Weekly Update – July 2nd – Mosquito Numbers Increase, but Stay Below Average
July 9, 2024
Weekly Update – July 23rd – Mosquito Numbers Decline, More Test Positive for West Nile virus
July 30, 2024
After a gradual increase for several weeks, overall mosquito numbers began to decline in the latest surveillance.
Late summer is typically when mosquito numbers begin to decline and that seems to be the case in the latest round of surveillance.
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.
We may have already seen the peak of mosquito season in Minnesota and there are several weeks of summer left. Based on the 10-year average, mosquitoes in the Twin Cities peak the first half of July and then begin a gradual decline through the rest of the summer. This year, despite lots of rain, we didn't see as much of an early July spike in mosquitoes. Prior to this week the numbers had been gradually increasing, but mosquitoes collected in CO2 traps on July 15-16th were fewer than the week before and with (slightly) drier weather over the past couple of weeks it looks like things may be beginning to slow down.
The main reason that mosquitoes didn't surge amidst this year's above-average rain is that our main nuisance species - the cattail mosquito (Coquillettidia perturbans) and the summer floodwater mosquito (Aedes vexans) have not been very present this year, likely not quite recovered from three years of drought. According to MMCD Entomologist Dr. Scott Larson, "Culex mosquitoes usually peak around 10-15 per trap, but this year we’re hitting a peak of almost 50 per trap. And these are abundant most everywhere. The gravid traps have likewise been pulling in tons of Culex mosquitoes. One gravid trap had a record-breaking 1,771 Culex in the collection." Culex mosquitoes are the main West Nile virus vector for Minnesota, so there abundance is a concern though West Nile has not become widespread in our area (see below for more on disease).
Black flies continue to be abundant in the South metro, though their numbers will hopefully start to decline now that the rivers have receded enough for us to resume treatment.
Below are the maps from Monday-Tuesday, July 15-16th:
West Nile vectors are abundant, but West Nile is not as of yet.
As mentioned above, Culex mosquitoes are the primary vector of West Nile virus in Minnesota and there numbers are much higher than normal this year. Our lab shared this graph depicting Culex numbers this year where you can see they are well-above average:
The far right line graph is 2024. Fortunately, despite very high numbers of Culex mosquitoes, incidence of West Nile virus is low. MMCD Vector Ecologist Kirk Johnson reports that there have been two positive samples of West Nile virus this year - from Dakota and Anoka Counties and a third that tested positive outside of the District. That's out of 472 samples that have been tested making a very low positivity rate. Outside of the District the Minnesota Department of Health did report its first human case of West Nile virus in western Minnesota (Yellow Medicine County).
It's still wise that people take the usual precautions like wearing bug spray and removing standing water as we head into the higher risk time of year.
Why do mosquitoes buzz? Good question!
Why do mosquitoes make that annoying buzzing noise? That was the weekly "Good Question" covered by WCCO/CBS Minnesota and MMCD, along with some local kids at a playground, helped provide the answer. Check it out below:
