https://www.cbc.ca/news/canada/ottawa/road-salt-environmental-impact-run-off-dragonflies-mosquitoes-1.5435761

Road salt is arguably a life-saving necessity during Canadian winters, but new research shows it may also indirectly boost the mosquito population come summertime.

According to a study published recently in the journal Frontiers of Ecology and Evolution, all that salt is leaching into roadside storm ponds, where both mosquito and dragonfly larvae grow.

Rosalind Murray, a post-doctoral fellow in biology at the University of Toronto and one of the study's authors, studied storm ponds in the GTA with her research team.

These man-made ponds are meant to catch road run-off before it drains into the watershed, but they're also full of tiny life. With no fish or frogs present, the wingless dragonfly larvae, which measure up to three centimetres in length, are the apex predators, and have a voracious appetite: a healthy one consumes an average of 11 mosquito larvae in two hours.

"But the salty dragonflies — the ones that were exposed to the highest level of salt for the longest amount of time — were eating significantly fewer mosquito larvae," Murray told the CBC's Ottawa Morning — as few as seven every two hours.

"That might not seem like that big a difference, but if you think about how many ponds and how many dragonflies, this can have a huge impact on ... how many mosquitoes they're actually consuming."

The salt appears to have little effect on the hardy, adaptable mosquito larvae, leaving the population to thrive.

"If you have a very salty environment that has killed off a lot of the potential predators for the mosquitoes, then you might be exposed to even more mosquitoes," Murray said.

But dragonfly larvae also feed on each other, and salty ones are slower to heal, she said.

"Dragonfly larvae are hugely cannibalistic, so they might pull off a leg. When they're exposed to high levels of salt … they're not as quick at recovering from these wounds."

The researchers followed the salty dragonflies into adulthood, and found they were less healthy. As well as healing more slowly, the insects were also found to be more vulnerable to infection.

For humans, this poses a dilemma: using road salt in the wintertime means more mosquitoes in the summertime.

Murray said she's not advocating for an end to road salt, but until a healthier alternative is found, she wants municipalities to consider the impact excessive amounts are having on the ecosystem.