Mayflies







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MAYFLIES

article by Ron Newman


Scientific Name:

  • Class - Insecta. Order - Ephemeroptera. Suborders - Schistonota or Pannota

Common Names:

  • Mayflies, Mays, Upwings, Duns, Spinners, Dippers, Fish Flies


General:

  • On our Interior lakes, Mayflies are not as important a food source for trout as they are in some other regions of the country. Our Mays are generally on the smaller end of the size scale and tend not to hatch in the same quantities as elsewhere in North America. The many types of Mayflies are split into either the 'splitbacks' or the 'fusedbacks' with three super families each. Those are then designated as to whether the nymphs are swimmers, crawlers, burrowers, or clingers and often that is determined by their habitat. With all this variety, the following description is somewhat generic.

Life Cycle:

  • Adults swarm and mating flight. In these swarms the male adult will fly up and down (dippers) while the female flies through the swarm. In this pass-through, a male seizes a female and mating takes place. These mating swarms generally occur at dawn or dusk. Eggs may be laid while the female adult is in flight skimming over the water surface, or some species will submerge below the surface to lay their eggs among the vegetation. The egg hatches into a larva or nymph that will develop while hiding on or under the lake bottom vegetation, and often will burrow into the mud. The nymph will feed on organic matter while it grows. When the nymph reaches maturity it will transform into a 'sub-imago'. The sub-imago is what we see 'hatch' from nymphs. They are not an adult but are fully winged. Before flight the sub-imago will rest while the wings dry. The wings will be held upright and folded over the back. After taking flight, the sub-imago usually rests on the shoreline vegetation for 1 or 2 days while gradually transforming from sub-imago into adult (or imago or spinner). The adult will not feed. After this transformation, the adult will still take one to three weeks to become sexually mature. The sexually mature adult will then mate within a few days and then immediately die. The dead or dying adult will then lie on the waters surface with wings spread and, at that point, is referred to as 'spent'. Most Mayflies have one or two generations per year but this ranges from a couple to weeks to a couple of years.

Appearance:

  • Most of the Mayflies on our interior lakes have two tails but some three tailed species are present. These tails are visible throughout most of the developmental stages. In the adult the tails are often as long as the body. The sub-imago often sits on the water with its wings held upright over the back as they dry. In this position they appear very triangular in shape. After mating and laying their eggs, the adults die fairly quickly and lie in the water with their wings spread out. In most species the nymphs have gills along the sides of the abdomen, which tend to give them a flattened appearance. There are generally 7 to 9 abdominal segments with nine being the most common.

Size:

  • On our interior lakes, Mayflies are up to 20mm long (3/4 inch with tails) but generally average about 10 to 15 mm (0.4 to 0.6 inches with tails). On a few interior lakes, members of the larger species have been reported.

Colour:

  • Colors tend to darken as the life cycle progresses. For our interior species, the most frequently found colors tend to be reddish brown to brown, or to a slate color. However, they are found in a variety of other colors including lime green. The wings of adults are semi-transparent.

Movement:

  • Larva burrow into substrates, crawl or cling to bottom plants and debris, and often swim like a fish by sweeping motions of the abdomen and tail. In swarming flights, the adults often go through "dipping" motions where they seem to drop a ways and then regain altitude.

Habitat:

  • Mayflies require clean, unpolluted water that is generally well oxygenated. They are one of the first aquatic species to disappear when water pollution occurs. The use of gasoline motors on our lake and the resulting pollution keeps the Mayfly population on most of our lakes relatively low. Mayflies live in flowing or still water and occasionally can be found at depths to 200 feet. The nymph feeds on algae, diatoms, and vegetable or organic matter that have settled to the bottom. Since their food is mostly on the bottom the larva are usually found on the bottom. They tend to burrow into the bottom substrates and hide under rocks and vegetation.

