Wildlife & Nature

Vanishing Spring Squalls: The Uncertain Status of Local Mayfly Populations


Mayflies are a diverse, enormous insect family critical to a range of ecosystems. (istock)

Among endless environmental concerns is the mayfly family. While mostly known to poets and trout anglers, mayflies are a diverse, enormous insect family critical to a range of ecosystems as both biomass and nutrient loads. In short, a lot of stuff eats them, and that they’re struggling anywhere is fretful news.

What this means for southern New England is currently unknown. While not yet a pressing local worry, however, people are paying attention.

Kassi Donnelly, the wild and scenic rivers coordinator for the Wood-Pawcatuck River Watershed Association, samples the Wood River throughout the warm months with kids from grade school to college-aged. Mayfly and stonefly nymphs are among her favorites.

“They indicate high water quality,” Donnelly said. She noted her educational sampling isn’t detailed enough to notice trends.

“Based on all the data we currently have, it doesn’t appear that mayflies are declining locally,” said Katie DeGoosh, principal environmental scientist for the Rhode Island Department of Environmental Protection.

Mayflies have been little studied in Rhode Island, with the current database only having a few samplings from an array of sites from 2002-2014, leading DeGoosh to qualify any assessments.

“We don’t have enough data over a long enough time frame to show any type of trends, such as a decline,” she said.

Given troubling dips elsewhere, then, particularly in the Great Lakes and Upper Mississippi basins, along with far-flung places like India, further data collection is warranted, for any threat to the mayfly genus is the reddest flag to the larger webs they inhabit.

If plankton sustain ocean life, freshwater and terrestrial chains are founded on arthropods, or bugs, and as a bloc bugs are in trouble. Many people are aware of colony collapse in honeybees, or the monarch butterfly’s sad decline, but insects the world over are teetering. Scientists are sorting the details, but the consensus is that global insect populations are slipping if not nosediving.

A 2019 global scientific review found more than 40% of insect species are declining, and a third are endangered.

Two local declines might sound familiar. Anyone over roughly 40 likely has memories of firefly abundance, of twilit meadows glittering with green bulbs. While these insects certainly still exist, a decades-long decline has mostly relegated such sights to a few-here, a few-there experiences.

As a group, mayflies, stoneflies, and caddisflies are second. In what’s been dubbed the “windshield effect,” people remember driving through clouds of insects, with many dying on the windshield. Over decades, that phenomenon has dwindled nationwide to another rarity. If most people are unaware of such goings on, biologists are in tune.

“Aquatic invertebrates such as mayflies provide forage for many insectivores,” said Corey Pelletier, a DEM fisheries biologist and a colleague of DeGoosh’s. “Mayflies are in streams, rivers, lakes, and ponds and are a preferred taxa for many fish species. Beyond fish, adult mayflies provide forage for numerous species of birds.”

Fish such as eastern brook trout prefer mayfly nymphs — the juvenile, aquatic phase of mayflies — their whole lives. When mayflies emerge for their famously brief, winged adult lives, they become critical protein for nesting birds. Hosts of migratory species such as American redstarts, tree swallows, and scarlet tanagers time their New England arrival to feed nestlings on many emerging insects, including mayflies. Bats, too, gorge on them.

Many species depend on mayflies for food, including birds and bats. (istock)

“If mayfly species disappeared,” said Pelletier, “there’d be a massive impact. Since Rhode Island’s freshwater systems already suffer low productivity, mayfly disappearance would further impact predator species within these systems.”

While assessing local mayfly populations is important, however, it isn’t easy. Unlike booming habitats such as the Great Lakes, where mayfly biomass is charted by radar, even in optimal conditions southern New England mayfly numbers pale in comparison. DeGoosh points to Rhode Island’s low elevation and warmer waters as two reasons for this. Still, they’re here and relatively abundant, and getting a handle on their status is crucial.

Rhode Island is ocean rich and river poor, though what freshwater streams there are offer some of the state’s richest habitats. From the Wood, Blackstone, Pawtuxet, and Pawcatuck rivers down to hop-across affairs like South County’s Metatuxet and Meadow brooks, these waterways are biodiverse, historically abused, and delicate. Discerning whether local mayfly numbers are following downward global trends will contribute to understanding those larger declines and gauge the general health of local rivers. For now, the data is little, the worry great.

“Mayflies need cool, clean water,” said George Christie of the Rhode Island Natural History Survey. “Rhode Island’s watersheds have been highly polluted and water temperatures are increasing. This suggest that mayflies are vulnerable, but currently there’s just not enough data.”

Anecdotes aren’t data, of course, but they’re not nothing, and regarding aquatic insects, the memories of trout anglers are relevant. Rich Benson has fished Rhode Island waters for more than 50 years, with the Wood River being his favorite. While Benson understands the limits of memory matched against hard data, he has noticed a downtick in mayfly hatches.

“I think they have declined,” he said. “There are still nights with good hatches, but not with the same intensity. On the other hand, the Wood still has a good population of wild brook trout, so that indicates a healthy system.”

For now, it’s hard to say if local mayfly numbers are in the same peril as those in more abundant ecosystems. Pollution, farm and lawn runoff, and rising water temperatures, however, are suspected culprits in all those spirals, with southern New England experiencing the same, and though the mayfly family is not as prolific locally as elsewhere, it’s still a vital ecosystem component comprised of fascinating creatures.

When a species hatches in unison, it can turn a late spring evening into a seeming snow squall, with fish, birds, and bats abandoning all inhibition. In the world over, though, including here, that’s becoming an increasingly rare occurrence, and learning why will go far to understanding what ails our freshwater systems.


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  1. Thank you Mike for shining some light on a little known problem that most people haven’t heard about. As a freshwater fisherman, I can attest to the declining populations of flies and bugs in and around our rivers, ponds and streams. Our freshwater ecosystems are dying, but we humans can slow this degradation of our freshwaters systems by stopping use of pesticides (lawn and garden fertilizers for example). This simple behavior change can help keep fly and bug populations from disappearing completely. This is everyone’s problem and everyone can/should help contribute to taking immediate and persistent corrective actions.

  2. So what’s the plan moving forward? How do you determine if the numbers are dwindling if you’re not doing enough monitoring to determine trends? How do you think todays advanced residential “bug zappers” and spraying services affect “bugs”? Not good I imagine. Healthy wild brook trout populations in the Wood River? Based on what? Redd counts, historic survey data?

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