Tuesday, July 24, 2018

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: