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Mosquito numbers dropped for the week, but likely because it was raining during surveillance.
Average number of mosquitoes collected dropped to the second lowest point since surveillance began, but not because mosquitoes went away.
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.
Here is an example of data needing some context - according to our graph of weekly averages of human biting mosquitoes collected in CO2 traps, mosquito numbers dropped last week to their lowest point since May 14th. However, that doesn't exactly mean that mosquitoes are on the decline. Last week during much of our surveillance period it was windy and/or raining. Mosquitoes tend to be less active when it's raining and they are weak fliers that don't do well in heavy winds. This trend likely skewed the data and led to fewer collections than we would expect. As we often say, more rain leads to more mosquitoes, but if it doesn't stop raining mosquitoes stay in hiding until it's stopped.
MMCD assistant entomologist Dr. Scott Larson states that we are seeing more Culex mosquitoes than would be expected for this time of year given the conditions. He pointed out that several gravid traps collected what appear to be record numbers and stated that it's "unusual to see so many Culex collected in northern CO2 traps." Below on the left is a map of the gravid traps where you can see high numbers in parts of Anoka County.
Black fly numbers declined a bit as the spring transitions into summer. Several species are more dominant in the spring and less likely to appear as we head into late June and July. It's unclear yet what impact the high river levels will have on black fly abundance throughout the metro.
Here are the mosquito trap counts from last Monday-Tuesday, June 17th-18th:
First positive West Nile virus sample found in Anoka County.
A mosquito pool collected on June 17-18th in Anoka County has tested positive for West Nile virus. This is our first positive sample of 2024 out of 177 pools that have been tested.
According to MMCD Vector Ecologist, Kirk Johnson, "Beyond the one positive result is the number of [Culex] tarsalis pools tested from last week’s sampling, there were 21 and there is at least one more to be tested. With last year’s drought impacting the Cx. tarsalis population, we only tested 51 pools over the course of the entire year. This is just the beginning of the season for Cx. tarsalis and with the amount of standing water we now have, expect their numbers to grow considerably over the next couple of generations."
Cx. tarsalis are a concern in the transmission of West Nile virus because they feed on humans more often than some of the Culex species.
The tick risk meter remains at Medium for now. June is a month when nymph ticks emerge and become abundant, but according to MMCD Tick Specialist Janet Jarnefeld, we aren't seeing that many yet.
Helicopters will be active throughout the week responding to excessive rain.
We finally have a break from the rain and crews are taking advantage by using helicopters to treat all of the areas where mosquito larvae have been found to be abundant. More rain is expected in the coming days, so we are likely to stay busy and try to ensure everyone can get a break from mosquitoes heading into the week of the 4th of July. The list of cities with planned helicopter treatment is updated daily.
Last week Fox 9 aired a story about all the work that goes in to fighting mosquitoes in the District:
