Yesterday, May 8th, about 4pm, we witnessed something we have never seen before. Hundreds of dead or dying mayflies were floating downstream carried by the brisk current or were gathering in the stillness of our little bay. The local fish population were taking full advantage of the sudden bounty. I managed to scoop one insect up for photo identification and deposit it on the dock where to my surprise it soon recovered and started to crawl about. I’ve attached two of my pictures. This morning there is no sign of any Mayflies on the river at all.

I understand that mayflies usually swarm in June. If this is indeed the aftermath of a swarm, it would appear to be somewhat earlier than usual.

Stephenie Armstrong, Warsaw

Note: According to Don Sutherland, a zoologist at the Natural Heritage Information Centre in Peterborough, “this mayfly appears to be Ephemerella subvaria. It is a widespread and abundant species with the period of emergence covering the month of May. The nymphs float to the surface and the adults emerge from the nymphal skin on the surface of the water (i.e. the surface tension keeps them afloat). As they emerge and take flight they are weak fliers, as the wings have not yet fully dried and hardened. This life state is termed the imago. The reference to fish gorging on them is interesting, as this species is a favourite among fly fishermen, who refer to it as the Hendrickson. Another mass emergence at the end of the month and the first half of June will be Ephemera simulans (Brown Drake) which has more patterned wings. The imagos will coat everything near water. There are in excess of 170 species of mayflies in Ontario; emergence and flight spans the entire year with some species emerging in early fall.”

Ephemerella subvaria mayflies on Indian River – May 8, 2017 -Stephenie Armstrong

Ephemerella subvaria Mayfly – Indian River – May 8, 2017 -Stephenie  Armstrong


Drew Monkman

I am a retired teacher, naturalist and writer with a love for all aspects of the natural world, especially as they relate to seasonal change.