Weekly Update – July 23rd – Mosquito Numbers Decline, More Test Positive for West Nile virus
July 30, 2024
Weekly Update – August 6th – Cool Weather Brings Mosquito Numbers Down
August 13, 2024
Overall human-biting mosquitoes approach season low-point based on surveillance from last week.
The number of common, human-biting mosquitoes continues to decline while warm weather brings an increase in West Nile virus activity.
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 end of the summer has a somewhat predictable mosquito pattern in recent years, even when we see a lot of rain like we have in 2024. The overall number of nuisance mosquitoes or "human-biters" tends to decline while disease vectors - those capable of carrying West Nile virus and other diseases - increase. Warm weather can spur West Nile virus activity and we are seeing that this year with nine new positive samples collected in the past two weeks alone.
According to MMCD Entomologist Dr. Scott Larson, "the big story once again this week – Culex mosquitoes are abundant and our normal nuisance mosquitoes are not. Don’t forget we have been doing a ton of treatments lately, so we really do make an impact on the mosquito population."
Helicopters have been active almost every day over the past two weeks due to continuous heavy rainfall that has meant more mosquito eggs hatching. Staff have been busy, often working overtime, to ensure samples are collected, identified, and treatments are made.
Below are the maps from Monday-Tuesday, July 29-30th:
Nine new samples of mosquitoes test positive for West Nile virus
Warmer weather often increases activity of West Nile virus and the past few weeks have been hot and wet, which elevates the disease risk. Mosquitoes collected during the week of July 23rd and July 30th have tested positive for West Nile virus - 6 from Hennepin County (in Minneapolis and in the western suburbs), 2 from Anoka County, and 1 from Dakota County.
After a hot stretch, we are experiencing cooler temperatures this week which is good for curbing WNV activity. MMCD Vector Ecologist Kirk Johnson says, "This week’s cooler temperatures should be working in our favor to slow WNV amplification, and I expect the infection rate in mosquitoes we test in two to three weeks to be lower than it is now. That will be especially beneficial as the mid to late August weeks are typically some of the highest risk weeks of the year for human WNV infections."
Some good news regarding vector-borne disease is that the risk of tick-borne illness tends to decline in the later summer months as deer tick activity slows down. Tick activity picks back up in the fall, so don't count the season over yet!
Come See MMCD at an Upcoming Event!
As we head into Fair Season, MMCD will be busy! We have several educational opportunities coming up with an opportunity to learn more about mosquito biology, control activities, and more. Here are some of our upcoming events:
- Today - Sunday, August 11th - Carver County Fair
- Today - Sunday, August 11th - Dakota County Fair
- Friday-Sunday, August 9-11th and 16-18th - Game Fair
- Saturday, August 10th - Stockyard Days Parade (New Brighton)
- Sunday, August 11th - Pollinator Festival at Bruce Vento
MMCD Staff Get Into the Olympic Spirit!
