**4. Biological basis of pesticides resistance**

transferase, and carboxylesterase [16, 17]; all these enzymes hydrolyze or sequester different kinds of pesticides. Exposure to a pesticide can exert enough pressure to select an enzymatic system or a specific isozyme within each family. Esterase isozyme overexpression is generally accepted as a mechanism involved in OP resistance. However, in the Coatzacoalcos laboratory strain of *R*. *microplus*, designated as such to reflect the name of the village in Mexico where the original tick population sample was obtained, metabolic detoxication has been identified by its efficient esterase activity resulting from enzyme overexpression as a resistance mechanism for permethrin that belongs to the pyrethroid chemical class of pesticides. This strain has been toxicologically characterized using the larval packet test (LPT) [18], which helped to elucidate the esterase-based mechanism of resistance to permethrin. The *R*. *microplus* Coatzacoalcos strain exhibits a significant enhanced capacity to hydrolyze permethrin as well as an increased esterase activity. This suggests an esterase based metabolic mechanism as a main component of permethrin resistance [19]. The esterase gene responsible for permethrin resistance was identified and named *CzEST9*. It is also known that the overexpression mechanism of this isozyme is the result of *CzEST9* duplication in the Coatzacoalcos strain that leads to metabolic detoxication through the overexpression of esterase 9 activity in *R. microplus* [20]. The sequence of *CzEST9* gene has been determined and the recombinant product yielded a 62.8 kDa protein [19]. Since the Coatzacoalcos strain does not include the *Kdr* variation in the sodium channel gene found in other Mexican strains of *R*. *microplus*, it is suggested that there are two inde‐ pendent mechanisms of acaricide resistance to pyrethroids. However, common mechanisms of acaricide resistance to pyrethroids in Mexico apparently involve the presence of sequence

The sodium channel is the known target site of pyrethroids. Sequence variation in the sodium channel prevents pyrethroids from attaching to the target site due to an alteration in the sodium channel stereochemical structure. For this reason, the process is described as target site modification mechanism, or *Kdr*-type resistance (Knock down resistance). This is one of the

Two important allele variants occurring in the sodium channel gene associated with pyreth‐ roid resistance in the cattle tick *R. microplus* are the variation occurring in domain III segment 6 (III-S6) [22] and the variation occurring at the bridge joining segments 4–5 in domain II (II-S4-5) [23]. The former is a Phe-Ile substitution produced by a nucleotide variation at domain III-S6 that was first reported in Mexican tick strains. Its role and contribution to pyrethroid resistance has been confirmed [21]. The other is a Le-Ile substitution found thus far only in Australian tick strains; this variation is very similar to a variation found in the crop insect pest

Findings on the diversity of allele variants occurring in the sodium channel gene associated with pyrethroid resistance in the cattle tick *R. microplus* have been confirmed by experiments based on differences in melting temperatures (Tm) of sodium channel allele specific gene fragments obtained with single larvae DNA from México and Australia. These experiments revealed that substitution III-S6 (Phe-Ile) only occurs in Mexican tick populations whereas substitution II-S4-5 (Le-Ile) only occurs in Australian tick strains [24]. The information available suggests that there are at least two different and independent mechanisms involved based on

mechanisms of pesticide resistance in insects that is better understood.

variation in the sodium channel gene [21].

*Bemicia tabaci* [23].

338 Insecticides Resistance

Allelochemical interactions are defensive processes or are involved in food competition mechanisms that different species employ to inhibit the action of natural enemies. Plant– arthropod coevolution is a natural selection mechanism driven by allelochemical interactions between plants and arthropods over millions of years [7]. As a result of the reciprocal inter‐ actions between these two groups of organisms [8], arthropods have conserved within their genomes all those traits conferring them the ability to inhibit or avoid toxicants produced by plants that function as defensive mechanisms against herbivorous insects [9, 10]. Sophisticated metabolic detoxication mechanisms have been developed by herbivorous arthropods in order to survive the exposure to toxic plant metabolites [11], which represents a natural process of resistance to plant toxicants. The preservation of these components in the genome of arthro‐ pods provides the foundation of molecular systems that allow them to get adapted and become resistant against pesticides currently used.

Insects affect the survival and reproduction of plants. Secondary metabolites like phenolic acids, flavonoids, terpenoids, steroids, alkaloids, and organic cyanides are produced by plants as part of defensive mechanisms. A coevolutionary race is established through these interac‐ tions where mutual selection pressure has led to the process of speciation to preserve natural equilibrium.

Adaptation to new environmental conditions requires the development of defensive processes through natural selection. Plants are biological engines producing a wide variety of natural defensive chemicals including repellents, antifeeding molecules, and poisons, some of which have been used as natural strategies to protect cultivars from plagues or to control vector-borne diseases. These mechanisms evolved separately in different herbivorous insects driven by diverse insect–plant interactions. In general, herbivorous insects feed on few plant species and plant species are attacked by pests specialized to overcome natural defensive substances. Herbivorous insects have developed the molecular machinery to metabolize most of the toxic material produced by plants, but not all toxicants are metabolized by all pest species.

Specialized insects have an adaptive advantage because their biochemical systems evolved to detoxify one or few potentially harmful substrates. The metabolic system of a polyphagous species reflects more diverse detoxication mechanisms against a wide variety of chemically defined plants. Thus, polyphagous insects have a "higher metabolic load". The activity of mixed function oxidases in the intestines of moths and butterfly larvae is higher in polypha‐ gous species than in species restricted to a single family of plants. Pyrethrins are part of the wide variety of allelochemicals metabolized by this family of enzymes [25].
