Mayflies and midges: A study in contrast

By John Hageman
Contributing Writer

Sandusky, Ohio — Each spring and summer, those living or visiting the Lake Erie shoreline deal with a large number of annoying “bugs.” All are eaten by fish, birds, and other animals while available in their nymph, larval, and/or adult life stages.

When aquatic insects leave the lake, it is often erroneously called a “hatch,” when in fact, it is actually an “emergence.” The hatch occurs when a larva or naiad first exits its egg.

Although residents usually learn to ignore the insects during the short periods of peak abundance, visitors to the lakeshore are often seen panicking while swatting at the harmless midges and mayflies that appear each season.

Midges are families of insects that include biting and non-biting varieties. Travelers to the North Country have to deal with “midgies” or tiny “no-see-ums” that deliver painful bites and lingering welts.

Fortunately, the larger midges most often encountered on Lake Erie are non-biting varieties, locally known as muffleheads. About the size and shape of a mosquito, they differ by having a pair of large, fluffy antennae and no proboscis to draw blood through.

Their bright red larvae, commonly called “bloodworms,” contain hemoglobin in their blood, which allows them to tolerate low dissolved oxygen conditions that commonly occur in the soft mud that they inhabit. They contribute significantly to the diet of important fish species, notably yellow perch.

Midges become extremely numerous in sediments that do not support the insect larvae that require higher dissolved oxygen levels. Unfortunately, that includes most of the lake’s bottom.

Their swarms can be so thick that they may look like a tire or an oil fire burning along the shoreline treetops some nights at dusk, especially in May around the islands and later across the rest of the lake’s shoreline. They may also be flushed in distracting numbers when walking through grass or past buildings on mornings following a big emergence.

Midges sometimes continue to emerge into the fall.

Mayflies can be recognized by their large, lacy wings, long cerci (tail spines), and slow, deliberate flight. The larger ones seen here each summer are burrowing species, with other smaller species living upon shallow rocks. They can persist only when there are adequate year-round dissolved oxygen levels at the water/mud interface.

Locally, they are also called Canadian soldiers and fish flies. Their populations started to recover in the mid-1990s and scientists celebrated their return as further evidence that the lake had truly recovered.

Unfortunately, high dissolved phosphorus levels have reversed the lake’s short-lived recovery once again, causing a decline in larval mayfly densities. Massive algae blooms lead to oxygen depletion and dead zones as decomposer bacteria feed on dead cells at the bottom of the lake.

Wherever mayflies thrive in good numbers, yellow perch and other fish regularly consume the larva from the mud. Walleyes, white bass, and smallmouth bass concentrate on them as they emerge from the mud and rise to launch themselves from the water’s surface.

When the weak-flying insect emerges from the water as a sub-adult, they are often steered according to the prevailing wind direction to the shoreline or nearby islands. Their numbers in June and July can be staggering enough to even display on Doppler weather radar, resembling densities similar to a moderate rainfall.

Their swarms begin to show up on the weather radar in early to mid-June, then progressively chronologically appear eastward as lake surface water temperatures climb in that direction throughout July. Stragglers can be seen as late as October.

The populations of mayflies diminish east of the islands, due to the dead zones that often occur in the deeper water of the Central Basin.

After spending a day above water, typically perched off the ground, they shed their skins one more time to reach the adult stage. In the meantime, birds and other animals fill up on this readily available source of protein. There are no skinny birds, fish, spiders, or raccoons during mayfly season.

At dusk, a well-recognized male courtship dance, consisting of a repetitive short flight up followed by fluttering down begins. If he gets lucky, he will grab a female that gets too close and mate, before she goes to the surface of the water to release the fertilized eggs and dies.

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