**4. Development of resistance to chemical insecticides**

Insecticide resistance is defined as the development of the ability of a insect population to tolerate doses of an insecticide, which would be lethal to the majority of individuals in a normal population of the same species and is also the result of pressure of positive selection exerted by the insecticide on the low frequency genes initially present in the vector insect [95]. Therefore, the development of resistance by mosquito disease vectors is of international concern due to the increase worldwide exchange of plant matter that mosquitoes can spread to other parts of the world, spreading resistance genes of the plagues that they have.

Most mosquito vector control programs of diseases in humans are mainly based on the use of chemical insecticides by outdoor spraying, impregnated nets, or indoor residual spraying [96]. Thereon, the use of insecticides has helped to eradicate insect-borne diseases. In this regard, since 1950, different classes of insecticides have been successively used. Organophosphates and pyrethroid insecticides have been used to control mosquito populations in their larval and adult stages. However, more recently, the disease vector control programs are based largely on the use of synthetic pyrethroid insecticides, which are recommended by the WHO only for impregnated nets [97]. However, the massive use of pesticides has caused detrimental effects on the agroecosystem, such as the acquisition of resistance, pest resurgence, and environmental pollution. Resistance has developed in more than 84 species of mosquitoes for each of the groups of toxicological insecticides [98]. Furthermore, it was found that insecticide residues accumulated in plants often end up in water bodies where mosquito larvae feeding on such plant debris or grow in water bodies enriched with plant compounds and interactions between these xenobiotics generate tolerance to insecticides or promote detoxification pathways of these insecticides against mosquitoes [99]. In addition to abiotic factors, biotic interactions that occur among mosquitoes, the pathogens that they transmit and their microbiome (microbes living in the mosquito) may also occur [96]. These vary from symbionts to entomopathogen opportunistic organisms that are able to affect various physiological host processes, such as detoxification systems [100] or the opposite effect leading to the appearance of insecticide resistance [101]. Furthermore, allelochemicals inducing enzyme production in insects can increase their tolerance to pesticides [102]. On the other hand, other studies have shown that the degree of development of a plant can affect insecticide resistance in insects [103].

There are two main mechanisms by which mosquito vectors can develop resistance to insecticides: alterations in the target site of action and metabolic resistance, also called increased rate of detoxification of insecticides [19]. Other less common mechanisms that develop resistance in insects are the resistance per behavior and the resistance per decreased penetration through the cuticle or cross resistance [104].

#### **4.1. Resistance mechanisms**

Each insecticide triggers the selection of one or more mechanisms of resistance; in addition, an unknown number of behavioral changes in adults. For instance, changes in the target site of action are produced when no silent mutations occur in structural genes that produce an alteration of amino acids responsible for anchoring the insecticide at a specific site. For example, resistance have been reported by altering the voltage-dependent sodium channel that is the target site of action for pyrethroids and organochlorines, such as DDT, and in the insensitive acetylcholinesterase, which is the target site of action for organophosphate and carbamate [19]. Furthermore, the metabolic detoxification is an acquired resistance mecha‐ nism, which is regulated by the activity of certain oxidized enzymes such as mixed func‐ tion oxidase, esterases, glutathione S-transferases, and in specific cases DDTdehidroclorinase. Mixed function oxidase represents an important detoxification mechanism in the degradation of carbamates; moreover, esterases have an important role in the degrada‐ tion of phosphorus insecticides [105]. Meanwhile, the metabolic resistance occurs through the increase in the detoxification of the insecticide. The most important form of metabolic resistance is given by detoxifying enzymes type glutathione S-transferase, mixed function oxidases, and esterases [78].

On the other hand, cross resistance can occur in two ways, positive and negative. The positive cross resistance refers to resistance to several insecticides due to expression in a single resistance mechanism [106]. Therefore, cross resistance occurs when a single gene confers resistance to a number of chemicals in the same group, such as *kdr* gene conferring resist‐ ance to DDT and pyrethroids [95]. Meanwhile, negative cross resistance occurs due to an increase in susceptibility to the insecticide "A," caused by the development of resistance to insecticidal "B" and vice versa. For example, in *Culex pipiens quinquefasciatus* larvae, it was found that resistance to organophosphorus insecticides increases susceptibility to pyreth‐ roid insecticides [107].

Furthermore, multiple resistances occur when two or more resistance mechanisms independ‐ ently selected are operating in the same insect [19]. However, the term multiple resistances not necessarily involve the cross-resistance term because an insect may be resistant to two or more insecticides, and each resistance can be attributed to different mechanisms [78]. Consequently, each additional mechanism of resistance leads to a wide cross resistance, which restricts the number of possible alternatives for the control and in extreme conditions, leading to highly resistant populations to virtually all available insecticides [108].
