The COVID-19 pandemic may have played a role in making 2020 a particularly bad year for mosquitoes.
Weather and river conditions were the chief drivers of one of the worst mosquito seasons in recent memory. But occupancy limits in the helicopters that apply apply pesticides to mosquito breeding grounds may also have left some of the blood-sucking pests’ prime habitats under-treated.
A report to the Fraser Valley Regional District, notes that the season was shaping up fairly normally through June 5, when the river initially peaked. But instead of slowly receding over the summer, river levels dipped, then started to rise again, peaking once again on June 30 – above the level of the first peak.
Every year, the FVRD hires a private company to apply larvae treatments to mosquito breeding grounds along the river. In 2019, those wrapped up on June 21. But in 2020, the persistently high water required mosquitoes kept hatching through August. The last treatments were applied on August 17.
In all, just shy of 20 tonnes of one pesticide agent were used by Morrow BioScience, the company tasked with treating and eliminating mosquitoes. (The pesticides aren’t sprayed; rather a larvicide that the FVRD says is environmentally friendly is applied in granular or liquid form. The mosquito larvae ingest a toxic protein that kills them.) Even in 2012, another bad mosquito year, 14,489 kg were applied – 30 per cent less than this year’s total.
Each application of the pesticide kills about 90 per cent of larvae. But there were just too many mosquitoes in 2020.
“Based on the extremely high larvae abundance in 2020,” the FVRD report says, “despite the intensive treatment effort, the remaining 5-15% of surviving larvae that hatched into adults were enough to cause unfortunate annoyance in many lowland areas near the Fraser River.”
Last year, only four people called a mosquito hotline on which people can report the pests. This year, 130 calls were received. Of those, the vast majority – 99 – were from Chilliwack residents. And most of those were from Fairfield Island, the residential area of Chilliwack closest to the Fraser River and surrounded by sloughs.
The mosquitoes on Fairfield Island could be an indirect result of the COVID-19 pandemic.
Helicopter and boat crews were limited due to COVID-19 safety protocols. And while the report by FVRD staff said “these limitations are not believed to have significantly impacted the treatment efficacy,” a separate Morrow BioScience report suggested some potential effect.
“It is possible that sites were not treated as thoroughly as usual,” Morrow BioScience report said. “The high number of concern calls associated with Fairfield Island [in Chilliwack] indicate the possibility that sites on that island may have been over-looked.”
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