**1. Introduction**

#### **1.1. Insecticides and mode of action**

Insecticides are traditionally employed in several human activities with the purpose of eliminating or controlling the density of undesired insect populations. At present, albeit the obvious environmental impact, the control of agricultural pests and disease vectors is still largely based on the use of those substances. Moreover, in several cases, chemical compounds represent the principal approach to interrupt the transmission of pathogens. Before the Second World War, most insecticides were constituted of inorganic compounds, and a few organic substances, such as nicotine, pyrethrin, and rotenone [1]. The modern era of organic insecti‐ cides began in the 1940s, a period known as the age of the "pesticide revolution", when DDT (dichlorodiphenyltrichloroethane) was used for the first time as an insecticide [2].

Currently, there are 25 groups of insecticides and acaricides based on available evidence about their target sites and mode of action, according to the Insecticide Resistance Action Committee (IRAC) [3]. The World Health Organization Pesticide Scheme (WHOPES) promotes and coordinates the testing and evaluation of pesticides for public health purposes, since 1960. Its recommendations are generally adopted for national campaigns in several countries. The main insecticide classes used for vector control are: organochlorine (OC), organophosphates (OP), carbamates (CA), pyrethroids (PY), insect growth regulators (IGR), spinosyns (SP), and toxins derived from bacteria (*Bacillus thuringiensis* var. *israelensis* and *Bacillus sphaericus*) [4, 5]. The classes OC, OP, CA, PY, and SP include a broad range of compounds that act on the insect central nervous system and, thus, have an immediate effect.

The target site of OP and CA is the acetylcholinesterase (AChE), a conserved enzyme present in a wide variety of animals, including mammals, birds, reptiles, fish, and insects. This enzyme is responsible for the rapid hydrolytic degradation of the acetylcholine neurotransmitter at synapses, causing momentary interruption of the nerve impulse. The OP and CA insecticides bind in the AChE active site, compromising the acetylcholine hydrolysis and then accumulat‐ ing the neurotransmitter at the synapses, causing repetitive nerve impulses.

The PY and OC (DDT and analogues) maintain the sodium channels in their opened confor‐ mation, generating a continuous influx of ions throughout the axons. Cyclodienes, another group of OC insecticides, act directly on the gamma-aminobutyric acid receptor (GABA), preventing the normal input of chlorine ions in the neurons, just after the nervous impulse. In all cases, regardless of the target site, OP, OC, and PY promote a continuous nerve impulse transmission that culminates in paralysis, convulsions, and death [6].

Unlike neurotoxic insecticides, IGRs do not induce an immediate death of the insects. How‐ ever, they are toxic mainly against immature stages, affecting the moulting, metamorphosis processes, besides commitments in viability and reproduction of adults [7]. Based on the mode of action, the IGRs are classified into three major categories: (i) juvenile hormone mimics; (ii) ecdysone agonists; and (iii) chitin synthesis inhibitors [8].

Concerning the bacterial toxins, *Bacillus thuringiensis* (*Bs*) var. *israelensis* (*Bti*) and *B. sphaeri‐ cus* are the most employed as insecticides. When ingested by larvae, the *Bt* toxins are activated by insect proteases and bind to specific receptors in the larvae midgut epithelia. The final effect is an osmotic stress that leads to the disruption of midgut membranes and, consequently, to death [9]1 .
