Monospecific farming and pesticides

Contemporary agriculture is generally based on monospecific crops for productive purposes. The biological systems thus created are highly unbalanced.
One of the main imbalances comes precisely from its monospecific character. The fact that a single species is repeated with high density in a given area leads to pests, such as certain insects, worms, various parasites, etc. These pests have more facility to reproduce due to the proximity between individual plants.
This nearness and high density leads to the multiplication of diverse pests by rapidly destroying crops or decreasing productive yields.
To prevent the progress of pests, some biocide chemicals are usually used to eliminate them.
For this, especially since the beginning of the 20th century, a series of organophosphorus chemicals have been developed that have biocidal properties and that allow the elimination of agricultural pests.
One of the first and best known organophosphorus insecticides was DDT (dichloro diphenyl trichloroethane). 
DDT was used systematically during the first half of the 20th century to reduce insect populations in some marsh ecosystems and to combat insect pests in agricultural plantations. Because toxic effects were found in workers and agricultural communities, their use was reduced until it was banned in most countries.
In the following years other insecticides and organophosphorus pesticides were developed that replaced DDT in the agricultural application and are still used (some of them as well).
Among them are the following:
Parathion is the common name of an organophosphorus insecticide that was used in the past in the United States and is still used in other countries to control insects and mites that suck or chew plants in a wide variety of crops.
In pure form, parathion is a pale yellow liquid with a slight smell of phenol. The technical quality preparation is a pale yellow to dark brown liquid.
Malathion is an organophosphorus insecticide that does not occur naturally. Pure malathion is a colorless liquid, and technical grade malathion, which contains> 90% malathion and impurities in a solvent, is a yellowish-brown liquid that smells like garlic. Malathion is used to kill insects in agricultural crops and gardens, to treat head lice in humans and to treat fleas in domestic animals. It is also used to kill mosquitoes and the fruit fly in extensive outdoor areas.
Chlorpyrifos is another organophosphorus pesticide that has been widely used in housing and agriculture. Since August 2008 its use for biocidal preparations for environmental use or in the food industry is not allowed. Its use has been limited to agriculture and at the domestic level in gardens. In agriculture, it is considered a non-systemic insecticide that acts by contact and ingestion with great shock effect. '
It is applied by spraying the affected areas for the control of crop pests, particularly insects.
Diazinon or Dimpilato is the common name of an organophosphorus insecticide used to control insects in the soil, in ornamental plants and in fruit and vegetable crops.
Biological insecticides
In addition to these chemical insecticides, biological insecticides have recently been used, such as Bt or bacillus thuringiensis.
Bacillus thringiensis is a Gram-positive bacterium that lives in the soil, and is commonly used as a biological alternative to the pesticide. The Cry toxin can also be extracted and used as a pesticide. B. thuringiensis also occurs naturally in the intestine of caterpillars of different types of moths and butterflies, as well as on poorly lit surfaces of plants.
During sporulation, many strains of Bt produce protein crystals known as δ-endotoxins, which possess insecticidal properties.
Genetically Modified Seeds
For this reason, Bt has been used as an insecticide and, more recently, to produce genetically modified organisms.
Bt was introduced into the genome of plants that thus develop insecticidal properties in their tissues and prevent the plant from being affected by pests. In this way, Bt potatoes, Bt corn, Bt cotton and other similar GM organisms have been created. The formulas of these organisms are protected by intellectual property rules that allow the control of seeds and GM crops to remain under the ownership power of the companies that produce them (eg Monsanto). A similar procedure has been developed with the crops resistant to the glyphosate herbicide with analogous consequences.
Conclusion
The toxicity of the aforementioned pesticides is directed directly to various organisms (eg insects, etc.) but the toxicity of glyphosate (herbicide) is particularly targeted to "weeds" and may have some indirect impact on animals and humans. But it is important to note that the toxicity of pesticides is focused on organisms and can have direct and obviously much more intense effect on various vertebrates, such as birds and mammals including humans.
These new developments of GM species have in addition to the environmental consequences or toxicity on people, effects on those who control the seeds and consequently of control of crops and world agriculture.
To a large extent, these new agricultural technologies have allowed a few multinational corporations (eg Bayer, Montsanto, Syngenta, BASF, Dow, Dupont)  to take over the products of biotechnology and to be able to control the vast spaces of agricultural production on the planet.