**4. Persistence in soil and water**

When pesticides are manually or aerially sprayed on seeds, soil, or even directly on plants, they can last for days, months, or even years. They might also filter through the soil into surface and deep waterways, polluting food and water sources for living beings by coming into contact with animal and plant life. **Table 1** illustrates the soil-water partition coefficients (Koc) and octanol–water partition coefficients (Kow), which are used to characterize the mobility and bioaccumulation properties of pesticides, respectively. While these coefficients are not the only indicators used to determine pesticide behavior in the environment and in organisms, they do serve as referents for pesticide toxicity.

The Koc is a coefficient that is used to determine the pesticide concentration "attached" to soil particles as well as the phase present in the solution, i.e., dissolved in the same soil's water. As a result, the lower the temperature, the higher the Koc of the pesticide in solution, and the greater the likelihood of it leaching into groundwater. The Kow is a coefficient that is used to calculate pesticide concentrations in octanol and water. Pesticides having a high Kow, which are more soluble in octanol and less soluble in water, have been found to accumulate in organisms [23]. Chlorpyrifos accumulates greater in organisms than carbaryl and imidacloprid, as shown in **Table 1**. It does, however, have a lesser tendency to leak into the soil as compared to them. In this sense, imidacloprid would pose a greater risk as a groundwater pollutant.

To estimate a substance's environmental fate in diverse environments, scientists must first determine its degradation half-life, or DT50, which is the time it takes for


#### **Table 1.**

*Crucial physicochemical characteristics for insecticides are chlorpyrifos, carbaryl, and imidacloprid.*

#### *Neurotoxic Effects of Insecticides Chlorpyrifos, Carbaryl, Imidacloprid, in Different Animal… DOI: http://dx.doi.org/10.5772/intechopen.100527*

50% of a chemical to degrade or disappear from water or soil [7]. For the purposes of this review, the three pesticides DT50 examined will be provided below, depending on their average persistence in soil and water,

Chlorpyrifos can have a long persistence even in arctic regions, where its presence has been assessed in samples of ice, snow, a microcosm of water, sediments, air, and flora. The persistence of this pesticide (due to its high resistance to hydrolysis) has been reported to be greater in aquatic habitats than in soil. However, the LD50 in soil, has a wide range of values as reported in the literature, ranging from a few days to four years. It is also suggested to be more stable in low-pH soils, dark settings, and cold environments [7]. Chlorpyrifos DT50 has been found to last from 1 to 120days in the field and up to 180 days in the soil in the absence of light. It is worth noting that in organic soils, the half-life is longer than in mineral soils. A DT50 of 150 to 200 days has been documented in anaerobic pond sediments, while 106 + 54 days has been reported in experimental circumstances of wetland and anaerobic sediments. Chlorpyrifos has a DT50 of 18.7 days in freshwater and 49.4 days in seawater at 10°C, which decreases with increasing temperature [24].

In the case of carbaryl, its DT50 in the soil ranges from 17 to 28 days. It is considered to have low persistence, where it is degraded mainly by the action of light and bacteria. In sandy soil conditions, its half-life is 7 to 14 days, while in clay soil it ranges from 14 to 28 days, hydrolyzing itself rapidly in alkaline soils. The DT50 in water is highly variable, increased in acidic conditions; for example, in acidic water with a pH of 5, degradation is slow and can persist for up to 1500 days [23]. The DT50 of carbaryl in soil has recently been reported to be 16 days, while it can reach 12 and 5.8 days in water and sediments, respectively [25].

Neonicotinoids have a high DT50, which means they can last a long time in the soil, with values in the range from 6.7 to 1230 days, while imidacloprid has the highest DT50, with a value of 35.9 to 1230 days. Though it should be noted that the degradation of neonicotinoids and other pesticides in soil is dependent on pH, temperature, humidity, chemical concentration, and even the presence of microorganisms [26]. As evidence, imidacloprid has been found to remain for 42 to 129 days in vegetated soils and more than 180 days in soils free of vegetation [27]. The data on this insecticide's water persistence is varied, with half-lives ranging from 1 to 3 hours, 48 hours, and even 31 to 43 days [28].

## **5. Neurotoxic effects in different animal species**

The lethal dose 50 or LD50, is a measure that in toxicology is used to estimate the dose of a test substance that produces 50% of death in a certain animal species. It is used as a reference to determine how toxic it is to humans [29]. The LC50 or lethal concentration 50, corresponds to the concentration of a chemical substance in the


**Table 2.**

*Toxicological classification for pesticides with moderate toxic effects.*


**Table 3.**

*Effect of LD50 or LC50: chlorpyrifos, carbaryl, and imidacloprid in different animal species.*

air or in the water that causes half of the exposed animals to die [30]. According to the WHO toxicological classification for pesticides (**Table 2**) [31], both imidacloprid and carbaryl are in class II, which includes those pesticides with moderate toxic effects, while chlorpyrifos is located in class 1b since its LD50 is below 200 mg. Therefore, it is considered highly dangerous. In **Table 3**, the LD50 or LC50 for chlorpyrifos, carbaryl, and imidacloprid in different animal species are illustrated.

