The Quiet Collapse: Mayflies Are Disappearing in the Midwest, and Why It Matters

The Fly Box LLC
Aug 11, 2025
2 min read

Fly fishers know the mayfly not just as a fly pattern, but as a seasonal signal. When mayflies hatch, trout look up. Anglers wait for those moments like holidays. But in parts of the Midwest, those hatches are no longer what they once were, and it’s not just anecdotal.

Scientific research confirms that mayfly populations are plummeting in some of the most important river systems in North America.

What the Numbers Say

A 2020 study published in the Proceedings of the National Academy of Sciences (PNAS) found that from 2012 to 2019:

Mayfly populations in the Upper Mississippi River Basin declined by approximately 52%.
In western Lake Erie, the decline was even sharper — an estimated 84% drop over the same period.

These numbers come from light trap data; a long-standing method for monitoring emergent aquatic insects like mayflies. The dramatic loss of biomass points to systemic environmental changes.

What’s Causing the Collapse?

Researchers and ecologists point to a combination of stressors:

Nutrient Pollution: Runoff from farms, especially nitrogen and phosphorus, feeds massive algal blooms. When the algae die, their decomposition consumes oxygen, creating hypoxic conditions deadly for mayfly larvae.
Neonicotinoid Pesticides: These common agricultural chemicals are known to be toxic to aquatic insects, even in small concentrations.
Climate Change: Rising water temperatures and altered seasonal flows disrupt mayfly life cycles. Some species are highly temperature-sensitive, and even small shifts can delay or suppress emergence.

Why It Matters for Fly Fishing

Mayflies are one of the foundational food sources in freshwater ecosystems. When their numbers crash, trout and other insectivorous fish lose critical calories. That means smaller fish, fewer rises, and an overall less productive fishery.

It also means the iconic dry fly fishing experience, sight-fishing rising trout during a hatch, could become less frequent or even vanish in some regions.

More Than Fishing: An Ecological Red Flag

Mayflies are bioindicators. When they disappear, it means the water is sick. Their decline isn't just a fly fishing problem, it's an environmental one.

This is a warning. One backed not by hunches or forum posts, but by published data.

What Can Be Done?

Organizations like the Upper Mississippi River Restoration Program and state-level conservation groups are working to improve water quality and reduce nutrient runoff. But real progress requires broader agricultural reforms and pesticide regulation.