Pesticides
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This post presents set of most common chemicals used as pesticides. Their production needs to be decentralised with the flow chemistry like processes in a fully decentralised future scenario or alternatively biological methods are used.
Pesticides
Pesticides are substances that are meant to control pests. Pest are any organisms that are harmful to humans or humans interests, i.e., they damage crops, livestock and forestry or cause a nuisance to people, especially at home. The term is an umbrella for herbicides, insecticides, fungicides and any substances against other pests like bacteria, rodents and other animals.
A pesticide consists of an active ingredient (molecule) that kills the insect, fungus or rodent and inert ingredients. The inert ingredient there is for helping to spraying and coating the target plant.
Active ingredients were originally distilled from natural substances. Nowadays they are largely synthesised in a laboratory. The feedstocks and starting materials used in the synthesis of pesticides can vary widely depending on the specific type of pesticide. Hydrocarbons are used as feedstock but also inorganic compounds like salts, minerals etc. Chlorine, oxygen, sulfur, phosphorus, nitrogen, and bromine are most common.
Common types of active ingredients include:
Insecticides: These chemicals target insects and can include organophosphates, pyrethroids, neonicotinoids, and others.
Herbicides: Herbicides are designed to control or kill weeds and can contain various active ingredients, such as glyphosate, atrazine, or 2,4-D.
Fungicides: Fungicides control fungal diseases and may contain chemicals like azoles or copper compounds.
Rodenticides: Rodenticides are used to control rodents and can include anticoagulants and other toxic substances.
Pesticides usually contain inert ingredients, which are non-active substances that help with the formulation, stability, and effectiveness. Inert ingredients can be many substances. Pesticides are formulated in various ways, including as liquids, powders, granules, or gases, to ensure effective application and distribution.
Liquid pesticides have traditionally used kerosene or some other petroleum distillate as a carrier, though water has recently begun to replace them. Emulsifiers (such as soap) are also added to distribute the active ingredient evenly. A powder or dust pesticide will typically contain vegetable matter such as ground up nut shells or corn cobs, clays such as diatomite or attapulgite, or powdered minerals such as talc or calcium carbonate. These cause the pesticide to stick better.
Herbicides
Modern herbicides act by restricting growth. They inhibit some action in cells that is essential to the growth of the plant. Exception is auxins, that is overstimulating growth. Auxins are natural plant hormones. Other herbicides include:
Glycines like glyphosate affect broad leaved weeds and grasses. Glyphosate is the most commonly manufactured herbicide. They are absent in mammals having little toxicity. It gets absorbed by leaves and goes into the roots where it inhibits enzyme action needed for the production of aromatic amino acids essential for protein synthesis. Roundup is a common marketing name.
Bipyridyliums. These interfere with photosynthesis (i.e. they are non-selective killing also crops). Paraquat is an example.
Sulfonylureas. These block the biosynthesis of essential amino acids (valine and isoleucine) preventing cell division and cell growth.
Herbicides can be selective or non-selective. Non-selective herbicides kill crops as well as weeds. With selective herbicides the weed is mainly affected.
Insecticides
Insecticides tend to be compounds that interfere with the nervous system of pest invertebrates.
Organophosphorus compounds (OPs) are an old class of insecticides and still economically very important and diverse class of insecticide with more than 100 different ingredients belonging. One of the most successful OPs is considered to be chlorpyrifos. It affects the stomach and respiratory action of the pest.
All OPs act by binding irreversibly to the enzyme acetylcholinesterase (AChE). This prevents the hydrolysis of the neurotransmitter acetylcholine in the central nervous system and leads to prolonged periods of nerve excitation leading to paralysis and subsequently death of the insects.
The major disadvantage of many OPs is their toxicity to vertebrates (us).
A different class are methyl carbamates that are less toxic than OPs. Pirimicarb is used for aphids (plant louses) and is harmless for pollinators and aphid predators. Other widely are carbamates are aldicarb against soil insects and bendiocarb against malaria-transmitting adult mosquitoes.
Macrocyclic lactones disturb the systems which control the flow of chloride ions shutting down electrical impulses in the nerve cells of the target.
Fipronil is best known phenylpyrazole. It disturbs the chloride ion concentrations in cells and works against Lepidoptera (moths), Coleoptera (beetles) and Diptera (flies, mosquitoes). Its also used against cockroaches, ants and termites and for treating dogs and cats for ticks and fleas.
Nereistoxin analogues and neonicotinoids prevent nicotinic acetylcholine receptor (nAChR) from functioning correctly. It affects the passage of sodium and potassium ions in the nerve cells and thus prevents the central nervous system from working well. Examples of these insecticides include cartap, thiosultap, bensultap and thiocyclam.
Diamides like phthalic acid diamides and the anthranilic diamides (chlorantraniliprole) act on the cell's ability to control the flow of calcium ions.
Pyrethroids are one of most important insecticides. They affect the flow of sodium ions in the nervous system. Present in Chrysanthemum and originally dried flower heads were used as insecticide.
Fungicides
The 1,2,4-triazoles are one of the most important classes of fungicide. They inhibit important biochemical pathways that produce sterols that are essential components of cell membranes. They are used to protect against fungi in cereals, rice, beet, trees, vegetables and flowers.
Strobilurins interfere with the production of adenosine-5'-triphosphate (ATP). This prevents germination and growth of fungi. ATP is a molecule storing and transferring energy within cells and is essential for various cellular processes. The strobilurins combat most major fungal diseases found on grasses (turf), vines, fruit and particularly on cereals.
Carboxamides also interfere with the production of ATP and are used to combat powdery mildew on fruit and vegetables.
Some fungicides act act on a number of enzymes rather then one making them not very sensitive to resistance problems. They are all protectant fungicides (act before fungal infection), working on contact with the fungal spores on the surface of the leaves.
Some examples of them are inorganic compounds such as elemental sulfur and some copper salts Examples of organic compounds with a multi-site action are salts of dithiocarbamates such as chlorothalonil and propineb. They are used on of plant pathogens in in grapes, apples, tomatoes and potatoes as example.
Alternatives
As an alternative to chemicals, it is also possible to use biological pesticides – perhaps the most common example releasing ladybugs to eat aphids and other harmful insects. Biological pesticides can be either very small micro-organisms which could be natural bacteria, fungi or viruses of the pest in question. They can be also macro-organisms like the pest’s natural predators.
In controlled environments like vertical farming the use of biological pesticides is easier than with plants growing outdoors. In greenhouses (or vertical farms) positive pressure ventilation is a technique to force air in to create a positive pressure inside. This positive pressure inside reduces risks of of insects and other harmful organisms getting in as air is always flowing from higher pressure (inside) out and insects would have to move against it.
One more, albeit pricey, approach to avoid using pesticides, is growing plants on floating platforms on the sea. There are very few insects and no pests if you go a few tens of meters away from the shore. Winds may carry still some. The costs of building such platforms are however much larger than growing on land, but for growing food completely devoid of any chemicals for people willing to pay for it, they are possible and technologies tend to drop in price over time.
To probe further:
http://www.madehow.com/Volume-1/Pesticide.html
https://www.essentialchemicalindustry.org/materials-and-applications/crop-protection-chemicals.html