquid in their body to get below freezing temperatures without freezing). The physiology of these processes is extraordinarily complex, but in general, because air temperatures generally get much colder during winter than water temperatures, most terrestrial bugs that are exposed to the frigid air temperatures are freeze-tolerant and most aquatic bugs (like winter stoneflies) that are exposed to more moderate water temperatures are freeze-avoidant. Because winter stoneflies keep close to the water and ice (which acts to insulate the water), they do not experience temperatures much below freezing and can avoid freezing. As they are exposed to air, cryobiologist Dr. Richard Lee and his research team from University of Miami – Ohio found that they can increase the levels of anti-freeze in their bodies to avoid freezing! In addition, they seek out protection under snow pockets (which are slightly warmer) and walk on the tips of their feet to avoid ice crystals. Why go through all this trouble? Emerging to mate during the winter allows them to avoid predators that are plentiful during the summer.
Winter stoneflies are clearly a wonderful winter phenomena. What makes them even more special is their role in determining water quality and predicting the health of stream communities. Winter stoneflies have less efficient respiration than other aquatic insects, so require high rates of dissolved oxygen. Dissolved oxygen levels decrease with warmer water temperatures, slow water flow and nutrient pollution. Because they are so sensitive to poor water quality, monitoring where they are, and how many there are, helps determine the health of a stream. While many humans welcome warmer winter temperatures, warm temperatures may negatively affect winter stoneflies (as well as other members of the stream community). Winter stoneflies also have relatively low rates of reproduction and a poor ability to disperse to find better habitats with high dissolved oxygen. As such, they are at risk in areas with nutrient pollution and warmer waters.
To understand winter stonefly numbers and also predict stream health, there are several watershed organizations that hold a winter stonefly search (e.g., Huron River Watershed Council, http://www.hrwc.org/volunteer/stonefly/; Friends of the Rouge, http://www.therouge.org/our-work/volunteer/benthic-macroinvertebrate-sampling/bug-hunt-events-and-trainings). No experience needed; most groups welcome volunteers, including children accompanied by an adult. Volunteers are paired with an experienced researcher to look for and learn more about these fascinating riparian inhabitants. For those that are interested in helping in the search, try contacting your local watershed organization. Most searches occur in January, with an early-mid January registration deadline.