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iSeeChange: Declining moth populations in Delaware


Over the last fifty years, there’s been reported declines of moths all over North America. And there’s no simple answer for what’s driving down their populations. Human development has led to deforestation and habitat loss. The spread of artificial light at night makes them easy for their predators to see. Changing climates are causing moth species and their natural enemies, like birds and bats, to expand or shift their geographical ranges.

Delaware Public Media’s science reporter Eli Chen went to find out what’s behind the dwindling moth populations in Delaware in our latest edition of iSeeChange.



One of the most vivid memories Jeff Gordon had growing up in Delaware was the sight of moths at night.

“When I was just a kid in the mid to late 60s,” said Gordon, “my grandparents bought a place on Rehoboth Bay, at that time, it was really out in the woods. And I remember one of the magical things as a kid, in the evenings, we had a screen porch and the lights there would attract, just this carpet of moths, and not just tiny little brown moths, there were luna moths, and rosy maple moths and prometheus and polyphemus and big, spectacular, intricately colored moths. It’s really one of the things I feel most strongly awakened an interest in nature in me, it set me on a path that I followed to the rest of my life.”

Jeff Gordon is now the president of the American Birding Association, a national birdwatching organization that’s headquartered in Delaware City. These days, he doesn’t see as many moths as he used to when he was a kid.

“It’s just so radically different now, the numbers and diversity,” said Gordon. “I think most of those species are still present, but it used to be like a blizzard. Now, it’s a few flurries. I hate to say this but unfortunately, one of the recent encounters I had with the silk moths in that area was finding one flopping around under a 24-hour gas station.”

To Gordon, the factors that could be making the moths disappear seem obvious. Human development has removed forests and added more artificial lighting.

“My concern is that the number and brightness of light is increasing mortality on these moths. It’s drawing them out to places where they get killed and are unable to reproduce successfully.”

Here’s what Gordon asked iSeeChange.

“Do the observation of scientists square with my own memories? Because my memories seem vivid and dramatic,” said Gordon.

Three years ago, David Wagner, an ecology professor at University of Connecticut, published a paper on this very subject. Wagner notes that moths are already vulnerable animals. While typically only a single pair of moths are required to replace the previous generation, their mortality rates are incredibly high, greater than 95 percent. Wagner writes that “even modest changes to climate, local ecology, natural enemy complexes, etc., could doom a population’s existence.”

In Connecticut, where Wagner has studied moths for 25 years, many large moth species are in decline and some have even extirpated, or gone extinct in its local area. Among the people who feel it the most are the local cocoon collectors.

“If we call some of the old timers up, we could talk to them about when they were a boy and used to go out with their shopping bags and collect giant silk moth cocoons with their Dad or their buddies and fill them up in half a day with promethea or cecropia cocoons. That phenomenon just doesn’t occur in the East Coast anymore,” said Wagner.

The reasons behind the decline of large moths on the East Coast are various and complex. From a distance, we can say that human development has caused habitat loss. Changing climates could create unsuitable conditions.

But if we just look at one declining species, such as the giant silkmoth, the cause of its decline gets complicated if you look at the data as closely as David Wagner does. The rates of declining silkmoth populations aren’t consistent everywhere and there are places that appear to have plenty of silkmoths.

“There are places in New Jersey that are still extremely good and seem to be intact.

and places in southeastern Ohio that have spectacular silkmoth faunas. And in the East, one place that’s really important is Martha’s Vineyard -- Martha’s Vineyard has a fantastic and seemingly intact fauna at the present time,” said Wagner.

Wagner says the likely reason behind this has to do with what biologists call the “natural enemy complex,” which are basically the network of pathogens, predators and parasites that can harm a species.  

“There’s something special about the natural enemy complex in New England, across the Northeast and Delaware, for that matter, that is challenging silk moths in a way they weren’t before,” said Wagner.

He definitely has some culprits in mind. Species that have been introduced by the government to as a means of pest control, like the ladybug, could be feasting on the moth eggs. There are also animals whose geographic ranges have changed because of climate or other factors, leading them to feed and deplete northeastern moths.

“The tufted titmouse is a serious cocoon predator,” said Wagner. “They are very, very savvy predators, and they can devastate all the cocoons in the area, particularly cecropia. And that's one of the species that have had the biggest collapse in our region and that moved north from the south part of the U.S. into New England over the 1950s. So this very collapse in Delaware could be due to differences in bird fauna.”

In turn, scientists have also seen the decline of birds that were already feeding on moths in the area, songbirds being among them.

Artificial lighting, as Gordon pointed out, is a major problem for moths. Wagner says there’s no doubt they play a role in their decline. They get drawn to the headlights of cars that hit them. Lights make them easier for insectivorous bats to see and prey on them.

But it’s not clear how exactly light has dragged down their numbers, and there hasn’t been enough data to suggest that a lower density of artificial light in an area would help revive moth populations.

“There’s places like Martha’s Vineyard and northern New Jersey where moths are still intact. And I would think there’s a lot of light pollution in those areas,” said Wagner.

Wagner says that if we want to tease out what’s really bringing down moth populations and why random places like Martha’s Vineyard seem to be doing better than others, we need much more data.

“This is a case where citizen scientists and people taking notes in their backyard could generate the data we need to solve this conundrum,” said Wagner.

That’s the underlining message of Wagner’s paper. There’s enough data to suggest that something is happening to moths in the northeastern U.S. But it’s not enough to pinpoint the exact causes, which leaves those like Jeff Gordon, who are wondering about the moths, without a precise answer.

“It’s scary sometimes to see these large-scale changes going on and say this is what we’re seeing. What’s going on that we’re not immediately seeing?” said Gordon.

Whether it’s a new predator or a change in climate, being able to explain what’s happening to the moths might start with channeling that curious kid inside of us, who wants to just out by that screen porch and admire the moths, as they dance around the light.

This story is part of iSeeChange, a national initiative to crowdsource and investigate the climate and environmental changes you see in your local communities. If you’ve got a question about something curious you’re seeing in your backyard or you want to show us the moths that you’re seeing in your neck of the woods -- post it at iSeeChange dot org.

Sound credit: (snardin42), (overtone2),

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