Chemical substances that exert a lethal action on the insects, being used, for that reason in the control of injurious species for the cultivated plants and, directly or indirectly, the human being and the animals.

Classification.

The insecticides can be classified of very diverse ways, like, for example, by their chemical base, its toxicológica action or its way of penetration in the insect. To classify insecticides by mechanism physiological of his performance is only exact when insect studies single or groups compatible of such, in such a way that not classification will be able to make by his performance physiological, since this mechanism is extremely variable forehead to the great diversity of insects and phases of its development that almost never is unique, but multiple. In addition, its quantitative value in front of experiences great differences the diverse insects. The classification that is going away here to make will consider if the effects have been demonstrated after an ingestion, a contact or an inhalation. Most of organic insecticides they penetrate of anyone of the forms before enunciated, although they are those that better are distinguished by their chemical base.

Insecticides by ingestion.

This type of insecticide only acts if it is ingested by the mouth of the insect that feeds itself on weaves of the plants on which the insecticide in fine and adherent form has dispersed, or those to which it has been made them absorb the compound toxic. They are indicated for adapted insects provided with jaw to the mastication. The derivatives of arsenic, sulfur, the cyanide, mercury and phosphorus are representing of this type of insecticides. They are gradually being replaced by the organic insecticides, which are less detrimental for the man and other animals.

Insecticides by contact.

The insecticides by contact can be divided in two great groups: natural and organosintéticos. Between first they are possible to be emphasized the nicotine, obtained of the plant of the tobacco, the piretinas, obtained of a particular type of crisantemos, and oils derived from petroleum. Some of the active principles responsible for the insecticide action of these products have been synthesized in laboratory. This type of insecticides presents/displays the particularitity of not remaining active too much time, with which they will not be most appropriate for prolonged plagues.

The insecticides by contact of organosintético origin are those that at the present time are used in greater proportion, due to the rapidity with which they penetrate in the insect and to the great fan of species to which they fight. The main group of this type of insecticide conforms the derivatives of chlorinated hydrocarbons, organic phosphates and carbamates. Most of them they are very stable in the time and its insecticide action lasts during great periods of time, with which they are most appropriate for lasting plagues, to which it disturbs the nervous system strongly. The derivatives of chlorinated hydrocarbons began to use after the proven effectiveness of the DDT, developed in the Forties. Nevertheless, at the present time they are most versatile the organofosforados insecticides and those than therefore are used with more frequency. They are most effective against the sucking insects of the juice of the plants. The insecticide action obtains when scattering product in the surface of the leaves, or applying insecticide solutions to the ground, with which the insecticide will penetrate in the plant through the roots. Those earth whose tolerance to insecticides is limited that these have little residual action, are the propitious ones for the use, in them, of this type of insecticides. The organofosforados insecticides along with the carbamato (insecticide organic relatively modern) present/display a degree of toxicity greater than chlorinated hydrocarbons, being their form to act enough similar: they produce the inhibition of an essential enzyme for the nervous system.

The use of this type of organic insecticides has increased the production of considerable form in all the regions of the globe, to the power to make a much more exhaustive control of plagues. They have as well allowed to control of much more efficient form carrying insects of infectocontagiosas diseases, such as the one of the malaria, the yellow fever and the typhus.

Respiratory insecticides.

They correspond to the employees in fumigation. By mixture with the air, they also create toxic atmospheres in which, according to it is the concentration, not only succumbs the insect, but all the sensible insects to the toxic diluted in the air. Their main representatives are: anhídrido sulfuroso, acid hydrocyanic and ethylene oxide.

Also he has, from the point of view of the physiological action, insectífugos products, whose mission is to repel the insects of its proximities. In general, they are not toxic or its toxicity is small, although they present/display the disadvantage that its scent is strong and tenacious for the human sense of smell.

Toxicity.

The characteristics and degree of toxicity of insecticides can be deduced of the experiments with animals witnesses. The main indications are:

- parenteral, intravenous, subcutaneous or intramuscular Toxicity acute, that shows the effect produced by the administration of a dose of the product by oral route.

- subcutaneous or cumulative Toxicity, that determines the effect of the repeated administration of the product in sublethal daily doses, during some time.

- chronic Toxicity, originated when extending the experiment of subacute toxicity during one or two years, deepening the farmacológicas and histo-anatomopatológicas investigations and extending the investigation to controls on the cancerigenic power of the product and the determination of the effects of the same one in the reproduction of the animals.

- skin or precutaneous Toxicity, that measures the toxicity of the substance applied on the skin of the animals.

- skin Irritability and of the mucous, that tend to clarify the reactions of allergic type.

- Toxicity inhalatoria, that measures the toxicity of the product provided through the respiratory routes.

The acute toxicity of insecticides varies within very ample limits. The products equipped with greater acute toxicity preferredly are between phosphoric organic insecticides and the chlorinated cíciclos. The chronic toxicity is a characteristic of orgánicoclorados, aromatic and cyclical insecticides, with the exception of few, like methoxicloro. The phosphoric organic insecticides, the carbámicos and those of vegetal origin do not give rise to phenomena of chronic toxicity, becoming inactive more or less quickly and being expelled.

In certain cases, the relation between the toxicity of insecticides for the animals of hot blood and the toxicity of such presents/displays great interest in relation to the insects. In fact, when the product is lethal for some species of insects and inoffensive for the animals of hot blood, it is possible to use the insecticide of an unusual way, administering it to bovines, ovines or other mammals directly by oral route or injection, in order to obtain in the blood lethal concentrations for the species of insects that feed themselves on her or which they are developed in the meats of these animals. The insecticides equipped with such property denominate "zoosistémicos".

