Insect Update – Week of August 16th – Surveillance shows West Nile virus risk is present
August 24, 2021
Insect Update – Week of August 30th – Mosquitoes Starting to Appear After Recent Rains
September 10, 2021
Summer floodwater mosquitoes along the Minnesota River led to an increase for the first time in weeks.
Results from the August 23rd surveillance show that even before the big rains from the end of last week, mosquito numbers were ticking up in some areas
Every Monday night starting in mid-May through September, the Metropolitan Mosquito Control District sets out a variety of mosquito and black fly traps throughout the district and employs a network of sweep net collections done by employees and volunteers. Any insects collected are brought back to our lab on Tuesday to be identified throughout the week with maps published to show current mosquito and black fly activity.
For the first time this year the number of human-biting mosquitoes collected in a week surpassed the 10-year average, though not by that much. This increase was mainly driven by CO2 traps along the Minnesota River which collected over 1,000 summer floodwater mosquitoes (Aedes vexans) during the Monday night surveillance.
At the end of last week most of the Twin Cities metro received more than 3 inches of rain and crews have been out all weekend checking to see where larvae has developed. Quite a few ponds and wetlands that were mostly dry have now seen an increase in water level and many are holding mosquito larvae. Helicopters will be up throughout the early parts of this week to respond to the most recent brood of mosquitoes. We anticipate mosquito numbers this week and next week will spike throughout the District.
We are still seeing elevated numbers of Culex mosquitoes, which are the genus most likely to transmit West Nile virus to humans. However, according to MMCD assistant entomologist Scott Larson, we are trending in the right direction: "Culex tarsalis numbers were down last week with only 4 CO2 traps catching more than 10 females - compared to more than 20 traps the previous week." It remains to be seen how the rain will impact Culex numbers, but Larson says it may have helped flush some of their larvae out of catch basins.
West Nile virus still remains a threat to residents of the District at this point in the year, however. MMCD vector ecologist Kirk Johnson reported that 9 more pools of mosquitoes that were collected last week have tested positive for West Nile virus - 4 from Ramsey County, 2 from Hennepin County, 2 from Dakota County, and 1 from Anoka County. This brings the season total to 41 positive samples collected in almost every county in the District. Residents are advised to lower their risk of mosquito-borne disease by wearing insect repellent and avoiding peak feeding times of dusk and dawn.
Here are the mosquito trap counts from last Monday, August 23rd:
Field staff is busy responding to most recent brood and West Nile threat
At the end of last week and over the weekend, the rain came in bursts with dry periods in between. This means that in many cases, crews had to check sites more than once to confirm the presence of larvae and get an accurate count. Facilities began doing helicopter larval control treatments on Saturday and will continue through the early parts of this week. The list of cities that will be treated by our helicopters is updated daily.
Staff will also be conducting efforts to reduce the West Nile virus threat by eliminating habitat including waste tires, using larval control in permanent water sources like catch basins, and in some cases using adult mosquito control efforts to reduce actively infected mosquitoes. Any adult mosquito treatments are posted on the Scheduled Treatments page of our website.
Please help us out by reducing mosquito habitat on your property - check your yard after the weekend rain to see if there is any standing water and eliminate it promptly.
For the latest updates on mosquito control, disease activity, and planned treatments, follow MMCD on Twitter or Facebook.
