First the Smoke. Then the Bugs.

It’s been described as a plague and a sign of the end times, turning New York City into the “Bug Apple.”

Insects may descend on populated places in unusual swarms for many reasons, such as mating season or in search of food. But New Yorkers pestered by clouds of small insects this week have wondered whether they were being dealt another consequence of the smothering smoke drifting in from Canadian wildfires.

As swarms of bugs were changing the city’s horizon, layering the muted skyline of skyscrapers with a swirling, street-level cloud of insects, some New Yorkers complained they could not open their mouths to breathe as they walked down the street, for fear of inhaling a big gulp of them.

Pedestrians swatted as they walked. Diners at outdoor patios in Brooklyn fanned the air.

Gothamist reported that “residents of the Bug Apple want these unwelcome tourists to skip town.”

WABC amplified the city’s pain: “No, you are not ‘bugging-out.’ A swarm of insects has seemingly taken over the city.”

But for every offhand, on-the-street reference to a Biblical-style plague, be it a swarm of locusts or gnats or hail and thunder, there is a scientist.

Dr. Corrie Moreau, a Cornell University professor in entomology, who looked at images of the insects, which can be green or white, said she believed they were aphids looking to mate.

Insect swarms often emerge to coordinate reproduction, which she believed could be attributed to the current appearance in New York City.

“It is unusual that there are so many of these aphids swarming this year,” she said. “It is because of the mild winter.”

Aphids are a non-stinging insect that normally sucks juice from plants. They are generally wingless, but will appear in winged form in the spring and fall.

The mild winter and rainy spring have made aphids optimistic about procreating. Moisture means flowers and plants, a sign for them to go into reproductive mode in swarms, increasing the potential gene pool.

“It increases the probability that they are going to be highly successful,” Dr. Moreau said.

Doug Currie, the senior curator of entomology at the Royal Ontario Museum, said he had not heard of any similar swarms in Canada.

“Trust me, the Canadian media would be contacting me,” he said.

His most memorable aphid swarm in Canada was in 2001, when soybean aphids clustered so heavily around the lights of a stadium, then known as the SkyDome, that the Toronto Blue Jays game against the Baltimore Orioles paused in the third inning.

“It caused them to close the roof to get the game going again,” he said.

But in New York, there might be some slim connection between the aphids and the overhanging smoke.

“I think that the smoke is amplifying our ability to see them well,” Dr. Moreau said. “Because I can’t see that far, I tend to focus on things that are closer.”

Aphids are probably not harmful if accidentally ingested, which some city types have been expressing is a concern if they walk down the street with jaws agape. So forget the “bug” pollution. It’s the smoke to be concerned about.

“There have been enough Manhattanites who have been biking. These are not toxic in any way,” Dr. Moreau said. “They should have masks on anyway, from the air quality.”