Where is West Nile virus this year?

If it seems you're hearing less about West Nile virus (WNV) this summer, you may not be imagining it.  Although mosquitoes have been abundant in north Texas this year, for some reason the virus has remained relatively quiet.

Where has WNV gone?

A paper written by epidemiologist Dr. Wendy Chung and colleagues in 2013 may offer some insights on the absence of the virus this summer. Those of us who lived in Dallas in 2012 may remember that summer as the worst human outbreak of WNV ever.  Nearly 400 cases were reported in Dallas County alone, and 19 people died of the disease. The epidemic was so bad that Dallas county resorted to spraying the entire county for mosquitoes by plane--something not seen in north Texas since an encephalitis outbreak in 1966.

Chung and colleagues charted the course of the disease during 2012 and saw high infection rates of mosquitoes early in the summer, followed by a rapid increase in human cases. Looking back over previous years and case numbers, the researchers concluded that an unusually mild winter followed by rainfall patterns ideal for mosquito breeding in the spring (and a very hot summer--West Nile virus multiplies quickly in mosquitoes at higher temperatures) created ideal conditions for an outbreak.

So what's different about 2019? We had a relatively mild winter, with only three days at or below 28° F, and a wet spring--both conditions mosquitoes love. But the summer, at least by Dallas standards, has so far been cool.  Until this week, the DFW Airport weather station saw only two days over 100° F. By the end of July the area usually has experienced more than seven days over 100° F.

These graphs show 2019 mosquito abundance and Vector Index (V.I.) estimates compared to previous years. Although mosquito numbers are high this year, the V.I. has remained low for both Tarrant (=Fort Worth-top) and Dallas counties (bottom). In 2012 the V.I. exceeded the danger level of 0.5 for multiple weeks (blue dotted line). Source: Tarrant County Public Health and Dallas County Health and Human Services.

Predicting WNV

One of the tools used by health departments to predict disease risk for WNV is a statistic called the vector index (V.I.).  The V.I. is calculated weekly from mosquito trap data, and combines information on both average abundance of Culex quinquefasciatus (the main carrier of WNV) and disease incidence in the trapped mosquitoes.  A V.I. of 0.5 or higher for two or more weeks is considered a crisis indicator by some health officials.

The graphs shown here are provided by epidemiologists in Dallas and Fort Worth, and show both mosquito abundance and V.I. estimates for both counties. Despite higher mosquito numbers, the V.I. hasn't ventured above 0.1 for either Dallas or Tarrant counties this summer. Most of the season the V.I. has been closer to zero, hence less need for mosquito spraying and fewer people getting sick. In Dallas county this year there have been no human cases of WNV. Tarrant County (Fort Worth) reports only one case this year with a very low V.I., near zero most weeks (top graph).

According to statistics from the Texas Department of State Health Services, low WNV incidence seems to be true for the whole state this year with no reported human cases as of the end of July.  Harris County (Houston) also reports a light year for WNV, according to the acting director of Mosquito and Vector Control, Chris Fredregill.

Looking Ahead

With this week's string of 100° days in many areas will risk go up?  Certainly West Nile virus remains a threat to all of us through the end of the summer and into the fall; but this late in the season the chance of a major outbreak is probably low. On the other hand, hot weather favors the virus. It's no time to forget about mosquitoes. I expect Aedes mosquitoes (yellow fever mosquito and Asian tiger mosquito) to become more abundant after last weekend's rains.  This week is a good time to get out and dump standing water.  Although Aedes mosquitoes are not major disease risks, they cause most of the itchy mosquito bites we get during the day--and we don't want that.

Why Surveillance Reports?

Integrated pest management is just as relevant for mosquito control as it is for all other forms of pest control.  One of the principles of IPM is to base treatments on pest numbers.  Because mosquito monitoring is expensive and requires special expertise beyond what most PMPs possess, few companies monitor mosquito numbers or disease. However, high quality data may be available from your local health authorities, depending where you live. A pest management company can use this data to alert customers to times of higher disease risk and changes in mosquito abundance.

Every community's mosquito situation will be different.  If you are doing business in a larger metropolitan area, or a mosquito control district, you may have access to the kind of data shown here. To find out, contact your local or regional health department and ask if they provide reports of mosquito abundance and disease prevalence.

In Dallas, weekly reports may be obtained by emailing Epidemiology@dallascounty.org and requesting to subscribe to the weekly Arbovirus Surveillance Report. For Tarrant County, email RWHill2@tarrantcounty.com and request to receive the Arbovirus Surveillance Report Weekly.  Unfortunately, not all counties have equivalent reporting systems. Harris County provides mapping of areas with virus detection.  And the Texas Department of State Health Services provides weekly reports throughout the summer for the whole state.

An additional source of information for both PMPs and your customers is the Mosquito Safari website. At the Safari you can take a virtual tour of a field and a backyard and learn important facts about mosquitoes. 

If you need more intensive training, our Extension medical entomologist, Dr. Sonja Swiger, is offering classes this year for pesticide applicators wanting to prepare for their Public Health (Category 12) license.  In the fall she also offers several 3-day Master Vector Borne Disease Management Courses around the state.  To learn more, or to register, go to https://livestockvetento.tamu.edu/workshop-registration/ .

