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Soon after a major Calgary water main burst, Mayor Jyoti Gondek urged city residents to “get any type of container” out in their backyard to collect as much rainwater as possible.

Behind the advice, however, lay a conflict: collecting rainwater also offered a haven for mosquitoes to lay their eggs. Entomologists now warn that putting buckets out to catch rain will lead to an unavoidable consequence — more mosquitoes.

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Open water surfaces create a breeding ground for the insects, said Dr. Nadir Erbilgin, who teaches entomology at the University of Alberta.

“As long as the water is stable, mosquitoes will thrive.”

Calgary’s mosquitoes are mainly floodwater species, meaning their eggs need stable aquatic habitats to hatch. Sometimes, small quantities of water is enough. After their developmental stages, the larvae emerge from the water into fully developed mosquitoes — a process that occurs within 20 days of hatching, said Erbilgin.

These species include the Anopheles quadrimaculatus and Culex pipiens, some of which carry viruses such as malaria, Zika and West Nile.

The number of mosquitoes usually peaks in late summer, said Alex Coker, a pest management technician with the city, with high rainfall and warmer temperatures as the main culprits.

The season also offers species such as Culex pipiens the perfect opportunity to develop within five days after hatching, said John Swann, an entomologist with consultancy company ABI Environmental Services. And the amount of water required is minimal.

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“It does really well in urban environments — breeding in small standing water, including bird baths,” Swann said.

The city reported a low to average number of mosquitoes in May, thanks to colder weather, and the estimated levels are similar for June. However, their population is expected to grow with more heat and rainfall in the coming weeks.

To estimate the number of mosquitoes in Calgary, officials use four traps across the city that contain dry ice — carbon dioxide — to attract the insects, and a fan that pushes them into a bag.

After about 18 hours, specialists count the number of mosquitoes trapped in the contraption. Coker said the number of mosquitoes per trap usually ranges from 50 to 20,000. The most recent level is between 50 and 100.

Coker said the city also monitors for mosquito larvae throughout Calgary while using rainfall levels to guide their search.

To eliminate the risk of mosquitoes, the city routinely tells residents to “remove any shallow standing water” on their property, and “regularly drain any containers which collect rainwater like bird baths, empty flower pots, rain barrels and eavestroughs.”

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However, as the city urges residents to find creative ways to conserve water while the feeder main is being restored, it has shifted its directives from draining away standing water to covering containers, which it says, “will ensure the safety of kids, pets and wildlife, while also keeping the water free of debris. This will also prevent mosquitoes from using the water as egg-laying sites.”

However, Erbilgin is more cautious, saying covering such open surfaces is insufficient to keep mosquitoes from breeding.

“They might limit the number of mosquitoes laying eggs, but when it rains, the covers will come out and there’s no guarantee the mosquitoes won’t use that opportunity,” he said. He suggested screens available at a few greenhouses whose mesh size is too narrow for mosquitoes to get out. But even that isn’t foolproof.

“They will find a way,” he said. “They are quite resourceful.”

The city also recommends using the collected water within five days, which Erbilgin said is an effective solution.

“Any interference will be detrimental to the mosquitoes that lay eggs or the larva inside the water,” he said.

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Swann emphasized that residents must drain the entire rain barrel.

“They need to be emptied about every five days,” he said. “That little inch in the bottom below the spigot that you use to drain your rain barrel — that’s still enough water for mosquitoes and larvae to breathe in.”

Meanwhile, the city didn’t comment on whether it would stop encouraging people to collect rainwater after the feeder main is restored, especially as the city is poised to debate whether to roll out restrictions to address a drier-than-average winter that has pinched water supply.

“It’s simply that mosquitoes are a pest we all have to put up with and, traditionally, Calgary has been dry enough in the late summer that mosquitoes generally subside,” Swann said.

“Now we’re getting the rains at just the right frequency for the mosquito populations to grow in the summer.”

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