Weekly Update – September 17th – Mosquitoes Slowing Down, Some Vectors Still Present
September 24, 2024
The last week of September brings an end to mosquito surveillance and essentially an end to mosquitoes for the year.
Mosquito numbers wind down at the end of September in the last week of adult trap and sweep net collections.
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.
Despite the fact that September has been warmer than usual, mosquitoes have not stuck around. Thanks to a combination of minimal rainfall, cooler overnight temperatures, and the early sunset, mosquito numbers have reached the lowest overall total since we began collections in mid-May. Only three CO2 traps in the District collected over 100 mosquitoes, with most collecting fewer than 20 and many collecting none at all.
Reflecting back on the year it is somewhat surprising that despite above average rainfall in June, July, and August most of the season had mosquito numbers that were below the 10-year average. MMCD Entomologist Dr. Scott Larson says that one reason is because of a single species that didn't yet make a comeback: "The 3 years of drought (2021, 2022, 2023) really put a dent in the Aedes vexans populations and while they recovered by the end of this season, they didn’t start out very abundant earlier this year. Another way to look at it is that we had only 56% of the 10-year average for this species. My prediction is that they’ll be back to pre-drought levels next year."
Bar graph depicting 10-year prevalence of Aedes vexans mosquitoes in the District.
Here are the maps from Monday-Tuesday, September 23rd-24th:
Several species showed up in unusually high numbers in 2024
Every year has some variation in the mosquito species that are present and the MMCD entomology lab keeps a running list of species occurrence from rare to ubiquitous. This year two species that had previously been considered "uncommon" made big appearances in our surveillance. Culex salinarius and Aedes dorsalis are two species that typically have fewer than 100 collections in a normal year, but in 2024 they each saw a many-fold increase. The graphs below were created by Dr. Scott Larson in the entomology lab:
Both of these mosquito species are saltwater tolerant, which does not explain why they were abundant in Minnesota this year as we don't have salt marshes in our area. Culex salinarius prefer birds for their source of bloodmeal, but may become opportunistic and bite mammals when in higher numbers. Aedes dorsalis are aggressive human-biting mosquitoes that are often active during daylight hours. There is still lots of data to gather, but it will be interesting to see if these species stick around next year or if this was a one-time phenomenon.
Disease activity slows down as mosquitoes enter diapause
As human-biting mosquito numbers begin to dwindle, so does the risk of mosquito-borne disease. One more pool of mosquitoes tested positive for West Nile virus last week and remaining samples will be tested this week. There is always a delay in the reporting of human cases of mosquito-borne disease so it is likely we won't know the impact of the 2024 season for some time yet.
As mosquitoes go away for the year, it's important to remember that ticks will be sticking around for a while. Ticks tend to re-emerge in the Fall and remain active as long as temperatures are above freezing with limited snow cover. During last year's warm winter we had reports of ticks being found well into December. Be sure to continue to take preventative measures to avoid tick bites as long as the weather conditions permit.
For the latest updates on mosquito control, disease activity, and planned treatments, follow MMCD on Twitter or Facebook.
