Weekly Update – September 3rd – A Slight Increase to Begin the Month, but Not Likely to Last
September 9, 2025
Weekly Update – September 16th – Mosquitoes Stay Steady As Surveillance Wraps Up
September 22, 2025
Overall mosquito numbers stayed about the same this week and close to the 10-year average.
Mosquito numbers stayed close to the 10-year average this year, but the distribution was not equal and a few areas had high numbers.
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.
Our "false fall" has come and gone and we have been back to temperatures and levels of humidity that feel like summer for the past two weeks. Last week those temperatures contributed to about the same number of mosquitoes collected in our CO2 traps as the week prior, though distribution was not equal and a few very high trap numbers raised the average for the whole District. For instance, one trap in Eagan near the Mississippi River collected 2,624 mosquitoes alone! Other traps along the Mississippi River near St. Paul collected over 300 mosquitoes making that area the hot spot for the week. The majority of traps collected fewer than 100 mosquitoes.
Below are maps showing all human-biting mosquitoes (left) and Culex mosquitoes (right) collected in CO2 and Gravid traps on September 8th-9th:
West Nile virus activity decreased, but it will not be gone completely for the year until the mosquitoes are.
The positivity rate for West Nile virus in mosquito pools declined again last week as did the number of vector species caught in traps. However, there were still 10 positive trap samples collected last week, mostly in Hennepin County with two each in Ramsey and Scott Counties.
According to MMCD Vector Ecologist, Dr. Jordan Mandli, "The composition of positive pools has shifted almost entirely to Culex pipiens, which is expected given its late-season dominance over Cx. restuans and Cx. tarsalis. We will continue trapping and testing mosquitoes until overnight temperatures consistently fall to levels that inhibit mosquito activity."
Based on timing and temperatures it is likely that our last week to collect mosquitoes will be next week, meaning we will have two more weeks of surveillance updates. Continue to take the usual precautions to protect yourself from West Nile virus including wearing insect repellent.
Deer ticks re-emerge in the Fall - Check for Ticks
As we write this the temperature is close to 90 degrees outside in mid-September and it doesn't feel like fall, but cooler temperatures will be coming. When mosquitoes and other insects tend to enter diapause and cease their activity for the year, it's easy to let your guard down. However, it's important to remember that ticks will remain active until temperatures are consistently below freezing, which may not be until November or December.
As you head out for hunting excursions, leaf-peeping, or just enjoying a nice fall hike, be sure to check yourself, your kids, and your pets for ticks when you get home!
Below is an example of the deer tick life cycle from the CDC:
