By Elizabeth Willott
The benefits of restoring wetlands are widely appreciated. Wetlands help purify our waters, control flooding, promote biodiversity, and provide aesthetic benefits. The federal government has a policy of "no net loss" of wetlands, and a key way to achieve that goal is to restore or create wetlands. However, there is a downside. Wetlands are generally good habitat for mosquitoes, some of which may transmit harmful viruses (such as yellow fever or West Nile) or pathogens (such as malaria). Malaria and other mosquito-vectored diseases are a brutal form of population control.
Since at least the time of the Romans, people have associated malaria and wetlands and have worked to reduce rates of malaria by draining swamps and marshes.1 Although most people now associate malaria with the tropics, historically malaria was present as far north as England and the Scandinavian countries.
Malaria, yellow fever, and other mosquito-vectored diseases have significantly affected U.S. history. For example, in 1802, Napoleon sent 33,000 men to conquer Haiti and the Mississippi River region. Twenty-nine thousand died of yellow fever. Unable to sustain such losses, France sold Louisiana to the U.S. government in 1803. Malaria reached epidemic proportions in the Midwest up to the latter part of the 1800s; southern states saw epidemics until the early 1900s. Even the desert Southwest suffered: several Arizona territorial Army camps closed due to malaria.
Malaria's decline in the United States and Europe in the late 1800s was due mainly to draining swamps and removing mill ponds, improvements in housing, and isolation of sick people in mosquito-proof areas. Draining swamps also exposed good agricultural land, enabling people to afford better houses and thus isolate the sick. Actuarial statistics for North Carolina in 1910 (the earliest figures available) show annual mortality from malaria was 32.2 per 100,000 people; by 1932, it was 1.6 per 100,000.
With our current standard of living, it is unlikely that malaria or yellow fever could return to historical levels. Still, there is reason for concern. West Nile virus, introduced into New York in 1999, killed 264 people in 2003 and seriously sickened at least 9,862, according to the Centers for Disease Control. Other viruses present elsewhere in the world could similarly be introduced into the United States, as well as new species of mosquitoes. In the mid-1980s Aedes albopictus arrived; in the late 1990s Ochlerotatus japonicus.
In only a few years, a newly opened wetland can have a mosquito problem. Between 1974 and 1988, at least five of nine pilot water treatment plants using aquatic plants closed because of mosquito problems (Eldridge and Martin 1987; Martin and Eldridge 1989). Mosquitoes increased a hundredfold after operation started at a constructed, surface-flow wastewater treatment wetland in Tucson (Karpiscak et al. 2004).
In listening to students and colleagues, I received the impression that introductory courses in biology and conservation biology largely ignore mosquitoes (and other similar potential disvalues in nature). Checking web-posted syllabi for conservation biology courses, I discovered that the most common textbooks were Meffe and Carroll (1994, 1997) and Primack (2000). These texts barely mention mosquitoes or suggest that restoring nature likely will have some negative consequences. Yet both texts discuss the negatives arising from the loss of wetlands.
Mosquitoes are also only cursorily addressed in key professional publications for restoration biologists and others managing or creating wetlands for water treatment. For example, a key reference on wetland construction is Treatment Wetlands (Kadlec and Knight 1996). "Mosquitoes" is not in the index. With searching, one finds mosquitoes discussed on two of the 893 pages. The authors conclude that mosquitoes are not usually much of a problem unless organic loadings are excessive, or bulrush or cattail growth gets too dense, or there is debris (such as floating cattails) on the surface.
In practice, vegetation grows, debris accumulates, and mosquito-eating fish die or cannot be used. I searched three key journals that address wetlands. The Journal of the American Water Resources Association does not include "mosquito" as a searchable term. The one paper I found that came close to addressing mosquito problems said that education may be needed to help neighbors with "perceived vector problems" (Sauter and Leonard 1997, 162).
In Wetlands, fifteen of 254 papers mentioned mosquitoes. Only four of these could be classified as significant for people interested in mosquitoes-they do connect mosquitoes to site design and they cite some relevant papers. But only one of the three potentially relevant papers to which they direct the reader was published in a fully refereed journal, and that is not a wetlands journal but the Journal of the American Mosquito Control Association.
Of the 560 papers published since 1992 in Ecological Engineering, only three address primarily mosquitoes and wetlands; only one is about a U.S. wetland (Thullen et al. 2002). (One does give a useful Australian perspective, however.) In sum, in three key journals with more than 1,000 research papers, only one was devoted to managing mosquitoes in the United States.
