Throughout history, mosquitoes have tormented humans in their search for blood protein necessary for egg development. Annoyance is one aspect, but when discoveries were made linking mosquitoes to various human pathogens such as viruses and parasites, an organized battle against mosquitoes was launched to protect public health. This battle continues today throughout the world.
One of the many jobs conducted by the California Department of Public Health and the dozens of local mosquito and vector control agencies statewide (and nationwide) is public education in an effort to reduce potential mosquito habitat in urban environments. Species of mosquitoes that transmit viruses such as West Nile virus thrive in artificial habitats with standing water in residential backyards (e.g., neglected pools, bird baths, buckets), commercial/industrial properties (e.g., swales, basins, belowground BMPs), and city infrastructure (e.g., street gutters, catch basins, and storm sewers).
Public education on this subject can be frustrating. A number of problems emerge, including complacency, defiance, and finger-pointing; however, misinformation has become one of the biggest challenges. This leads to one of the most misunderstood topics: mosquitoes and belowground stormwater structures.
On April 29, 2010, a widely circulated cartoon arrived in e-mail boxes throughout the country. The cartoon and the associated web-based blog contained misinformation about mosquitoes and mosquito breeding habitat. Although we do not believe this was intentional, the following points were made in response to contents of the cartoon and blog in an attempt to provide the best possible information based on published science.
Cartoon Claim: “Mosquitoes use sight to locate shallow water breeding grounds.” False.
Answer: Adult mosquitoes primarily use smell to locate suitable places to lay their eggs, which explains how certain species can find belowground sources of standing water.
Cartoon Claim: “Mosquitoes need protection in suburban environments, and love damp forests.” Partially true.
Answer: With 53 mosquito species in California and over 200 nationwide, habitat preferences vary widely. Adult mosquitoes do seek protection in vegetation, including trees, but manmade structures such as homes and stormwater infrastructure (including belowground BMPs) can provide shelter as well.
Cartoon Claim: “They prefer enough light to detect potential predators in the water.” False.
Answer: Adult female mosquitoes have the ability to detect certain potential aquatic predators before they lay eggs. Mosquito larvae detect potential predators using a variety of senses (including shadows during the day), but do not rely solely on light. In fact, in belowground habitats mosquito larvae often live in complete darkness.
Cartoon Claim: “Mosquitoes aren’t likely to find underground BMPs that hospitable—especially with the captured oil that rests on the water’s surface.” Rarely true.
Answer: Extensive monitoring of belowground BMPs in California has shown that 1) the vast majority of these structures that contain standing water also produce mosquitoes, 2) the buffered temperatures (i.e., cooler in summer and warmer in winter) allow mosquitoes to breed for longer periods than their aboveground counterparts, 3) presence and quantity of oil is rarely sufficient to prevent entry of adult mosquitoes (except in certain specialized oil-capturing devices), and 4) oil sheens on the water surface are rarely uniform and plenty of gaps exist for mosquito larvae to obtain surface air.
Blog Statement: “I vary the depths of permanent pools, creating some areas that will attract natural predators of mosquitoes such as fish, frogs, birds and dragonflies.”
Answer: This concept is good, but there are many myths regarding predators of mosquitoes. Certain species of top-feeding fish are voracious predators of larvae and can do an excellent job of controlling populations (e.g. Gambusia mosquitofish), but others species of fish may not provide a strong level of control. There are few other larval predators that can control populations adequately (i.e., from a public health perspective) in habitats conducive to mosquito production. Regarding adult mosquitoes, there are no known effective predators. Frogs, birds, dragonflies, and bats are often cited and encouraged as “natural mosquito controls,” but mosquitoes constitute only a tiny portion of their diet, making their impact undetectable.
Blog Statement: “We're lucky in Canada and the US; due to our temperate climate and access to technology, we are protected from common mosquito-borne diseases such as malaria and yellow fever.”
Answer: Organized mosquito control, modern medicine, housing, and lifestyle were responsible for the eradication of certain diseases such as malaria and yellow fever from Canada and the USA. Unfortunately, the temperate climate was never a barrier to these diseases, as they historically ranged widely in North America. The risk of reintroduction is a constant concern, and does occur along the US-Mexico border intermittently.
Mosquito production is an unfortunate and unintended consequence associated with the installation of many types of stormwater treatment BMPs, above- and belowground. Most people outside the mosquito control arena do not realize that belowground sources of standing water often represent the largest sources of mosquitoes in urban environments and are among the most difficult to control. It’s good to see that the mosquito problem is being discussed in the stormwater community; however, it is imperative that misconceptions are not perpetuated. This response was prepared in the spirit of dispelling some of those misconceptions. I strongly recommend everyone in the stormwater community consider collaborating with local mosquito and vector control agencies to help combat these persistent insect foes. Let’s work together to protect people from mosquitoes and the diseases they carry.