23/04/2015
Pesticides are substances meant for
attracting, seducing, and then destroying,
or mitigating any pest. [1] They are a class
of biocide. The most common use of
pesticides is as plant protection products
(also known as crop protection products),
which in general protect plants from
damaging influences such as weeds, plant
diseases or insects. This use of pesticides
is so common that the term pesticide is
often treated as synonymous with plant
protection product, although it is in fact a
broader term, as pesticides are also used
for non-agricultural purposes. The term
pesticide includes all of the following:
herbicide, insecticide, insect growth
regulator, nematicide , termiticide,
molluscicide, piscicide, avicide, rodenticide,
predacide, bactericide, insect repellent,
animal repellent, antimicrobial , fungicide,
disinfectant (antimicrobial ), and sanitizer.
[2]
In general, a pesticide is a chemical or
biological agent (such as a virus ,
bacterium , antimicrobial , or disinfectant )
that deters, incapacitates, kills, or
otherwise discourages pests. Target pests
can include insects, plant pathogens ,
weeds, mollusks , birds , mammals , fish,
nematodes (roundworms ), and microbes
that destroy property, cause nuisance, or
spread disease, or are disease vectors .
Although pesticides have benefits, some
also have drawbacks, such as potential
toxicity to humans and other desired
species. According to the Stockholm
Convention on Persistent Organic
Pollutants, 9 of the 12 most dangerous and
persistent organic chemicals are
organochlorine pesticides. [3][4]
Definition
The Food and Agriculture Organization
(FAO) has defined pesticide as:
any substance or mixture of substances
intended for preventing, destroying, or
controlling any pest, including vectors of
human or animal disease, unwanted
species of plants or animals, causing
harm during or otherwise interfering with
the production, processing, storage,
transport, or marketing of food,
agricultural commodities, wood and
wood products or animal feedstuffs, or
substances that may be administered to
animals for the control of insects,
arachnids, or other pests in or on their
bodies. The term includes substances
intended for use as a plant growth
regulator, defoliant, desiccant, or agent
for thinning fruit or preventing the
premature fall of fruit. Also used as
substances applied to crops either before
or after harvest to protect the
commodity from deterioration during
storage and transport. [5]
Type of pesticide Target pest group
Herbicides Plant
Algicides or
Algaecides Algae
Avicides Birds
Bactericides Bacteria
Fungicides Fungi and
Oomycetes
Insecticides Insects
Miticides or
Acaricides Mites
Molluscicides Snails
Nematicides Nematodes
Rodenticides Rodents
Virucides Viruses
Pesticides can be classified by target
organism (e.g., herbicides , insecticides ,
fungicides, rodenticides, and pediculicides
[4][6] - see table), chemical structure (e.g.,
organic, inorganic, synthetic, or biological
(biopesticide) ,[7] although the distinction
can sometimes blur), and physical state
(e.g. gaseous (fumigant) ). [7] Biopesticides
include microbial pesticides and
biochemical pesticides. [8] Plant-derived
pesticides, or "botanicals", have been
developing quickly. These include the
pyrethroids , rotenoids , nicotinoids, and a
fourth group that includes strychnine and
scilliroside. [9] :15
Many pesticides can be grouped into
chemical families. Prominent insecticide
families include organochlorines,
organophosphates , and carbamates .
Organochlorine hydrocarbons (e.g., DDT )
could be separated into
dichlorodiphenylethanes, cyclodiene
compounds, and other related compounds.
They operate by disrupting the sodium/
potassium balance of the nerve fiber,
forcing the nerve to transmit continuously.
Their toxicities vary greatly, but they have
been phased out because of their
persistence and potential to bioaccumulate.