Importance to Fly Fishing:

  • In most parts of North America the Mayfly has always been considered as a main or 'the' main staple food source of trout and many other species of fish. I can't speak to historical records, but I know that this is not the case on the interior lakes of British Columbia. Currently they rank as seventh among the seven most important species for fly fishers. Mayflies represent only two percent of the total feeding samples for fish I have caught on interior lakes over the last twenty or so years. However, in terms of 'observed' food they range around 10 to 14% of the feed, which is fairly significant. They are obviously fed on by the trout less often than they are available. Mayflies are taken slightly better in the evening or after dark than they are during the daytime.
  • I'm not sure exactly why but the majority of fish feeding on mayflies, in all stages of their life cycle, tend to be the smaller fish (generally less than a pound in size). To me, that indicates 'opportunistic' feeding by the growing immature fish rather than a 'seeking out' by all fish as a primary food source.

Hatches:

  • 'Hatches' from nymph to sub-imago can begin in the last half of April at lower elevations. These hatches peak about April 28th and the moderate to high elevation hatches peak about June 10th. However, hatches continue throughout the season. These hatches are known to happen right up until the first fall frosts in about mid October. A secondary peak in hatches occurs about August 3rd but feeding samples indicate that feeding on this hatch actually peaks about July 23rd while still in the nymph stage (but both are dependent on elevation). A significant number of other hatches occur throughout the season but not of sufficient importance to report as a separate incidence. It should be noted that a brood of Mayflies may hatch "in mass" in a few hours or it may take several weeks for the brood to complete a hatch.
  • Hint: Your are more likely to have success fishing Mayfly patterns during the secondary hatch in late July to early August than during the peak hatches. During the peak hatches, other food sources are also abundant. In the late summer hatch, many of those other food sources are not readily available to the trout.

Adults:

  • The sub-imagos and the spent adult are important to the fly fisher primarily as a dry fly. Fly patterns for the sub-imago should be tied with the wings upright as if ready to take flight. As a spent adult the fly should be tied as if the insect were dying, with the wings flat to the water. Don't forget to tie in two or three tails on these dry flies. Both types of patterns are effective and the trout will readily feed on each of these stages of the life cycle.
  • Hint: If fishing is slow, and you see some rises in the shallows, and you have seen some Mayflies, and you are willing to catch the smaller fish, try a dry Mayfly pattern (sub-imago) in the shallows. It will often provide some excitement on what could otherwise be a slow day. On occasion, even that larger fish may be taking the dry May.

Nymphs:

  • The nymph of the Mayfly is also an important stage to imitate for the fly fisher. Larger fish are more likely to feed on the nymph than the sub-imago or the spent adult on the surface. The nymph is available most of the season, usually in shallow waters, on or near the bottom. A good nymph pattern presented just at the start of a hatch can produce excellent results for the fly fisher. The best success often comes when the nymph is fished in the evening on the shallow shoals. After a major hatch has commenced, it is a mixture of small to large fish that feed on the nymphs and almost entirely the smaller fish that feed on mayfly sub-imagos or adults. Get in at the beginning of a hatch to get the larger fish.

Additional Notes:

  • With their susceptibility to pollution, Mayflies are a good indicator species for the health of a lake. Since Mayfly populations are a main food source in most parts of the country and not on our interior lakes, I hope this isn't a statement on the condition of our interior lakes. Time will tell. In the meantime maybe each of us can help prevent any further lake pollution. Please don't use gasoline motors and avoid the use of all motors in shallow waters. Don't pee over the side and avoid introducing anything into the lake but your anchor. Mayflies can become a primary food source for the large trout but it will take some work by all of us.

Recommended Fly Patterns:

  • Larva (Nymph):
    • Tiny Terror
    • Little Green
    • Copper Creek
    • Western Brown
    • Halfback
  • Dun:
    • Tom Thumb
    • Mayfly Dun
    • Blue Dun
    • Green Mayfly
  • Spinner:
    • Mayfly Spinner

Be sure to read other articles by Ron Newman

Study Other Insects | Tip & Techniques | Study Fly Patterns


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