Insect Update – Week of August 2nd – Low mosquitoes overall, high West Nile vectors

Results from the August 2nd surveillance show that overall mosquito numbers are at their lowest point since May, but West Nile virus vectors are 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.

As of last week most of the state of Minnesota was in a "severe" or "extreme" drought as categorized by the U.S. drought monitor and the result is a continued lack of mosquitoes as sources of standing water dry up. Results from last week's mosquito surveillance brought the lowest counts of human-biting mosquitoes since our first trap week on May 17th. According to MMCD assistant entomologist Dr. Scott Larson, "we averaged only ~22 human-biting mosquitoes per CO2 trap. Only 4 traps (out of 137) collected over 100 mosquitoes."

While overall mosquito numbers are down substantially, the numbers of Culex mosquitoes, the primary vector of West Nile virus in our area, is still high. According to Larson, "Of note, there were 586 Culex mosquitoes in a St. Paul catch basin gravid trap. There were 194 Culex tarsalis mosquitoes in an Eden Prairie CO2 trap (I don’t believe that’s a record for the District, but it is the highest number recorded for this location since at least 2015 and possibly much longer)."

Last week MMCD vector ecologist Kirk Johnson reported that three more mosquito pools tested positive for West Nile virus including the first two positive results from Scott and Anoka counties. The third was from a trap in Dakota county. To date there have been 11 positive samples found throughout the district with West Nile positive mosquitoes found in Ramsey and Hennepin counties within the past several weeks.

Residents are advised to lower their risk of mosquito-borne disease by wearing insect repellent and avoiding peak feeding times of dusk and dawn. With lower annoyance mosquitoes, people may feel less inclined to take precautionary measures, but Minnesota is heading into the peak time of year for West Nile virus transmission.

Here are the mosquito trap counts from last Monday, August 2nd: