Microsoft word - pesticides used in mosquito control _2_.doc

At Hillsborough County Mosquito and Aquatic Weed Control, we make decisions to use pesticides based on an
evaluation of the risks to the general public from diseases transmitted by mosquitoes, and on an evaluation of
the nuisance level that the community can tolerate from a mosquito infestation. All pesticides used are
evaluated and registered by the Environmental Protection Agency (EPA) to ensure that they can be used
safely. To evaluate any pesticide, EPA assesses a wide variety of tests to determine whether a pesticide has
the potential to cause adverse effects on humans, wildlife, fish and plants, including endangered species and
non-target organisms. Of the pesticides available to us, we make every effort to choose the ones that are least
harmful to people, animals and the environment, while selecting those most effective for the conditions and the
target organisms present. We also routinely rotate pesticide use in order to avoid developing resistance
among the indigenous mosquito population.
What are larvicides? Larvicides kill mosquito larvae. They include biological insecticides, such as the
microbial larvicides Bacillus sphaericus (Bs) and Bacillus thurningiensis israelinsis,(Bti) as well as other
pesticides, such as temephos, methoprene, oils, and monomolecular films.
What are Microbial Larvicides? They contain bacteria that are registered as pesticides for control of
mosquito larvae in outdoor areas such as irrigation ditches, flood water, standing ponds, woodland pools,
pastures, tidal water, fresh or saltwater marshes, and storm water retention areas. They work by essentially
disrupting the gut of the mosquito larva when ingested, and pose no harm to any other organisms apart from
black fly larvae. The bacteria of choice is based on species of mosquito targeted, weather conditions, habitat to
be treated, water quality, and desire for residual effects.
Do Microbial Larvicides pose risks to human health? The microbial pesticides have undergone extensive
testing prior to registration with the Environmental Protection Agency (EPA). They are essentially nontoxic to
humans, so there are no concerns for human health effects when used according to label directions.
What about the environment? Extensive testing shows that microbial larvicides do not pose risks to wildlife,
nontarget species, or the environment, when used according to label directions. At Hillsborough County
Mosquito and Aquatic Weed Control, we consider larviciding our first line of defense, and make extensive use
of the microbial larvicides in order to impact the environment as little as possible.
What is Methoprene? Methoprene is an insect growth regulator (IGR) that prevents the larvae from maturing
to the adult stage. We apply it in the form of briquets to small areas of water that we know to be regular
mosquito larval habitat, such as storm drains, ditches and small retention ponds. Methoprene has been shown
to pose low toxicity to mammals, and there is little opportunity for human exposure, since the material is
applied directly to ditches, ponds, marshes or flooded areas that are not drinking water sources. Toxicity of
methoprene to birds and fish is also low, and it is nontoxic to bees. Methoprene breaks down quickly in water
and soil and will not leach into ground water.
What is Temephos? Temephos is an organophosphate (OP) pesticide registered by the EPA in 1965 to
control mosquito larvae, and is an important resistance management tool for mosquito control programs. Its
use helps prevent mosquitoes from developing resistance to biological larvicides. As part of our larval control
program, we use Temephos to treat roadside ditches containing large amounts of mosquito larvae. The
amount of temephos used is very small in relation to the area covered – less than one ounce of active
ingredient per acre – and it breaks down within a few days in water. Since it does pose some risk to nontarget
aquatic species, we are very selective about where we use this pesticide.

