Pesticide Information

How do larvicides affect the environment?

Fredville treats standing water with three larvicides: methoprene, Bti and Bs. Each one is aimed narrowly at the larvae of mosquitoes and black flies. Methoprene imitates an insect juvenile hormone and acts as a growth regulator. It does not kill mosquitoes outright; instead, it keeps the larvae from maturing into biting adults. Bti and Bs are bacteria that live naturally in soil, and mammals, birds, reptiles, amphibians and almost every kind of aquatic invertebrate can pass them through their bodies without harm.

These larvicides let the city reduce mosquito numbers at the larval stage for a modest cost and without damaging the surrounding environment. No meaningful environmental problems have been tied to their use.

What is permethrin?

Permethrin is an insecticide that belongs to the pyrethroid family. Pyrethroids are synthetic chemicals modeled on pyrethrins, the natural compounds in pyrethrum that come from chrysanthemum flowers and have been valued for centuries as insect killers. Compared with many other classes of insecticide, pyrethroids cause fewer unintended effects on non-target species.

Public health programs rely on pyrethroids because they are relatively safe for people, work at very low doses and knock insects down quickly. They were first developed in the early 1970s and hold up better in sunlight than natural pyrethrum. The U.S. Environmental Protection Agency (EPA) first registered permethrin in 1979 and re-registered it in 2006.

Where is permethrin used?

Permethrin is approved for a wide range of settings: many food and feed crops, livestock and the barns that house them, vehicles and other transportation, buildings and structures including food-handling businesses, mosquito abatement run by public health agencies, and many uses around the home, indoors and out, on pets, and on clothing (both pre-treated fabric and ready-to-use sprays).

EPA figures show roughly 2 million pounds of permethrin applied each year across farm, home and public health uses. More than 70 percent goes to non-agricultural settings. Of the total, about 55 percent is applied by professionals, 41 percent by homeowners around their residences, and only 4 percent in mosquito abatement.

For crop uses and wide-area treatments such as nurseries and sod farms, permethrin is a restricted-use pesticide because it is highly toxic to aquatic life; wide-area mosquito adulticiding is the exception to that rule. For homes and industrial sites it is classified for general use. Outside of pesticide law, permethrin is also a medicine for head lice and scabies, and the FDA, the federal drug regulator, approves those medicated products under federal drug law.

How much product does Fredville apply when it sprays?

During adult mosquito spraying, the city applies no more than 0.007 pounds of active ingredient per acre. By comparison, typical agricultural applications run from 0.2 to 0.4 pounds per acre.

Does spraying harm other insects?

Research has found that ultra-low-volume (ULV) fogging aimed at adult mosquitoes has little effect on other insects. ULV equipment releases less than one ounce per acre as microscopic droplets of a low-toxicity product. The city further limits impacts on non-target species by spraying only when nuisance levels or disease risk are high, and by scheduling applications for evening hours when most other insects are inactive. Permethrin is toxic to bees, but most bees are not out flying or pollinating when mosquito spraying takes place.

How does permethrin affect people?

Pyrethroids do not build up in the body and are cleared fairly quickly, even after repeated exposure; usually about 90 percent of a dose leaves the body in urine and feces within a week. Permethrin has a long history of use with no reported human poisonings, and there is no sign that it seriously harms people when it is used according to label directions. Workers exposed to it have reported skin sensations and tingling that clear up within one to two days. When permethrin has been applied to the skin to treat head lice, a small share of people have noticed brief numbness, itching, tingling or burning (World Health Organization, 1990).

For mosquito spraying, staying indoors with the windows shut during an application keeps most residents from any exposure at all. ULV spray lingers in the air for only two to three hours before settling onto vegetation, so there is no permethrin left to breathe the morning after a treatment.

How long does permethrin last in the environment?

In soil with oxygen present, permethrin has an average half-life of 39.5 days, ranging from about 12 to 113 days. It binds firmly to soil particles and is broken down mostly by microorganisms, with some help from sunlight. In water, part of the product degrades and sunlight breaks down more of it in the water column, but most of it attaches to sediment. Its half-life in the water column is roughly 19 to 27 hours, although permethrin held in sediment can remain for over a year. Because it dissolves poorly in water and clings to soil, permethrin is unlikely to reach groundwater.

How much exposure is considered safe?

The EPA has set a Reference Dose (RfD) for permethrin of 0.25 mg per kilogram of body weight per day, for both short-term and long-term dietary exposure. An RfD estimates how much of a chemical a person could take in every day over a lifetime without a meaningful risk of harm, and it is expressed in milligrams of chemical for each kilogram of body weight per day. Available studies have not documented long-term effects of permethrin in people.

No federal Maximum Contaminant Level (MCL) has been set for permethrin in drinking water. The World Health Organization (WHO) does recommend a guideline limit of 0.3 mg/L when permethrin is added to water for mosquito control.

The federal Agency for Toxic Substances and Disease Registry has set minimum risk levels for swallowing technical-grade permethrin: 0.3 mg/kg/day for acute exposure of 14 days or less, and, for intermediate exposure of 15 to 364 days, a lower level of 0.2 mg/kg/day.

Can garlic repel or kill mosquitoes?

Garlic has shown some activity against mosquitoes, and its active compound is allicin. Allicin has an LD50 of 60, while permethrin's is 4,000; a lower LD50 means a more toxic substance. For personal protection, the city recommends an EPA-registered repellent.

No research directly shows that garlic works as an adulticide, either. Garlic products have not gone through EPA registration and are not recognized as a registered adulticide for controlling adult mosquitoes.

Is permethrin behind honey bee losses?

Researchers and experts from around the country are now working together on funded comparative studies of Colony Collapse Disorder (CCD). CCD drew wide attention around 2006 and was first associated with varroa mites. Study of declining bee populations is still in its early stages, and pathogens, environmental chemicals and poor nutrition are among the newer suspected causes. Neonicotinoids, a class of pesticides known to be highly toxic to honey bees, are getting particular scrutiny from beekeepers and the media. Residents are encouraged to follow current research to stay informed.