Weekly Update – June 26th – Drought Impacts Nuisance Mosquitoes; More Test Positive for West Nile
July 5, 2023
How MMCD Monitors and Controls Biting Black Flies
July 11, 2023
For the first time in 2023 the average mosquitoes per trap is lower than it was in 2022.
For the third year in a row there was no 4th of July mosquito peak as adult numbers continue to decline due to lack of rain.
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.
The 4th of July is historically the mosquito peak in Minnesota when there is normal rainfall thanks in part to the timing coinciding with cattail mosquito emergence. For the third year in a row the 4th of July was a rather quiet time for adult mosquitoes thanks to a lack of rain and drought conditions increasing throughout Minnesota.
According to MMCD assistant entomologist Dr. Scott Larson, MMCD surveillance data "suggests that the peak of Cq. perturbans (cattail mosquitoes) has already past for 2023 – about two weeks earlier than normal."
Larson also adds that spring Aedes have been collected in much higher numbers than usual. The graphs below demonstrate the anomaly that 2023 has been. The graph on the left shows spring Aedes collections (black line is 2023, gray line is the 22-year average).
Larson says, "it might be hard to tell, but the (apparent) peak of cattail mosquitoes this year on 6/21 (49.71/trap) was only barely higher than average number of springs collected that week (49.00trap). That is very abnormal!"
Here are the mosquito trap counts from last Wednesday, July 5th:
One new mosquito sample tests positive for West Nile virus in Ramsey County.
MMCD Vector Ecologist Kirk Johnson reports that one new mosquito pool tested positive for West Nile virus of the 23 samples tested. To date, mosquito samples have tested positive for WNV in Anoka County (4), Ramsey County (2), and Scott County (1).
Johnson says, "our WNV findings in mosquitoes this year provide a good example of how temperature influences amplification of the virus. We often see increases in the WNV infection rate of the mosquitoes we test two weeks after a substantial increase in weekly temperatures and a decrease in the infection rate following cool periods. The mean weekly temperatures for weeks 22 & 23 were 76.6 F & 74.4 F respectively and our infection rate for week 25 was 5.79/1000 mosquitoes tested. The mean weekly temperature for week 24 dropped to 68.8 F and the infection rate for mosquitoes from week 26 fell to 1.35/1000. Weeks 25 & 26 were warm again at 78.5 F and 75.9 F, so the WNV infection rate will likely rise."
Ensure you are protecting yourself from mosquito-borne disease by using bug spray, wearing long, light, and loose clothing, and avoiding the peak feeding times of dawn and dusk.