What are Monomolecular Films? Monomolecular films are low-toxicity pesticides that spread a thin film on
the surface of the water that makes it difficult for mosquito larvae, pupae, and emerging adults to attach to the
water’s surface, causing them to drown. Films may remain active typically for 10-14 days on standing water,
have very low toxicity, and pose mimimal risk to the environment. We generally apply them to roadside
ditches, which contain few nontarget organisms.
What are Oils? Oils, like films, are pesticides used to form a coating on top of water to drown larvae, pupae,
and emerging adult mosquitoes. They are specially derived from petroleum distillates and have been used for
many years in the United States to kill aphids on crops and orchard trees, and to control mosquitoes. Used
according to label directions for larva and pupa control, they do not pose a risk to human health. If misapplied,
however, oils may be toxic to fish and other aquatic organisms, so we are very careful (as with all pesticides) to
follow label and EPA guidance about how we use these products.
What are adulticides? Adulticides are chemicals released in an aerosol form to kill flying adult mosquitoes,
applied by truck-mounted or aircraft-mounted sprayers. Modern adulticides are applied as ultra-low volume
(ULV) spray – very fine aerosol droplets that stay aloft and kill mosquitoes on contact. ULV applications
involve small quantities of pesticide active ingredient in relation to the size of the area treated. We apply Naled
aerially at a rate of one half ounce per acre, and Permethrin by ground vehicle at the rate of one quarter ounce
per acre. It should be noted that adulticiding is only employed when certain criteria are met to indicate a high
level of nuisance and/or public health risk, as mandated by State and Federal regulations. Adulticiding is just
one part of an effective Integrated Pest Management (IPM) program, which includes source reduction,
education, larviciding and barrier sprays.
What is Naled? Naled is an organophosphate (OP) insecticide that has been registered since 1959 for use in
the United States. It is used primarily for controlling adult mosquitoes, but is also used on food and feed crops,
and in greenhouses. When applied in accordance with the rate of application and the safety precautions
specified on the label, naled can be used to kill mosquitoes without posing unreasonable risks to human health
or the environment. Although the label rate allows us to use up to one full ounce per acre, by achieving
uniform droplets in the optimum size range we are able to increase our efficiency and use the lower end of the
label rate.
Does Naled pose risks to human health? The EPA has estimated the exposure and risks to both adults and
children posed by ULV aerial and ground applications of naled. Because of the very small amount of active
ingredient released per acre of ground, the estimates found that for all scenarios considered, exposures were
hundreds or even thousands of times below an amount that might pose a health concern. These estimates
assumed several spraying events over a period of weeks, and also assumed that a toddler would ingest some
soil and grass in addition to skin and inhalation exposure. Our use of naled is limited to aerial applications
from 300 feet above ground level (AGL), and the size of the droplets are engineered to remain aloft until they
contact mosquitoes, rather than deposit out on the ground and other surfaces.
What about the environment Naled degrades rapidly in the environment, and displays low toxicity to birds
and mammals. Acute and chronic risk to fish is not expected, but there is potential for risks to invertebrates
from the repeated use of naled (we do not spray over bodies of water). Naled is highly toxic to insects,
including beneficial insects such as honeybees. For that reason, EPA has established specific precautions on
the label to reduce such risk. We limit our adulticiding to hours of darkness, or dusk and dawn, in accordance
with State and Federal regulations.
What is Permethrin? Permethrin is a broad spectrum insecticide, used to kill a variety of insects. Permethrin
is referred to as a synthetic pyrethoid insecticide because, while manmade, it resembles naturally-occurring
chemicals with insecticidal properties, called pyrethroids. Pyrethroids are found especially in chrysanthemums,
and are one of the oldest classes of organic insecticides known. They work by quickly paralyzing the nervous
system of insects, producing a quick “knockdown” effect on insect pest populations.
Does Permethrin pose risks to human health? Permethrin has low mammalian toxicity, however, contact
with eyes, skin or clothing should be avoided and handlers of permethrin should wash thoroughly after
handling. Breathing of the spray mist or vapors of permethrin should be avoided.
What about the environment? Permethrin is practically non-toxic to birds, however it is toxic to fish and
should be kept out of all bodies of water. Permethrin is extremely toxic to bees, and somewhat toxic to wildlife.
It should not be applied or allowed to drift to crops or weeds in which active foraging takes place. For this
reason, our spray trucks stay on paved roads of established neighborhoods, and do not spray in rural areas.
Because permethrin binds very strongly to soil particles and is nearly insoluble in water, it is not expected to
leach or to contaminate groundwater. It is readily broken down, or degraded in most soils, with the help of
naturally occurring soil micro-organisms.
What are Barrier Sprays? A barrier spray is a pesticide used to coat the underside of the leaves of foliage
surrounding a home or business in order to provide some relief from biting mosquitoes. We use a permethrin
based chemical which acts as a repellant in addition to killing mosquitoes on contact. By coating the underside
of the foliage (the shady side) we rob the mosquitoes of a resting area. When not exposed to direct sunlight,
the insecticidal activity of this material can last up to 12 weeks after application.
REFERENCES: Environmental Protection Agency website:, Pesticide Management
Education Program at Cornell University website:


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