The busy lives of aquatic insects in autumn
By: Mike Miller, Stream Ecologist, Wisconsin Department of Natural Resources
(Photos by Jessica Orlofske)
Autumnal equinox on September 22nd marks the day when the sun tracks directly along the equator and daylight and darkness are of equal duration. We become more aware of cooling temperatures and shortening daylengths, having already lost more than two hours of daylight relative to the long days of July. In the terrestrial world, hummingbirds are heading south, monarch butterflies are taking advantage of late blooming fall asters, and ground squirrels have already gone to bed for a long winter’s rest.
Seasonal changes are occurring in and along Wisconsin’s streams and rivers as well. Green darner dragonflies (pictured) begin southerly migrations, flying upwards of 90 miles per day and often following the same Lake Michigan and Mississippi flyways as waterfowl. While these adult dragonflies will never again return to Wisconsin, their offspring will.
Other adult aquatic insects are similarly on the move but often to more local habitats. True bugs (Order Hemiptera to give them their proper name) such as giant water bugs (pictured) and water boatmen will fly to larger rivers and lakes to overwinter in shoreland detritus.
Some of the aquatic insect species that have two generations per year (bivoltinism), such as some caddisflies and mayflies, see their second broods emerging to mate and lay eggs, well past the more common emergences of spring and summer.
Leaf off is a key seasonal event for streams. Small headwater streams tend to be more shaded than larger streams and rivers and rely more heavily on leaf material falling or washing into streams as a primary energy source, relative to the greater amount of sunlight that energizes wider streams and rivers. When leaves end up in streams, tannins, alkaloids, and other chemical compounds that ward-off terrestrial insects are quickly washed out of the leaves, and soon the leaves become covered with a biofilm comprised of diatoms and other types of algae, becoming a nutritious food source for larval insects and other aquatic invertebrates. By analogy, this food source is like us eating crackers spread with peanut butter; while the invertebrates derive some nutrition from the leaves, the biofilm provides the bulk of the nutrition.
The leaf litter energy source persists long into the following year, being wedged between rocks, or caught in tree branches overhanging into the water. Volunteer stream monitors routinely find the skeletal remains (leaves with little left but their venation) from the previous autumn when prospecting for invertebrate specimens well into summer and fall. Certain insect larvae (e.g., craneflies, some stoneflies, mayflies, others) are known as “shredders” as they primarily feed on this coarse leaf material. Being sloppy eaters, much of the shredded leaf matter washes downstream and provides a key food source for “collector” insects like the ubiquitous net-spinning caddisflies (family Hydropsychidae).
As we progress towards winter, many people believe that Wisconsin streams “shut-down” for the winter, when in fact this is quite untrue. Those intrepid enough (and ideally having neoprene waders and gloves) to collect invertebrates in the winter months will quickly see that streams are alive and well, with most of the same invertebrate animals found in summer being quite active in winter as well. Wisconsin’s groundwater flows out of springs, seeps, and the streambeds themselves, at approximately 52º F year-round, preventing many “spring creeks” from freezing over. While the water cools as it flows downstream, it does allow these streams to remain relatively warm compared to the ambient air temperatures and support an active community of stream invertebrates.
With perhaps fewer time commitments and distractions of summer, the fall season (and for hardy souls, winter) is an interesting and invigorating time to explore the flowing waters of the state.