#### **6. Neurotoxic effects of chlorpyrifos, carbaryl, and imidacloprid**

Although insecticides are substances designed to kill some kinds of insects that cause pests, for decades it has been documented that they can also kill insects that should not be the target of their toxic effects and that overall, are essential for life on planet Earth. The most documented case is the decrease in pollinator populations and its possible association with insecticides utilization. In recent reviews, information supporting that insecticides can interfere with localization capacity, alteration of foraging and motor behavior, olfactory learning, and flight ability has been gathered. Additionally, they negatively impact the immune system and increase the death rate, among other toxic effects in bees [32–35], bumblebees [36–38], butterflies and moths [39–42], ants [43, 44], earthworms [39, 45] and various aquatic invertebrates [46–48]. They have also been associated with neuronal and colony performance alterations in bumblebees [32]. Insecticides such as dichlorvos, imidacloprid, and malathion, among others, can harm butterfly populations, resulting in decreased survival and changes in feeding and oviposition patterns [49].

Therefore, the effects on non-target insects have received special attention. According to studies on these species, an environmental emergency has been declared due to the decline in their populations. It is worth noting that insecticides have effects not confined to insects, which exacerbates the existing problem because all living beings are exposed to varying degrees of insecticides, making humans vulnerable to their toxic effects. Following, there is a brief overview of the effects identified in the last five years for each of the insecticides that have been the subject of this chapter, grouped into three different types of effects: behavioral, neurochemical, and cellular (**Tables 4**–**6**). However, for more detailed information, consider the present bibliography.

#### **6.1 Chlorpyrifos**

The recent literature regarding chlorpyrifos toxic effects in different species is extensive. However, this chapter has focused on those that are associated with effects


*Neurotoxic Effects of Insecticides Chlorpyrifos, Carbaryl, Imidacloprid, in Different Animal… DOI: http://dx.doi.org/10.5772/intechopen.100527*


*Behavioral effects of chlorpyrifos, carbaryl and imidacloprid on five animal species.*

**68**


*Neurotoxic Effects of Insecticides Chlorpyrifos, Carbaryl, Imidacloprid, in Different Animal… DOI: http://dx.doi.org/10.5772/intechopen.100527*



*Neurotoxic Effects of Insecticides Chlorpyrifos, Carbaryl, Imidacloprid, in Different Animal… DOI: http://dx.doi.org/10.5772/intechopen.100527*


#### **Table 6.**

*Effects on the cellular level of chlorpyrifos, carbaryl and imidacloprid on five animal species.*

on the nervous system. For example, in non-target insects, such as bees, it has been observed that it can have adverse effects on caste differentiation [50], as well as on olfactory learning and memory retention [51, 53]; in cockroaches [52] and mosquito larvae [54] has been associated with locomotor alterations (**Table 4**) [143]. It has also been documented that chlorpyrifos can cause alterations in acetylcholinesterase activity and induce oxidative stress in different insects [52, 54, 103] and annelids (**Table 5**). On the other hand, in aquatic organisms such as mollusks, crustaceans, amphibians, and fish, it has been reported that it can cause alterations in locomotor activity [61, 63, 64, 144], inhibit acolinesterase in shrimp [62, 144], copepods [145], common carp [106], tadpoles [63] and snails [61, 107], as well as causing neuronal degeneration in catfish [64]. In toxicity studies carried out in broilers, it has been described that it can cause nervous signs such as salivation, tearing, panting, frequent defecation, tremors, and seizures [77], in sparrows, it can alter the migratory orientation [78] and inhibit acetylcholinesterase activity in broilers [77] and quail (**Tables 4** and **5**) [113]. Regarding its cellular effects, in repeated studies, chlorpyrifos has been reported to be associated with neurodegeneration in broilers [127, 128]. The neurotoxic effects of chlorpyrifos scale to small mammal species. In fact, in rodents under experimental conditions, it has been seen that it can have anxiogenic effects [83, 86] and cause alterations in the memory of recognition [84] and reference [86] in locomotor activity [85, 87], in social behavior (**Table 4**) [84, 89].

While, acute poisonings are associated with signs of piloerection, tremors, seizures, and hypoactivity, among other neurological manifestations [88]. Regarding brain neurochemistry in experimental rodents, it has been reported that chlorpyrifos can alter the activity of acetylcholinesterase. It participates in the downregulation of genes related to Parkinson's disease, causes oxidative stress and decreases dopamine and serotonin levels [86, 87, 117, 118]. Overall, it has also been associated with neurodegeneration in rodents for experimentation [85, 131, 132]. In humans, it has been reported that chlorpyrifos can alter social and motor function in children (**Table 5**) [96, 97]. As well as having fallout related to neurobehavioral deficits in workers exposed to the insecticide [98]. At the neurochemical level, in an in vitro study with human cells, it was shown that it can decrease intracellular levels of ATP and cause mitochondrial dysfunction [121]. Finally, at the cellular level, it has been reported to cause inhibition of activated calcium channels by voltage [137], alter

morphology [138], and induce apoptosis in vitro [139]. In human cells exposed to chlorpyrifos, a recently published study reported that it may be associated with alterations in the morphology of different brain regions in children exposed to the substance (**Table 6**) [140].
