Keeping up with mosquitoes

Every knowledgeable PMP knows that assessment and monitoring is a crucial part of integrated pest management (IPM). Yet practically no one in the mosquito control business monitors mosquitoes. Why is that?

BG Sentinel traps are among the most popular
traps for sampling nuisance biting mosquitoes
like Asian tiger and yellow fever mosquitoes.

Well, mosquito surveillance isn't easy. It's time consuming, requires expensive sampling equipment and specially trained personnel.  Disease surveillance means sending mosquitoes to an outside laboratory or else conducting expensive, highly technical lab tests. So, it's no surprise, really, that few pest control companies include monitoring services, beyond perhaps backyard inspections for breeding sites.

Yet, monitoring remains important to any PMP wanting to implement an IPM/IMM program.  Without monitoring you don't know what mosquitoes are biting, what diseases are circulating, and when human risk is highest.  So what's a mosquito technician to do?

Good news. Many counties, mosquito abatement districts and cities have area-wide mosquito surveillance programs.  While not necessarily focused on your customer's property, these area-wide programs are often quite good. Some even provide neighbor-hood level detail in their public reports.  They may chart Aedes mosquito numbers collected from traps like the carbon dioxide-baited BG Sentinel trap.  And they may provide both Culex mosquito numbers and disease prevalence from gravid traps.

The gravid trap is an especially powerful monitoring tool.  West Nile carrying Culex mosquitoes are drawn to these traps like cats to catnip. They are typically baited with polluted water known as "stink water." While every health department has their own formula for stink water, it's usually made by steeping grass clippings in water for 2 weeks or so.  The foul smell of the water draws gravid female mosquitoes as they seek stagnant water for egg laying.  Gravid traps help health departments measure the number of potential WNV-carrying mosquitoes present.  Mosquitoes collected in these traps are also collected into groups, or "pools," and sent to laboratories for virus testing. Some health departments publish this data in up-to-the-week reports that can tell you and your customer about their local, seasonal risk for West Nile and related viruses.

Gravid traps consist of a tray with "stink water," with a suction
trap positioned a few inches above the water. Female mosquitoes
drawn to the water for egg-laying are sucked into the trap, where
they are collected, counted and prepared for testing.

If you live in or near one of these health departments you should be able to get useful information about your community's mosquito status by visiting their website. Some health departments even provide weekly email updates on West Nile virus and Zika surveillance. Usually all you have to do is ask to subscribe to these reports.  For those of you in north Texas, the Dallas County Health and Human Services (DCHHS) and Tarrant County Public Health departments offer some of the best mosquito epidemiology reports I've seen.  To see a sample report, click here.

There's a lot of data in these reports, but some of the key things I look for include the following.  Where in my community has WNV been detected in mosquitoes? How abundant are Aedes mosquitoes (these are the daytime biting mosquitoes most noticed by your customers)? How abundant are Culex mosquitoes (responsible for transmitting WNV to people)?  And how is the Vector Index changing?

A July 2018 graph from Dallas County Health and Human Services shows the average abundance of West Nile virus
(WNV) mosquitoes, and a calculated Vector Index (V.I.) in both 2012 and to date in 2018.  A V.I. greater than 0.5
indicates high risk of a WNV epidemic, and may serve as a threshold for aerial spraying against mosquitoes. 
The blue dotted line is the V.I. from 2012, the year of the most severe WNV outbreak in Dallas with over
400 cases and 20 deaths. This year's data (in red) shows high mosquito abundance, equivalent to 2012,
but a relatively low V.I. suggesting that human risk from WNV is still relatively low.  Graphs like these
provide a weekly snapshot of public health risk due to mosquito-borne disease.

The Vector Index (V.I.) is a mathematically calculated number that combines information about both mosquito abundance and percent of mosquitoes carrying the West Nile virus.  If numbers of mosquitoes are high, but disease incidence is low, the V.I. will show low risk.  If mosquito numbers and disease incidence are high, the V.I. will be high.  The DCHHS considers a V.I. of 0.5 or higher to constitute an unacceptable high risk of mosquitoes. For Dallas county a high V.I. will trigger emergency public health measures, possibly including aerial spraying of insecticides.