Formulations.

The insecticides, so that in the substrata that are tried to protect they constitute active deposits and they develop to his Maxima effectiveness without producing damage, must be diluted in the suitable proportion. For it, the products are put under formulation, so and as they are obtained industrially, mixing them and diluting them with auxiliary substances like dissolvents and extenders. This way different types from product-formula are obtained. These products are diluted in water at the moment of their use, although diverse product-formulas exist (the poisoned aerosols and, of general way, baits) that, due to his low poison concentration, they do not require of his water dissolution.

Disadvantages.

Although it is not necessary to doubt the benefits derived from the insecticide use in the control of plagues and in the improvement in the yield of the fields, the nonselective use and in great amounts of insecticides it has brought with himself the appearance of a series of serious problems. Most serious of all of them he is the one that can pronounce in the ecosystem, because, when introducing modifications that lead to the practical disappearance of certain insects, it for sure does not know the effects that on other insects and animals can carry, either of direct form, by its exhibition to the product, or of indirect form, when modifying itself the nutritional chain. A diminution in the population of prey birds has been observed, such as eagles or hawks, in regions exposed to great amounts of DDT (organ-chlorinated compound), demonstrating that all the nutritional chain can be seen affected. It is possible that the insecticide concentration is so that disables, by certain periods of time, the as much human life as animal (is had documented many cases of envenenamiento of humans by insecticides). Although part of the used insecticide can be metabolizado by the animals that ingest it, will always be left other than it remains in the ground or atmosphere, that, even, could get to contaminate underground waters. In fact, many birds and mammalian savages with great amounts of DDT in their organism have been found. The use of the DDT began to restrict itself in the decade of the Sixties. A decade later many countries would prohibit their use.

Another important problem provoked with the massive insecticide use is the relative one to the tendency presented/displayed by certain insects to resist the action of these. Certain insects of a population, when they offer this resistance, are multiplied quickly, being able to get to be the most numerous group of the colony. It is then when the insecticide will not present/display injurious effect some on the insect. It is also probable that if has developed a resistance to a particular type of insecticide, also the action of some other resists that has a form to act similar.

The reality of these problems has caused that many agriculturists practice what ecological agriculture or biological control is known like, where is minimum or null the use of organic insecticides and in which the insecticide action is carried out by other animals, in whose diet are the insects to eliminate.

Control of plagues.

All a series of interventions in the medio.ambiente is understood by control of plagues directed to make disappear or to at least reduce the effects of the plagues of insects in the agricultural production. Also part of the control of plagues will be considered the fight against the bad grass and any other pathogenic element that can affect the plants. The procedures used for such control incorporate a series of mechanisms that include physical procedures as biological and chemical as much, emphasizing between all of them organic chemical insecticides.

Historical references.

Ever since the man practiced of conscious form seedtime and the consequent harvesting of the fruit, plagues and diseases have existed that made lose partly, when not everything, the harvest. The search on the part of the man of remedies against this type of infortunios is therefore as old as same agriculture. The first made effective control against the plagues was based on the selection for seedtime only of the most resistant specimens, action than it could be fitted within the biological control of plagues. Nevertheless, the scientific study of the problem did not begin to carry out until centuries XVII and XVIII. Later, in the middle of century XIX, it was managed to control the plague that invaded the European vitivinícolas cultures with the use of chemical agents. The plague had been concerned from America and brought Europe; when not inhabiting in the old continent his natural enemies, one extended mainly without no impediment Europe. The plague managed to stop using sulfur dusts. The inverse case also occurred, when causing an insect of European origin a plague of cultures to the other side of the Atlantic. By these same dates also the news of another plague were had that devastated the potato cultures, and although it had appeared in Ireland in 1845, quickly it affected cultures of all Europe and America.

The scientist Jonh Curtis wrote the first book on the plagues of insects in farms, moved by the economic impact that these produced in those farms that were contaminated with some plague. When the first colonos arrived at the region of Colorado, in North America, and planted the potato, they were with escarabajo that it ruined all the harvest. Was not remedy, or only he was it partial, until to an American entomólogo it was happened to dissolve certain compound chemistries to him (calcium and derivatives of arsenic) in water and to scatter it by the leaf of the culture. According to it was passing the time other many insecticides were being developed, using for it insecticide products with natural components such as the nicotine, petroleum and the piretinas. In the years passed from end of century XIX to first of century XX, products saw the light numerous that increased the possibilities of fighting successfully against many types of plague.

Biological control of plagues.

By biological control of plagues the use is understood of alive organisms in tasks of control of plagues. The problem consists of the search of a natural enemy of the insect to fight, as a predator, parasite or disease. Once found, it will be scattered by all the zone to protect. If the means are not to him adverse it will be able to be multiplied and to make an effective control of plagues. As example of biological control can be mentioned the one that was carried out to limit the increase of individuals in the increasing community of European rabbits in Australia, introducing the virus of the mixomatosis. Actually the cases are many in which an effective fight against the plagues with this type of biological control is carried out, helped by genetic engineering, implanting in the genetic material of the plants resistance genes which they in front of multiply the natural resistance of the cultures the diseases and insects. Another form to use genetic engineering for the control of plagues has been the creation or modification of some other virus whose objective is the insects of the plague to fight. Obtained this one it will be scattered by fumigation on the culture. The virus artificial and genetically modified is designed to be innocuous other insects, and to disappear when the attacked insect has been eliminated, to the being the this one last only animal in which it can reside. Many examples of the use of virus, fungi or protozoos for a selective control of plagues exist.

CRS.

Herbicida

or {m.} Any chemical agent that destroys the grass or prevents them to be multiplied.