[9] :239–240 Organophosphate and
carbamates largely replaced
organochlorines . Both operate through
inhibiting the enzyme acetylcholinesterase ,
allowing acetylcholine to transfer nerve
impulses indefinitely and causing a variety
of symptoms such as weakness or
paralysis. Organophosphates are quite
toxic to vertebrates, and have in some
cases been replaced by less toxic
carbamates. [9] :136–137 Thiocarbamate
and dithiocarbamates are subclasses of
carbamates. Prominent families of
herbicides include phenoxy and benzoic
acid herbicides (e.g. 2,4-D ), triazines (e.g.,
atrazine ), ureas (e.g., diuron ), and
Chloroacetanilides (e.g., alachlor ). Phenoxy
compounds tend to selectively kill broad-
leaf weeds rather than grasses. The
phenoxy and benzoic acid herbicides
function similar to plant growth hormones,
and grow cells without normal cell division,
crushing the plant's nutrient transport
system. [9] :300 Triazines interfere with
photosynthesis. [9] :335 Many commonly
used pesticides are not included in these
families, including glyphosate .
Pesticides can be classified based upon
their biological mechanism function or
application method. Most pesticides work
by poisoning pests. [10] A systemic
pesticide moves inside a plant following
absorption by the plant. With insecticides
and most fungicides, this movement is
usually upward (through the xylem ) and
outward. Increased efficiency may be a
result. Systemic insecticides, which poison
pollen and nectar in the
flowers[ citation needed ], may kill bees and
other needed pollinators[ citation needed ].
In 2009, the development of a new class of
fungicides called paldoxins was announced.
These work by taking advantage of natural
defense chemicals released by plants called
phytoalexins, which fungi then detoxify
using enzymes. The paldoxins inhibit the
fungi's detoxification enzymes. They are
believed to be safer and greener.[11]
Uses
Pesticides are used to control organisms
that are considered to be harmful. [12] For
example, they are used to kill mosquitoes
that can transmit potentially deadly
diseases like West Nile virus , yellow fever ,
and malaria. They can also kill bees , wasps
or ants that can cause allergic reactions.
Insecticides can protect animals from
illnesses that can be caused by parasites
such as fleas .[12] Pesticides can prevent
sickness in humans that could be caused
by moldy food or diseased produce.
Herbicides can be used to clear roadside
weeds, trees and brush. They can also kill
invasive weeds that may cause
environmental damage. Herbicides are
commonly applied in ponds and lakes to
control algae and plants such as water
grasses that can interfere with activities
like swimming and fishing and cause the
water to look or smell unpleasant. [13]
Uncontrolled pests such as termites and
mould can damage structures such as
houses. [12] Pesticides are used in grocery
stores and food storage facilities to
manage rodents and insects that infest
food such as grain. Each use of a pesticide
carries some associated risk. Proper
pesticide use decreases these associated
risks to a level deemed acceptable by
pesticide regulatory agencies such as the
United States Environmental Protection
Agency (EPA) and the Pest Management
Regulatory Agency (PMRA) of Canada.
Pesticides can save farmers' money by
preventing crop losses to insects and other
pests; in the U.S., farmers get an estimated
fourfold return on money they spend on
pesticides. [14] One study found that not
using pesticides reduced crop yields by
about 10%. [15] Another study, conducted in
1999, found that a ban on pesticides in the
United States may result in a rise of food
prices, loss of jobs, and an increase in
world hunger.[16]
DDT , sprayed on the walls of houses, is an
organochloride that has been used to fight
malaria since the 1950s. Recent policy
statements by the World Health
Organization have given stronger support
to this approach. [17] Dr. Arata Kochi,
WHO's malaria chief, said, "One of the best
tools we have against malaria is indoor
residual house spraying. Of the dozen
insecticides WHO has approved as safe for
house spraying, the most effective is
DDT." [17] A later October 2007 study linked
breast cancer from exposure to DDT prior
to puberty. [18] Other studies have found no
link. [19] Symptoms include nervous
excitement, tremors, convulsions, or death.
Scientists estimate that DDT and other
chemicals in the organophosphate class of
pesticides have saved 7 million human
lives since 1945 by preventing the
transmission of diseases such as malaria ,
bubonic plague , sleeping sickness , and
typhus . [20] However, DDT use is not always
effective, as resistance to DDT was
identified in Africa as early as 1955, and by
1972 nineteen species of mosquito
worldwide were resistant to DDT.[21] [22]