Most of your customers will likely not be familiar with health department data, nor will they understand its interpretation. But by taking the time to get access to the data, and understanding its significance, your company can serve as an educational bridge to your community. By letting customers know when disease incidence and mosquito abundance is highest, everyone is reminded of when to take special steps to avoid mosquito contact, and the importance of their own mosquito control.

It's often said that information is power. Whether you include residential mosquito control as a special service, or simply an add-on benefit to your regular customers, you owe it to them to be informed about what mosquitoes are doing in your community.  A call to your local health department can provide you with a wealth of public health data, that can make you more a powerful advocate for your clients.

Some Texas and national WNV and mosquito information can be accessed online at:

• Dallas County: http://www.dallascounty.org/department/hhs/epistats.html
• Tarrant County: http://access.tarrantcounty.com/en/public-health/epidemiology-and-health-information/health-data-and-information/west-nile-virus/2017-arbovirus-season.html
• Harris County: http://publichealth.harriscountytx.gov/Resources/Mosquito-Borne-Illness
• Travis County: http://www.austintexas.gov/department/vector-control 
• National (Centers for Disease Control--will not be as timely as local reports, but offers national insights) https://www.cdc.gov/westnile/statsmaps/preliminarymapsdata2018/index.html 

West Nile virus risk high in north Texas

Residual insecticides applied via backpack mist blower
sprayer can provide 3-4 weeks of mosquito control  during
times of peak mosquito activity.

July and August are typically the months of highest risk from west Nile virus, and true to form the past few weeks Dallas and Tarrant counties have seen a major increase in not only mosquitoes themselves, but infections within the mosquitoes.

After the major outbreak of WNV in 2012 in north Texas, some health officials made a decision to use something called the Vector Index (VI) as a form of threshold to ramp up mosquito control efforts.  Based on when human cases started to soar in 2012, and on suggestion from the CDC, a VI of 0.5 was determined to be a good threshold to consider going from ground based spray efforts to aerial spraying.

Two weeks ago the VI exceeded that threshold in both Dallas and Tarrant counties.  Both counties publish very interesting reports, available to the public, that include graphs to show  the latest mosquito counts and VI numbers.  To see the trends in Dallas and Fort Worth areas, check out the graphs below.  In the first graph, the Vector Index is the heavy red line.  Last week it exceeded the 0.50 threshold, although there was a drop this week. Note also the numbers of mosquitoes this summer (red bars) compared to average trap catches in 2012 (for the past four weeks, higher than 2012 averages shown by the blue bars). In Tarrant County (Fort Worth and surrounding communities) the VI (green line with triangle points) was likewise up last week, over 0.60 (new data is not yet published).  Note that the most recent 1-2 data points are preliminary estimates and may change as all the data is calculated.

These data are why there is discussion about aerial spraying this week.  In 2012 the number of human cases of WNV in Dallas county reached almost 400, and there were 19 deaths attributed to WNV. Serious business. Last week DCHHS issued a health advisory to the public, and this week the Dallas County commissioners voted to authorize the health department to prepare for possible aerial spray operations should conditions warrant.  

Where does all this leave the PMP who provides residential mosquito control service?  Municipal mosquito spraying actually complements, rather than replaces, mosquito control work on the ground done by professionals.  Aerial spraying generally provides better coverage of the tree canopy where WNV carrying mosquitoes (Culex quinquefasciatus in north Texas) live and mostly feed. Municipal truck mounted ULV sprays provide some control of lower level mosquitoes (Aedes mosquitoes that potentially transmit Zika and dengue fever, among other diseases), but they typically do not provide high level control in backyards or areas protected from spray coverage.  In that sense, the best control of Aedes mosquitoes is accomplished by your boots on the ground, looking for and treating or eliminating mosquito breeding sites, treating doorways, and treating shrubbery and other mosquito resting sites that are difficult to reach from the street.

As you and your technicians visit mosquito control customers this summer, keep in mind that you carry some of the most effective tools in the war against mosquitoes.  This summer, with Zika fears and WNV threats, what you do is more important than ever.