Michigan insects in the garden – Week 4: Mayflies

The brief appearance of the mayfly.

A mayfly swarm congregating on a waterfront sign
Photo 1. A mayfly swarm congregating on a waterfront sign. Photo by U.S. Fish and Wildlife Service.

The mayfly crams its adulthood into a brief period. Within 48 hours, they develop wings, fly towards land, mate and die. If you reside near one of the Great Lakes, your garden or home and community may briefly be overwhelmed with up to 88 billion mayflies (Photo 1). At least 126 mayfly species occur in Michigan, and the giant mayfly (Hexagenia limbata) is one of the most widely distributed ones, found across much of the United States except for Alaska and the Southwest.

Mayfly life cycle

In their brief adult lives, a single mayfly will lay 4,000 to 8,000 eggs on the water surface. These eggs drop to the bottom of the water where they hatch into larvae known as naiads. The naiads will live underwater for one to two years while they feed on algae and other plant material. Most insects change from wingless immatures to winged adults. Mayflies are an exception to this rule. Their final immature life stage, also called a subimago, is winged and capable of flight. The subimagos fly towards the shoreline and within 24 hours, molt a final time into an adult.

Mayfly hatches occur in June or July in southern Michigan. When they enter land, mayfly populations are large enough to interfere with outdoor activities. With thin wings and two filamentous “tails” that are as long or longer than their body (Photo 2), mayflies can be as long as 1 inch including their tails.

A mayfly on the side of a gas station pump
Photo 2. A mayfly on the side of a gas station pump near Lake St. Clair in 2019. Photo by David Lowenstein, MSU Extension.

What are they doing in my garden?

As a mayfly spends 99% of its life in the water, their presence in the garden is the mayfly’s golden years and only chance to reproduce. Adults cannot feed since they lack mouthparts and have air-filled digestive systems. Their carcasses can pile up, be a nuisance, create slick surfaces and smell like rotting fish. If not swept or cleared, birds and bats will feed on dead adults. Alternatively, the dead adults can be collected and used as fishing bait.

Since mayflies are attracted to light, leave your porchlight off on the night of a swarm to reduce the impact of nuisance issues. No insecticides or other control efforts are necessary, as the adults are only in flight for one to two days.

Is seeing a lot of mayflies a good thing?

Mayfly naiads only thrive in lakes and rivers with good water quality. Large populations of mayflies are a bioindicator or a sign that indicates the health of an ecosystem. The type of insects living in aquatic environments characterize water health. One of these indices, known as the EPT index, bases water quality on the insect community found in a water sample. The presence of more mayflies (Ephemeroptera), stoneflies (Plecoptera) and caddisflies (Trichoptera) suggests a clean waterway.

Unfortunately, a recent study indicated a 50% decline in the number of mayflies emerging since 2012. One explanation is due to ongoing runoff of phosphorus from agriculture and residential use. The flow of excess nutrients into waterways triggers algal blooms, which lead to oxygen-limited areas that do not support mayflies.

Michigan State University Extension recommends using less phosphorus in home lawns and understanding the fertilizer needs of a lawn. Only apply the necessary nutrients in the interests of protecting water quality. Mayfly naiads and adults are an important food source for invertebrates, fish, birds and bats, and their declining numbers may affect other organisms that use them for food.

Spending time in a mayfly emergence is a minor nuisance and a unique experience. This video from Nat Geo Wild provides a perspective of what it feels like to spend time amidst a mayfly swarm. If you are lucky enough to live near a waterway with a mayfly swarm, spend an evening marveling at the large impact of an insect’s brief appearance on land.

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