Getting to know the Turkestan cockroach

Cockroaches have historically been a top pest and reliable source of business for PMPs.  Indeed, from day one most new technicians are taught to recognize the four most commonly encountered species:

• the German cockroach--one of our smaller cockroaches, bane of restaurants and homes
• the American cockroach (a fast and intimidating insect that looks twice as big as it really is when running across a floor or flying)
• the black, rather nasty Oriental cockroach--pest of sewers and the grounds around buildings
• the smoky brown cockroach, an outdoor cockroach unafraid to venture into homes. 

Other common species, depending on your part of the country, include the Australian, brown, brown-banded, Asian and field cockroaches.

Figure 1. The adult female S. lateralis (A) and adult female Oriental
cockroach (B) are similar in size and color. Arrow points
to the distinguishing light marking on the forewing margin
of the Turkestan cockroach. Photo modified from Kim and
Rust (2013).

In many parts of the country pest management professionals need to add the invasive Turkestan cockroach (Shelfordella lateralis Walker) to their watch list. This Asian invader is quickly making a name for itself and moving through Texas and other states.

Because the Turkestan cockroach looks similar to other species, you may already have seen it and not realized it was something new. Female Turkestans look like Oriental cockroaches. Male Turkestans look like small American cockroaches or perhaps an innocuous field roach.

Spread

The Turkestan cockroach has becoming a significant new pest since it was first reported in Shelford, California in 1978 and El Paso, Texas in 1979.  It has since spread through Arizona and New Mexico, across Texas and even to Georgia.  This week I got my first north Texas specimen, and tentatively identified an emailed photo from Tennessee as a Turkestan cockroach.

Figure 2. To my knowledge, this is the first Turkestan cockroach
recorded from north Texas. Collected from a hotel in Frisco,
TX (VII-29-2019). Note the pale, almost transparent border
at the margin of the forewing, and the size (28 mm/one inch),
which is smaller than a typical American cockroach. Photo
M. Merchant.

If you think this pest won't reach your area soon, think again. Internet commerce is also at work. Turkestan cockroaches are commonly sold online where they are well known in the pet trade as "red runners." They provide food for reptile, amphibian and small mammals. Pet owners like the fact that Turkestan cockroaches breed quickly, do well in captivity and don't climb glass (so are easy to keep in aquaria).

My first North Texas specimen of a Turkestan cockroach came this week courtesy of Emory Matts, with Rentokil Steritech.  Guests at a local hotel recently started complaining of roaches on several floors. Whether this was an invasion from outdoors (males can fly and are attracted to lights at night), or represented an indoor infestation could not be determined.  Though it's often referred to as an "outdoor" insect, the Turkestan is capable of establishing itself indoors, similar to Oriental and American cockroaches.

Competition

Figure 3. Turkestan (A) and Oriental cockroach nymphs.
Notice the reddish-brown thorax and dark abdomen of
the Turkestan nymph compared to the uniform brown color
of the Oriental. Photo from Kim and Rust (2013).

According to Kim and Rust (2013), the Turkestan is replacing the Oriental cockroach throughout much of the Southwest as the most important cockroach pest around the outsides of structures.  Common breeding sites are similar to those of Oriental cockroaches, including water meter and irrigation boxes, electrical boxes, hollow block walls, cracks and crevices in concrete, compost piles and potted plants.  However, it appears that in the warm climates of the Southwest, Turkestan cockroaches rush through their 5 nymph stages faster than the Oriental with its 7-10 nymph stages.  A female Turkestan cockroach will produce about 25 oothecae (egg cases) in her lifetime compared to 5-10 oothecae for an Oriental cockroach.  The numbers tell the story as to why Turkestan cockroaches are taking over.

Distinguishing Turkestan cockroaches

The immature Turkestan roach resembles both Oriental and American nymphs in general appearance (Fig. 3).  The Oriental cockroach, however, is uniformly dark-brown and the American cockroach is uniformly reddish brown.  The Turkestan, in contrast, is reddish-brown on the head and  thorax (pro- and meso-thorax) and dark-brown only on the rear of the body.

Figure 4. American (top) and Australian cockroaches. 
Note the bold markings on pronotum and the
forewings of the Australian cockroach, in contrast
with the American, which lacks forewing markings.
Photos, M. Merchant.

Besides size (the male Turkestan cockroach is smaller), American and Australian cockroaches can be distinguished by both the wing borders and markings on the prothorax (shield behind the head)(Fig. 4).  Forewing margins of the Turkestan cockroach are pale and almost transparent compared to the uniform-colored wings of the American, and the bold yellow margins of the Australian. Click here for another image of male and female Turkestans.

Control

Research on control methods for the Turkestan cockroach is still limited; however control methods should be similar as for the Oriental cockroach. Granular cockroach baits can be effective; however, Kim and Rust suggest that higher reproductive and growth rates for the Turkestan cockroach could mean that technicians should need more bait when treating.  Special attention should be paid to pest proofing doors and other building entry points to keep cockroaches outdoors where they pose the least trouble.

Reference:

Kim, T. and M. Rust. 2013. Life history and biology of the invasive Turkestan cockroach (Dictyoptera: Blattidae). J. Econ. Entomol. 106(6): 2428-2432.