Yet entomologists, restoration biologists, hydrologists, and others are actively working to manage mosquitoes. Several journals and conferences target people active in mosquito management. A gap exists. The knowledge of these field scientists is not included in the textbooks about conservation biology, nor do these scientists generally participate in projects to develop or restore wetlands.
One reason for this gap may be that restoration project funding is often short term, and a serious mosquito problem may emerge only later. Applicants also may fear loss of funding if they talk about possible negatives. Yet representatives of funding agencies have assured me that addressing an obvious problem strengthens a proposal.
Steps can be taken. The project can incorporate design elements to reduce the risk of mosquitoes becoming a major problem,2 and mosquitoes can be monitored. Advance planning may stimulate sufficient interest that some people will even volunteer to help.
Novel and better solutions can emerge. For example, mosquito control personnel often suggest adding the fish Gambusia, also called mosquito-fish, to ponds, since Gambusia eat mosquito larvae. However, Gambusia also eat some native fish species, including the endangered Gila top-minnow (which also eats mosquitoes).
Because the Gila top-minnow is endangered, it is illegal for pet stores to sell it and wetland managers who wish to add it face more regulation. Several individuals have been working, however, to overcome the obstacles, and soon, private citizens may be able to introduce Gila top-minnows to their property without incurring excessive legal liability.(3)
I want to close by moving from ecological to social considerations. When we teach, when we work as professionals, or when we go out to enjoy the products of others, we also create and maintain human social environments. We have a responsibility to remember relevant history, see the negatives as well as the positives, and realistically address problems, be they social or biological.
1. To access the literature citations on this and most topics in this article, please see the longer article, Willott (2004), on which this PERC Reports essay is based. Two other key resources are Gillett (1972) and Spielman and D'Antonio (2001).
2. A partial bibliography can be found online:research.biology.arizona.edu/mosquito/Ecology/WetlandBib.html.
3. This Safe Harbor Agreement can be accessed at:arizonaes.fws.gov/ then choose Document Library, then Safe Harbor Agreements.
Eldridge, B. F., and C. V. Martin. 1987. Mosquito Problems in Sewage Treatment Plants Using Aquatic Macrophytes in California. In Proceedings and Papers of the 55th Annual Conference of the California Mosquito and Vector Control Association. California Mosquito and Vector Control Association, Inc., January 25-28, Sacramento, CA, 87-91.
Gillett, J. D. 1972. The Mosquito: Its Life, Activities, and Impact on Human Affairs. Garden City, NY: Doubleday & Company.
Kadlec, R. H., and R. L. Knight. 1996. Treatment Wetlands. Boca Raton: CRC Lewis Publishers.
Karpiscak M. M., K. J. Kingsley, R. D. Wass, F. A. Amalfi, J. Friel, A. M. Stewart, J. Tabor, J. Zauderer. 2004. Constructed Wetland Technology and Mosquito Populations in Arizona. Journal of Arid Environments 56: 681-707.
Martin, C.V., and B. F. Eldridge. 1989. California's Experience with Mosquitoes in Aquatic Wastewater Treatment Systems. In Constructed Wetlands for Wastewater Treatment: Municipal, Industrial, and Agricultural, ed. D. A. Hammer. Chelsea, MI: Lewis Publishers, 393-98.
Meffe, G. K., and C. R. Carroll. 1994; 1997 (2nd edition). Principles of Conservation Biology. Sunderland, MA: Sinauer Associates.
Primack, R. B. 2000. A Primer of Conservation Biology, 2nd ed. Sunderland, MA: Sinauer Associates.
Sauter, G., and K. Leonard. 1997. Wetland Design Methods for Residential Wastewater Treatment. Journal of the American Water Resources Association 33: 155-62.
Spielman, A., and M. D'Antonio. 2001. Mosquito: The Story of Man's Deadliest Foe. New York: Hyperion.
Thullen, J. S., J. J. Sartoris, W. E. Walton. 2002. Effects of Vegetation Management in Constructed Wetland Treatment Cells on Water Quality and Mosquito Production. Ecological Engineering 18: 441-57.
Willott, E. 2004. Restoring Nature, Without Mosquitoes? Restoration Ecology 12(2):147-53.
Elizabeth Willott is assistant professor of entomology at the University of Arizona. This article is adapted from "Restoring Nature, Without Mosquitoes?" in Restoration Ecology 12(2):147-53, 2004. See: research.biology.arizona.edu/mosquito/Willott/Pubs/Restore.html.