Letters to the Editor

Surface Hydrocarbons vs. Mosquito Breeding

Editor:
Stormwater BMPs as a source for vectors is not a new issue (March/April 2002 Stormwater, “The Dark Side of Stormwater Runoff Management: Disease Vectors Associated With Structural BMPs” and “Stormwater, BMPs, and Vectors”). It has been suggested that because of the accumulation of hydrocarbons (e.g., motor oil, ethylene glycol, gasoline) in BMPs, standing water would be rendered unsuitable for larvae. The hydrocarbon film would prevent the larvae from being able to breath through specialized mouth parts. Is this true? How much oil is adequate? Was this observed during your survey of any of the BMPs?
Walter K. Caldwell
Environmental Specialist
Environmental Health Administration, Watershed Protection Division
Washington, DC

Two of the coauthors of the articles respond:
As a general rule, the accumulation of hydrocarbons on the surface of standing water in BMPs does not provide reliable mosquito prevention. However, to best answer this question, we first need to review some basic mosquito biology.

The life cycle of mosquitoes involves a process known as complete metamorphosis. This describes a process of dramatic change from egg to immature (larvae and pupae) to adult, where the immature stages do not even remotely resemble the adult stage. Perhaps the most well-known complete metamorphosis occurs in butterflies and moths when they change from eggs to caterpillars to winged adults. A pupal stage occurs between the larval and adult stages during which changes in physiology and morphology take place. When complete, adults emerge from the pupal skin and carry on life as sexually mature insects.

Although best known for the females’ need to feed on blood, mosquitoes spend most of their life as wingless immatures. Adult female mosquitoes lay their eggs in carefully selected locations either on the surface of standing water or in areas subject to flooding. After they hatch, the immature stages (larvae and pupae) are completely reliant on water. Larvae feed on microorganisms and organic material in the water and eventually develop into pupae, which are also aquatic but do not feed. Adults then emerge from the pupal skin onto the water surface from where they take flight, mate, and start the cycle over again. There are currently 176 recognized species of mosquitoes in the United States. Each has a preferred or specific habitat type.

Aquatic stages of nearly all species of mosquitoes breathe atmospheric air through specialized body structures called “siphons” in larvae and “trumpets” in pupae. These breathing structures function much like a diver’s snorkel: They are essentially hollow tubes that work by breaking the surface tension of the water and allowing air to enter the body. This is one of the main reasons mosquitoes require relatively tranquil standing-water habitats. Wave action, turbulence, or significant currents prevent mosquitoes from maintaining a connection with the water surface to breathe. This critical water-to-air connection needed by immature mosquitoes was recognized early on by mosquito control experts as a vulnerability that could be used in integrated control efforts. Hydrocarbon surface films, such as kerosene, were found to interfere with the immature mosquitoes’ ability to connect with the water surface, causing them to drown. There are several commercially available materials used today for professional mosquito control that work on this basic principle; one is a petroleum oil-based material and the other is classified as a monomolecular film.

However, oils that accumulate in sumps, catch basins, and vaults of BMP devices do not provide reliable mosquito prevention. Oily sheens present on the water surface are rarely uniform and usually contain a multitude of “breaks” through which mosquito larvae can access surface air. The La Brea Tar Pits, in western Los Angeles County, form natural ponds that produce mosquitoes despite the fact that crude oils seep into them from belowground sources. Likewise, oil-contaminated wastewater sumps in oil fields are often major mosquito breeding sources. Manmade habitats in storm sewer systems including catch basins and, more recently, in stormwater BMPs also frequently provide usable habitat for certain mosquito species despite the presence of oils. Unfortunately, the mosquitoes most likely to utilize “dirty water” are in the genus Culex and are both public nuisances and competent vectors of viruses, including St. Louis encephalitis and West Nile virus.

It should be concluded, then, that although “runoff-derived” hydrocarbon accumulations in stormwater BMPs, sumps, and other structures might occasionally inhibit or even prevent mosquito breeding from taking place, the efficacy of such accumulations in preventing breeding cannot be relied upon with any degree of confidence. Our research studies in southern California clearly support this, as mosquitoes are detected regularly in BMP devices that hold oil-contaminated urban water runoff. We are not aware of any public health or vector control agencies that rely upon these kinds of accumulations to inhibit mosquito production.

Stormwater BMPs, especially those that hold permanent sources of standing water by design, pose a difficult challenge for public health officials and vector control agencies. We feel very strongly that the best solution to the problem of mosquito breeding in stormwater structures lies in fostering cooperation between BMP designers, municipal planners, public health officials, and vector control agencies. It is essential that new stormwater BMP designs incorporate features that suppress or prevent vector breeding and harborage. Through creative engineering we might be able to eliminate or deny access to the habitat that mosquitoes and other vectors need from BMPs: standing water. The state or local public health/vector control agency can discuss specific vector issues in your area and provide input and consultation into siting, design, and maintenance of proposed BMPs.
Marco E. Metzger
Public Health Biologist
California Department of Health Services, Vector-Borne Disease Section
Ontario, CA
Susanne Kluh
Vector Ecologist
Greater Los Angeles County Vector Control District
Santa Fe Springs, CA

SW - January/February 2003