Pesticides

Definition:

A pesticide may be a chemical substance or biological agent (such as a virus or bacteria) used against pests including insects, plant pathogens, weeds, mollusks, birds, mammals, fish, nematodes (roundworms) and microbes that compete with humans for food, destroy property, spread disease or are a nuisance. Pesticides are usually, but not always, poisonous to humans.

History of pesticides:

Since before 500 BC, humans have used pesticides to prevent damage to their crops. The first known pesticide was sulfur. By the 15th century, toxic chemicals such as arsenic, mercury and lead were being applied to crops to kill pests. In the 17th century, nicotine sulfate was extracted from tobacco leaves for use as an insecticide. The 19th century saw the introduction of two more natural pesticides, pyrethrum which is derived from chrysanthemums, and rotenone which is derived from the roots of tropical legumes.

In 1939, Paul Müller discovered that DDT was a very effective insecticide. It quickly became the most widely-used pesticide in the world. However, in the 1960s, it was discovered that DDT was preventing many fish-eating birds from reproducing which was a huge threat to biodiversity. Rachel Carson wrote the best-selling book "Silent Spring". DDT is now banned in at least 86 countries, but it is still used in some developing nations to prevent malaria and other tropical diseases by killing mosquitos and other disease-carrying insects.

Pesticide use has increased 50-fold since 1950, and 2.5 million tons of industrial pesticides are now used each year.

Why do we need Pesticides:

Today's modern agriculture produces plentiful food, at a reasonable price, all year round. Most of us take it for granted that we can buy whatever food we want, whenever we want. We rightly expect our food to be safe and nutritious and we have also become used to food, particularly fruit and vegetables, not having any blemishes or other marks. We don’t tend to think about how farmers produce food or how it gets from the farm to the shops in "perfect" condition.

Farmers use pesticides to:

• protect crops from insect pests, weeds and fungal diseases while they are growing
• prevent rats, mice, flies and other insects from contaminating foods whilst they are being stored
• safeguard human health, by stopping food crops being contaminated by fungi

Type of pesticies:

1. Bactericides for the control of bacteria
2. Herbicides for the control of weeds
3. Fungicides for the control of fungi
4. Insecticides for the control of insects
5. Miticides for the control of mites
6. Nematicides for the control of worms
7. Rodenticides for the control of rodents
8. Virucides for the control of viruses


1. Bactericides for the control of bacteria

A bacteriocide or bactericide is a substance that kills bacteria and, preferably, nothing else. Bacteriocidal antibiotics kill bacteria; bacteriostatic antibiotics only slow their growth or reproduction.

Penicillin is a bactericide, as are cephalosporins. Aminoglycosidic antibiotics can act in both a bactericidic manner (by disrupting cell wall precursor leading to lysis) or bacteriostatic manner (by binding to 30s ribosomal subunit and reducing translation fidelity leading to inaccurate protein synthesis)

Other bactericidal antibiotics include the fluoroquinolones, nitrofurans, vancomycin, monobactams, co-trimoxazole, and metronidazole.


2. Herbicides for the control of weeds

A herbicide is a pesticide used to kill unwanted plants. Selective herbicides kill specific targets while leaving the desired crop relatively unharmed. Some of these act by interfering with the growth of the weed and are often based on plant hormones.

Herbicides used to clear waste ground are nonselective and kill all plant material with which they come into contact. Some plants produce natural herbicides, such as the genus Juglans (walnuts). Herbicides are widely used in agriculture and in landscape turf management. They are applied in total vegetation control (TVC) programs for maintenance of highways and railroads. Smaller quantities are used in forestry, pasture systems, and management of areas set aside as wildlife habitat.


3. Fungicides for the control of fungi

A Fungicide is one of three main methods of pest control- chemical control of fungi in this case. Fungicides are chemical compounds used to prevent the spread of fungi in gardens and crops, which can cause serious damage to the plants. Fungicides are also used to fight fungal infections.

Fungicides can either be contact or systemic. A contact fungicide kills fungi when sprayed on its surface. A systemic fungicide has to be absorbed by the fungi before the fungi dies. One example is the QoI.

