Weekly Update – July 24th – Surge in West Nile Activity While Floodwater Mosquitoes Continue to Drop
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Another dry year means mosquitoes are staying below the 10-year average by a significant amount.
Not a lot of mosquitoes are out, but there was a surge in samples testing positive for West Nile virus.
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.
It's not uncommon for mosquito numbers to drop as we progress through August in the Twin Cities. As you can see from the graph above, even in years with normal precipitation, mosquito numbers tend to decline throughout July and into August. While the more abundant floodwater mosquitoes are declining, the number of Culex mosquitoes tends to increase in August as they thrive in hot, dry conditions.
According to MMCD assistant entomologist Dr. Scott Larson, "We collected many Culex mosquitoes again last week. Thirty-four (34) out of the ~130 traps that collected a sample had more Culex mosquitoes than our typical human-biting genera (Aedes and Coquillettidia). That’s pretty unusual in our normal CO2 traps, but not so for the elevated CO2 traps."
Here are the mosquito trap counts from last Monday, August 7th:
A surge of new mosquito samples test positive for West Nile virus, especially in Ramsey and Hennepin Counties.
More mosquitoes collected have tested positive for West Nile virus over the past two weeks. Ramsey and Hennepin Counties have had the most positive samples this year with eleven more testing positive between the two counties in the past week.
It is common to see a surge in Culex mosquitoes in the Fall, even in dry years because this species is less impacted by drought conditions and thrives in man-made habitats like waste tires and catch basins. However, this year there has been more positive West Nile virus tests than the past several years.
MMCD Vector Ecologist Kirk Johnson reports that the species testing positive for West Nile virus are Culex pipiens or Culex restuans, which tend to bite birds and rarely humans. Culex tarsalis is the species most likely to transmit WNV to humans in the District, and their numbers this year have been relatively low. According to Johnson, "this year we have tested just 39 Cx. tarsalis pools with four positive for WNV."
This is a good reminder that even when nuisance mosquitoes seem to be low, it's important to take precautions to avoid mosquito bites. Wear insect repellent and cover up during peak mosquito activity periods.
Potential uptick in mosquitoes coming due to recent rains
Over the weekend and on Monday we saw significant rainfall throughout most of the Twin Cities and staff have been out checking sites where mosquito larvae tends to develop. Initially it appears several sites are holding enough mosquito larvae to warrant a treatment so helicopters will be out today and tomorrow in those areas.
Any increase in mosquitoes will likely not be apparent until next week because it takes about a week for most species to emerge from the water after the eggs hatch.
