When the flood waters recede from areas that have never been flooded before, there is likely to be pools and puddles of standing water left in places where we have never seen them. The abundance of standing water in stagnant puddles, pools and ditches will add to the flood cleanup misery by enormously increasing the number of mosquitoes from their already high population levels. Mosquito larvae that were in puddles and ditches before the flood have been flushed from the area. The next problem will come from new mosquito larvae that developed from eggs laid since the flood. The timing for the emergence of the next crop of mosquitoes is 5 to 10 days after the running water recedes and stagnant puddles form.
How much effort toward mosquito control is practical in the aftermath of the flood? This is a difficult question with no single, correct answer. Eliminating mosquito breeding sites is always desirable and is usually accomplished by filling or draining standing water sources. However, with such an over abundance of breeding sites, it may not be possible to eliminate enough standing water to make a difference in the mosquito population, especially in the next week when other, higher priority activities are commanding the attention of city officials and homeowners impacted by the flood. And because mosquitoes can migrate several miles after they emerge from their breeding site, a wide area puddle elimination project would be required.
Pesticides are available for treating standing water for mosquitoes. However, for the same reasons as just mentioned, it may not be practical to begin a larviciding effort that can treat only a small portion of the total number of breeding sites. An area wide abatement program might make a noticeable difference, but few municipalities or counties will have the resources and experience to accomplish this.
Weather, rather than management activities, may be the deciding factor in how bad the mosquitoes get. An occasional heavy rain that would flush water puddles and pools would reduce populations; frequent light rains would keep puddles full of water and mosquitoes. As with so many things right now, sunshine and gentle breezes would be greatly appreciated.
Fogging for adult mosquitoes is possible, but again, probably not practical nor extremely effective. Wide area treatment would be required for any long term, noticeable impact, and frequent fogging would be required to keep up with mosquito emergence and migration. Fogging by individuals or municipalities might be practical for short term relief for a special event but not for long term control. Special certification is required of commercial pesticide applicators making mosquito treatments.
Mosquito control was discussed in the Horticulture and Home Pest Newsletter, May 26, 1993, page 81 , and mosquito repellents on June 30, 1993 , page 107. Repellents remain our most effective way to cope with the annoyance of mosquitoes. Use repellents only according to label directions, paying special attention to safety precautions.
Encephalitis. Health officials and entomology research specialists have agreed that there is little threat of an encephalitis outbreak accompanying any post-flooding increase in mosquito populations. Encephalitis is an inflammation of the brain and nervous system caused by a virus that is transmitted by mosquitoes from infected birds to people. The species of mosquito we will notice following the flooding is not an effective vector of the virus and the virus is not likely to build up to dangerous levels this late in the summer.
This article originally appeared in the July 21, 1993 issue, pp. 1993 issue, pp. 122-123.