It is used in agriculture to avoid fungus growth because this can harm profits, Corn is often sprayed with fungicides and some fungicides are genitically modified to make sure that they get to work straight away by damaging all of the fungal cells.
In contexts other than agriculture, the term antifungal is often used for similar compounds.

Fungicides can also indirectly be harmful to human health as the cultivated crop is consumed and it can cause irritation as well as many symptoms such as headaches, diarrehea, damaged organs as well as severe disorders and maladies related to the nervous system. It is also a hazard to ecosystems as it can run off and can contaminate water bodies and as it bioaccumulates it can be increasingly toxic to living organisms in the ecosystem.


4. Insecticides for the control of insects

An insecticide is a pesticide used against insects in all developmental forms. They include ovicides and larvicides used against the eggs and larvae of insects. Insecticides are used in agriculture, medicine, industry and the household.

The use of insecticides is believed to be one of the major factors behind the increase in agricultural productivity in the 20th century. Nearly all insecticides have the potential to significantly alter ecosystems; many are toxic to humans; and others are concentrated in the food chain. It is necessary to balance agricultural needs with environmental and health issues when using them.


5. Miticides for the control of mites

Miticides are pesticides that kill mites. Antibiotic miticides, carbamate miticides, formamidine miticides, mite growth regulators, organochlorine, permethrin and organophosphate miticides are all in this category.

Acaricides: Methoprene is virtually harmless to non-insects, & the US EPA has exempted it from tolerance. It is widely available in supermarkets, ctc. Hydroprene is toxic to fish & perhaps birds. Both are for indoor use only, as they break down in sunlight. Methoprene is applied as a wetting spray, hydroprene as an aerosol space spray. Neither will affect adult insects; they work on future generations by preventing growth or maturation. Permethrin can be applied as a spray or in more targeted forms (e.g. Damminix TickTubes ) that attack the ticks and mites on mammalian hosts. Their effects are not limited to mites: cockroaches, fleas, mosquitos, & other insects will be aborted. Permethrin, however, is not known to harm mammals or birds, as it has a low mammalian toxicity and is poorly absorbed by skin.


6. Nematicides for the control of worms

A nematicide is a type of chemical pesticide used to kill parasitic nematodes (a phylum of worms).


7. Rodenticides for the control of rodents

Rat poisons are a category of pest control chemicals intended to kill rats.
Single feed baits are chemicals sufficiently dangerous that the first dose is sufficient to kill.

Rats and certain other vermin are difficult to kill with poisons because their feeding habits reflect their place as scavengers. They will eat a small bit of something and wait, and if they don't get sick, they continue. An effective rat poison must be tasteless and odorless in lethal concentrations, and have a delayed effect.

Effects of pesticides:

1. On environment
2. On farmers
3. On consumers

1. On environment:

The use of pesticides also decreases biodiversity in the soil. Not using them results in higher soil quality with the additional effect that more life in the soil allows for higher water retention. This helps increase yields for farms in drought years where there is less rain. For example, during drought years, organic farms have been found to have yields 20-40% higher than conventional farms.

2. On farmers:

There have been many studies of farmers with the goal of determining the health effects of pesticide exposure. Research in Bangldesh suggests that many farmers' do not need to apply pesticide to their rice fields, but continue to do so only because the pesticide is paid for by the government. Organophosphate pesticides have increased in use, and in addition to being more damaging to the environment they are more persistent than organochlorine pesticides. These are associated with acute health problems such as abdominal pain, dizziness, headaches, nausea, vomiting, as well as skin and eye problems.

Additionally, many studies have indicated that pesticide exposure is associated with long-term health problems such as respiratory problems, memory disorders, dermatologic conditions, cancer, depression, neurologic deficits, miscarriages, and birth defects. Summaries of peer-reviewed research have examined the link between pesticide exposure and neurologic outcomes and cancer, perhaps the two most significant things resulting in organophosphate-exposed workers.

3. On consumers:

the levels of organophosphorus pesticide exposure in 23 school children before and after replacing their diet with organic food (food grown without synthetic pesticides). In this study it was found that levels of organophosphorus pesticide exposure dropped dramatically and immediately when the children switched to an organic diet.

References:

1. http://en.wikipedia.org/wiki/
2. http://www.pesticides.gov.uk/
3. http://www.agf.gov.bc.ca/pesticides/g_1.htm