Mosquito Control with Pesticides
The recent crisis of West Nile encephalitis that first occurred in New York City and later was reported in surrounding areas has necessitated some type of program to control mosquitoes, the insect vector of this disease.
What you can do
There are a number of things that residents can do to lessen the pressure of mosquitoes in their area such as providing good drainage and not allowing water on their property to become stagnant. However, especially when nature provides weather conditions that promote mosquito breeding, even the best practices are not enough to control mosquitoes to acceptable levels.
Programs in Nassau and Suffolk County
Controlling the problem early
Both Nassau and Suffolk Counties have excellent programs for monitoring mosquito populations and detecting potential viruses that these insects carry. When high insect populations are detected early in their life cycle, pesticides such as Altosid (Methoprene) which is an insect growth regulator can be used to control the larva. This material is one of the least toxic pesticides for warm-blooded animals because it effects the growth stages of the insect (the larva cannot pass from one instar (growth stage) to the next. It is not effective at all in controlling the adult stage of the mosquito, which of course is the flying and biting stage.
Controlling the adult
For the control of the adult stage of mosquito, there are only a few options. The pesticides used in the fogging machine are Scourge (resmethrin) or Anvil (sumethrin) which are synthetic pyrethroids. These are among a number of pesticides that have been developed that are very similar in chemistry to the pyrethrum that is an extract of a chrysanthemum. These have a toxicity level very similar to the malathion.
Malathion
Malathion is a general use pesticide that has been the choice in many community insect control programs for the Mediterranean fruit fly in areas like California, and mosquitoes in many residential areas especially where diseases such as encephalitis is present.
There are several reasons that malathion is being used:
It has a relatively low mammalian (warm-blooded animal) toxicity.
It has a relatively short residual in that it breaks down into nontoxic components quickly.
It also has the capacity to temporarily stimulate flying of the adult mosquito making it more effective.
It is used at relatively low rates.
Understanding Toxicity Rates
All pesticides are measured by what is called LD50 (Lethal Dose 50). That is the amount needed to kill 50% of the test animal, usually rats or mice. A very toxic chemical that requires low doses to kill the test animal has a low LD50 while a less toxic chemical would rate a much higher LD50.
Toxicity of Malathion
Malathion that was fed to animals is rated at about 5500 mg/kg (milligrams per kilogram or parts per million of body weight) compared to sodium borate (Borax) with a rating of 2700. Remember, the higher the LD50 the less toxic the chemical. Of course you would not want to eat either of these chemicals but it does give a reference point. Although it is low in toxicity to warm blooded animals, it is fairly toxic to some species of fish at rates higher than those used for mosquito control.
What is Malathion
Malathion is a non-systemic, wide spectrum insecticide. It is classed as an organophosphate insecticide was introduced in 1950, so it has a long history of use. It is used primarily to control sucking and chewing insects on fruits and vegetables and for mosquitoes, flies, household insects, animal parasites and body and head lice. As an organophosphate it kills the target by attacking the nervous system. That is why exposure to great amounts of the chemical will cause the typical symptoms of nervousness, nausea, headache, tightness in the chest, vomiting, other involuntary movements, etc.
Rates of Use
Malathion is used at the rate of 3 ounces per acre for mosquito control and has a half life of six days under normal conditions. It does not accumulate in the body as DDT did. There have been cases of poisonings where humans have been exposed to this pesticide at high rates and over long periods of time. For this reason, everyone should take common sense precautions when exposed to the material.
Safety Precautions
If you come into contact with malathion as with most other chemicals, wash your skin with plenty of soap and water, the same for your clothes.
Avoid putting things into your mouth that have been sprayed.
Avoid breathing the fumes from the sprayer. Malathion is a chemical that contains mercaptan (sulfur) and has a noxious odor. This is an advantage because if you are exposed to it most people can smell the material readily.
Not all people react the same to the exposure. As with many types of exposure, people that are asthmatic, have a compromised immune system, the elderly and the very young may need extra precautions to avoid any complications.
Conclusion
At the rates used in the mosquito program, malathion is one of the most effective pesticides to control the adult mosquitoes and with proper precautions, problems should be minimal. The West Nile encephalitis is not as dangerous as several other virus diseases but there have been deaths attributed to this disease and our Health and Vector Control Departments are to be applauded for the constant monitoring and rapid response in protecting the health of our residents.
10/99 Prepared by : William Sanok, Agriculture Program Director at Cornell Cooperative Extension in Suffolk County and chairman of the Suffolk County Pest Management Committee.
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