Pesticides - different types

Pesticides are substances, most often chemical, designed for two purposes: to protect or control plants and to destroy or neutralize pests.

The most widely used pesticides are herbicides, insecticides and fungicides, which act respectively against unwanted weeds, harmful insects and diseases caused by fungi.
There are also rodenticides to control mice, rats, and other rodents, as well as wood preservatives and biocides to combat mold.

Classification

Pesticides are classified into main families according to a dual classification

Classification by purpose: there are four main families

Insecticides: These are designed to control insects. They interfere by killing or preventing the reproduction of insects; they are often the most toxic. Here are some examples:

• arsenic, widely used before World War II.
• POPs, in particular the famous DDT (dichlorodiphenyltrichloroethane), a very powerful insecticide that was widely used before its ban, are very persistent, very mobile and highly soluble, as traces of DDT have been found in ice and in mammals in the Arctic and Antarctic.

DDT was synthesized by Müller in 1939 (earning him a Nobel Prize in 1948) and used by the US military to combat lice during World War II. It was then widely used to control mosquitoes and in attempts to eradicate malaria. It was banned in the 1970s due to suspected carcinogenicity. After years of research, in September 2006, the WHO (World Health Organization) advocated its reintroduction in the form of DDT-impregnated mosquito nets. This decision followed devastating reports of several million deaths per year due to malaria in the poorest countries. However, DDT is very stable, highly mobile, and highly soluble, as traces of DDT have been found in Arctic and Antarctic ice and mammals.

• LINDANE (hexachlorocyclohexane HCH) from the organochlorine family of compounds has been banned since 1999. It is this family that contains the majority of organochlorine compounds.
• Carbaryl, infamous for causing the Bhopal disaster (December 1984) due to a leak of methyl isocyanate from the plant where it was manufactured.

Fungicides: These are designed to kill mold and parasites (fungi, etc.) on plants. The oldest fungicides are sulfur, copper, and its organic derivatives, such as Bordeaux mixture. Bordeaux mixture is a mixture of copper sulfate and calcium hydroxide (or slaked lime), traditionally used in vineyards since the 1880s. It is commercially available and is increasingly used in agricultural crops.

Synthetic fungicides (most often aromatic) are used prophylactically and therapeutically; their advantage is low toxicity and a broad spectrum of action.

A distinction is made between contact fungicides, which prevent fungi from penetrating the plant (e.g., zineb, captan, etc.), and systemic fungicides, which have a therapeutic effect (e.g., triadimefon, morpholine, etc.).

Herbicides: These are designed to control specific plants ("weeds") that compete with the plants being protected, inhibiting their growth. They differ significantly in nature from the other three families. For one, their action is not to interfere with the intruder (insect/parasite), but to control a different plant. Furthermore, their application method is different, as they are applied directly to the soil, unlike other products, which are sprayed onto the growing plant. The most well-known herbicides are sulfuric acid, which was used for weeding cereals as early as 1911, and phytohormones (2-4 D), as well as derivatives of 2-phenoxyethane acid (such as MCPP) and sulfonylureas.

In the 1930s, the first plant hormone (α-indole acetic acid or IAA) was identified. This was followed by a period of research into phytohormones; thus, phenoxyalkanoic acids such as 2,4-D-(2-(2,4-dichlorophenoxyethanoic acid) were synthesized.

Thus, we see how many types of pesticides there are and in what areas the use of certain types is relevant.