Tuesday, March 10, 2009

Bugging out: Mayflies will soon spring into action

March may seem a little early in the spring to talk about mayflies, but they are currently active on the bottom of virtually all of our streams, rivers and lakes.

Resident trout and whitefish here in Alaska use the many species of mayflies for food all year long. Because of this, mayflies are well-known to fly-fishermen, since they are the model for a large number of dry and nymph fly patterns.

It is also well-known that nymph patterns attract fish more frequently than dry flies. This is easily understandable since most mayfly species spend almost the entire year underwater in a nymphal form, and only a day or two as an aerial insect. The mayfly order name, Ephemeroptera, comes from the Greek “ephemeros,” which is a referral to the short-lived adult stage. While many mayflies can survive a couple days as an adult, some members of the group have adult stages lasting less than two hours. During that short aerial life stage, they have to molt once, find a mate, migrate to a water body and lay eggs. Talk about having a lot to do in a short time.

Most mayfly nymphs, or naiads, feed on fine particulate detritus or algae in the water. Some filter the tiny food particles out of the water column with hairs on their legs while others scrape the surface of rocks and gravel. Some species have brushlike mouth parts to scrub algae and detritus from almost any submerged substrate. The grazing of mayflies has been shown to significantly reduce and actually control the algal cover on submerged rocks.

With a very short time to find a mate and reproduce, most mayfly species choreograph their emergence so everyone emerges together. When the proper light and temperatures cues occur, the entire population can emerge in a matter of a few days.

My father used to tell of using snow shovels in the 1930s to clear walks and roads in July after the Hexagenia mayflies emerged en masse from the Mississippi River near La Crosse, Wis. Several years ago there was such a large emergence of these same mayflies from Lake Erie that the swarm was picked up on radar and were thought to be unidentified airplanes in the area.

Once the females have mated, they head to a stream, river or lake and fly just above the water surface to drop small packets of eggs. The flying female will often fly up and down over the water, each time touching her abdomen to the water surface and releasing a few more eggs. This behavior will go on until all her eggs, 500 to 3,000, are released. Incidentally, this is when trout rise and start taking insects off the surface of the water or start jumping out of the water after aerial insects. Because the female mayfly is carrying masses of energy-rich eggs, it is a sought-after meal by many fish species.

Mayfly eggs immediately start to absorb water once they are deposited. They become sticky and end up bound to rocks, leaves, twigs or vegetation. Depending on the species, they may hatch right away or possibly remain in diapauses for several months.

Depending on the species and the temperature regimes, some species can produce several generations a year. In colder climates like here in Alaska, most mayflies have only one generation per year. Sometimes, in cold years or very old habitats, it may take two years to complete one generation.

In the fast-moving current of streams, mayflies use a couple strategies to keep from being washed downstream.

Since currents are much reduced right up against a rock, one approach is to have their thorax, head and legs flattened so they can remain very close to the substrate. Others have modified thoracic gills that act like a giant suction cup, which then holds them against the solid substrate. Yet another group of mayflies use a disk of fine hairs on their abdomen that can also act like a suction cup to hold the insect in place in spite of the current. Incidentally, there are several species using this approach in the Kenai River.

Another survival approach for these aquatic insects is being streamlined or torpedo-shaped.

This shape allows fast-moving water to pass by the mayflies without washing them downstream. By wriggling their abdomen they can swim much like a fish and move about in fast-moving water. This same technique enables mayflies in lakes to quickly move to hiding places and escape predators.

Mayflies are one of the most common aquatic insect orders, and they are found in fresh water all over the world. Apparently their many adaptations for survival in different habitats have enabled them to survive for millions of years. They are among the oldest of the insects and date back to the Carboniferous period.

David Wartinbee, Ph.D, J.D., is a biology professor at Kenai Peninsula College’s Kenai River Campus. He is writing a series of columns on the ecology of the Kenai River watershed.

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