Weekly Update – September 10th – Mosquitoes Right at Average with a Few Weeks Left
September 13, 2024
Mosquito numbers are almost precisely in line with the 10-year average as we head into the last weeks of surveillance.
In the penultimate week of mosquito surveillance, we find that overall mosquito numbers are low, but some vectors are still present.
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.
A number of factors determine when the last night of surveillance will occur at MMCD including mosquito population, weather forecast, staffing levels, and more. This year we are ending our Monday night mosquito surveillance as of today (September 24th), which is two weeks later than last year. This is due in part to warm September temperatures, which may keep the bugs around a bit longer and also a fair amount of vector species still showing up as of last week. These include Culex mosquitoes, which are responsible for transmission of West Nile virus, and Culiseta melanura, which are the primary vector of Eastern Equine encephalitis (EEE) in our region.
Here are the maps from Monday-Tuesday, September 16th-17th:
Vectors of West Nile virus and EEE are still present in the Twin Cities
While the risk of mosquito-borne disease is low, there remains mosquitoes present in our collections that have the potential to transmit diseases that may affect humans. Last week another seven mosquitoes tested positive for West Nile virus from collections on the evening of September 10th or 11th. These samples were from Anoka, Hennepin, Ramsey, and Dakota Counties.
The Minnesota Department of Health reported to the CDC that two more metro area residents have tested positive for West Nile virus - one in Ramsey and one in Carver County. It's unclear at this point whether the disease was transmitted locally or during travel outside of the District by those residents. This brings the total number of human cases in Minnesota this year to 8 total.
In last week's collections there were also a few Culiseta melanura mosquitoes captured in northern Washington and Anoka Counties. These are being sent in to be tested for Eastern Equine encephalitis, a rare but dangerous disease which has been making headlines this year. The number of Cs. melanura mosquitoes is low and this species doesn't typically bite people, instead the disease is transmitted via a bridge vector like cattail mosquitoes, which are mostly absent as adults this time of year. There is not a significant risk of EEE to humans, but testing the few that we collect will help MMCD with our disease surveillance and preventative measures.
Look for our last surveillance update of the 2024 mosquito season coming next week!
