Beyond Catch Basins: Managing Culex Populations

Catch Basins and West Nile Virus Transmission

In urban environments, road-side catch basins are common larval habitats of Culex species mosquitoes and important targets of larval control in areas subject to West Nile virus (WNv) transmission. After WNv was first discovered in the US (New York) in 1999 with subsequent migration West across the US, hundreds of mosquito abatement districts around the US have invested and instituted operational programs that focus on controlling Culex spp. in catch basins/storm drains in addition to open, defined, polluted water bodies where Culex larvae can be found.

As such, authors from Emory University and the University of Georgia, in a study recently published in the Journal of Medical Entomology (McMillan et. al. 2018)1, investigated the impact of stand-alone catch basin treatments around urban parks in Atlanta, Georgia over two separate seasons. Treatments were done in parallel with WNv surveillance using gravid traps and aspiration of adults resting in catch basins. While larviciding led to > 90% reductions in Culex spp. larval and pupae in treated sites during the treatment periods, they did not observe significant reductions in Culex spp. collections in gravid traps or in adults collected resting in basins. In addition, WNv infection prevalence in Culex spp. mosquitoes was similar between treated and untreated sites.

Road-side catch basins are common larval habitats of Culex species mosquitoes and important targets of larval control in areas subject to West Nile virus transmission.

What happened? Potential for migration, insecticide resistance to space spray adulticides, trap bias, etc. could all have contributed to the outcomes observed in this study. However, it does raise the question, “Even in the most successful catch basin management programs, are we addressing the fundamental goal of reducing overall WNv vector populations and subsequently, reduced risk of WNv incidence within a given area?”

Even in the most successful catch basin management programs, are we addressing the fundamental goal of reducing overall WNv vector populations and subsequently, reduced risk of WNv incidence within a given area?

Management of Urban Culex Larval Habitats – More Diverse than Operationally Assumed?

Control of WNv vectors is a complicated endeavor, especially in urban areas where larval sources of Culex spp. vectors are both ubiquitous and diverse. Over the course of the 20 years since West Nile’s appearance in the U.S., vector control professionals across the country have gained valuable experience in effectively managing the visible and treatable Culex sources such as street catch basins, stormwater retention facilities, wetlands, and waste lagoons. Even hidden underground storm drainage sources have been managed with innovative aerosol application technologies that distribute larvicides throughout the underground systems. What difficult larval sources remain tend to be backyard containers beyond the capacity of inspection crews to identify and treat by hand.

Sewage Systems

These highly polluted habitats contain sewage or mixed sewage and rainwater. These habitats include cess pits, septic ditches, septic tanks, and blocked sewerage systems.

Drainage Systems

These habitats include catch basins, channels, roadside ditches, and underground storm water/drainage systems.

Open Habitats

Surface-water habitats that become stagnant and enriched to support Culex larval development, such as swamps, marshes, bogs, rice fields, and pastures.

Complicating matters, many vectors of WNv have developed resistance to pyrethroid insecticides, which are the front-line defense whenever virus activity is confirmed in mosquito pools in a community. Further evidence suggests that these vectors may be also developing resistance to organophosphates, which are heavily relied upon for aerial responses to elevated WNv risk.

Are Small, Cryptic Larval Habitats Significant Drivers of Culex Populations?

Unlike Aedes aegypti where there is published evidence regarding contribution of small, cryptic, numerous larval habitats to a given area’s overall Ae. aegypti abundance, there is little in the scientific literature that investigates at how these types of habitats contribute to a given area’s Culex populations. Factors contributing to this investigative challenge are oftentimes the difficulty for operational programs to access and evaluate sources on private property as well as general ecological variances that can occur within a small area.

For WNv vector control programs which thoroughly larvicide their traditional known and accessible larval habitats and adulticide in response to virus detection, important questions remain.

  • Where is the residual adult Culex population coming from?
  • Are small, cryptic larval habitats that are not reached by traditional treatments driving the problem?
  • Are “backyard” containers the last mile in WNv vector source control?

Compounding this issue in 2019 has been the high historical rainfall totals across the country which has consistently flooded these small, hidden potential habitats, increasing the potential for high Culex populations.

Container Sources

These habitats are generally man-made sources that will catch and hold water, such as waste tires and buckets.

West Nile Virus – Preliminary Maps & Data for 2019

CDC stated that as of September 17, 2019, a total of 46 states and the District of Columbia have reported West Nile virus infections (people, birds, or mosquitoes) in 2019. Overall, 468 cases of West Nile virus disease in people have been reported to CDC.

Cryptic Containers – Important Considerations to Understanding WNv ecology
A Management Gap?

In a paper entitled, “Larval development of Culex quinquefasciatus in water with low to moderate pollution levels”, published in the Dec 2015 edition of The Journal of Vector Ecology, the authors indicated that the continued maintenance of low level Culex populations is important because it establishes population reservoir that would allow populations to dramatically expand when high-nutrient habitats appear (as would be the case with tremendous rainfalls experienced in the United States entering the later summer where standing water would stagnate). The authors further claim that:

“WNv risk can remain even in the presence of larval habitats with low to moderate nutrients, which are more common in the landscape than the levels characteristic of combined sewer overflows. The numbers of Culex mosquitoes around residential areas might be maintained with standing water in containers such as flower pots, wheelbarrows, gutters, buckets, old tires, pool covers, pet water dishes, discarded tires, and bird baths.”

As such, it’s prudent that as mosquito abatement professionals move into the later-half of 2019 after a historical rain fall year giving consideration to larval sources beyond larger, polluted, stagnant water bodies and catch basins. A strategy to manage small, cryptic containers that can serve as potentially disease amplifying reservoirs should be part of the approach.

Historic Rainfalls – Expansion of Small, Cryptic Larval Habitats?

On July 9th, the National Oceanic and Atmospheric Administration stated that for the third consecutive time in 2019 (April, May and June) the past 12-month precipitation record had hit an all-time high. From July 2018 through June 2019, there was a new 12-month precipitation record of almost 38 inches according to NOAA.2

One can certainly say “it’s wet” in the United States. In fact, it was reported by the National Weather Service at the end of May 2019 “there had never been a wetter 12 months than the period that recently ended”, since such records have been tracked over the last 124 years. During that same period, the continental United States averaged 6 inches of precipitation above average during the one-year period.

The map from NASA’s Earth Observatory site indicates rain fall totals in May 2019. The colors depict the amount of groundwater on May 13, 2019, compared to all Mays from 1948 to 2012. Blue areas have more abundant groundwater than usual for the time of year, and orange and red areas have less.

1 McMillan, J. R., Blakney, R. A., Mead, D. G., Coker, S. M., Morran, L. T., Waller, L. A., … & Vazquez-Prokopec, G. M. (2018). Larviciding Culex spp.(Diptera: Culicidae) Populations in Catch Basins and Its Impact on West Nile Virus Transmission in Urban Parks in Atlanta, GA. Journal of medical entomology, 56(1), 222-232.
2 U.S. Has Its Wettest 12 Months on Record – Again https://www.noaa.gov/news/us-has-its-wettest-12-months-on-record-again/