**5. Discussion**

In principle, all life forms are immersed in an ionized environment. Ions bear an electric charge and thus an electric field may be influenced by another one. Thus, the electric fields from two bodies would interact in such a way that initially the ions would be driven or set into motion.

Our experiments showed that electrostatic charge in wheat seeds is weakly negative (−0.191 (±−7.15 × 10−2) pC/grain), the electrostatic charge of diatomaceous earth is slightly negative (−11.554 (±2.342) pC/grain) and nanostructured alumina bears a strong negative electrostatic charge (−93.91 (±2.62) pC/grain). These data indicate despite the negative charge of wheat kernels, other characteristics such as rugosity and hairs on wheat kernels' surface are determinant for the surface attachment of DE and NSA particles (**Figure 3a, b**). Thus, even the repelling force between like charged particles and wheat kernels, the low net charge density of these will not be relevant enough for particle detachment from the kernels and therefore, the smaller the particles the denser the wheat grain surface coverage (**Figure 3b**).

These differences in attachment effectiveness are evidenced by the fact that insects exposed to surfaces treated with NSA became massively and uniformly coated with NSA particles (**Figure 5a**). In contrast, insects exposed to surfaces treated with DE showed a scant and diffuse distribution of particles on their body surface (**Figure 5b**) demonstrating that DE are not

**Figure 3.** SEM image (300×) of a wheat grain *Triticum sativum* var. Baguette NIDERA. a – exposed to 125 ppm DE; b –

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Insecticidal inert powders in general, attach to the insect cuticle surface (**Figure 1**) damaging the cuticle and producing a negative effect on insect water balance [36]; furthermore insecticidal efficacy decreases as ambient humidity increases and this may negatively impact the efficacy of inorganic powder insecticides [26, 32]. This decrease in efficacy in at higher relative ambient humidity of abrasive powders as DE can be explained by a delayed drying process [80] due to a slower rate of water loss through the damaged insect cuticle [40, 81–84]. The natural transpiration rate of an insect into the surrounding air is dependent on water vapor pressure. With increasing relative humidity the vapor pressure increases in the air and the water discharge from the insect body surface tends to decrease. These results are consistent with earlier findings for abrasive insecticide powders which suggested that toxicity of insecticide powders on arthropods is a consequence of the "cuticular water flux" [84]. On the other hand, the loss of insecticide efficacy in sorbtive insecticide powders such as NSA at higher relative humidity can be explained by analyzing the effect of moisture on the interaction of tribocharged insect body surface and the small but high electrically charged particles of NSA as follows: at constant temperature, a substance absorbs moisture from the air until the material and humidity are in equilibrium attaining the adsorption isotherm of the substance [85]. As shown, triboelectric charge is the main reason for insecticide powder adhesion to insect body surface. In general, the importance of the triboelectric effect increases with low humidities and with smaller particles. High relative humidity can influence the interparticle forces when certain quantity of water is condensed on particle surface reducing the electrostatic forces by electrostatic discharge [86]. Initially, at lower humidity (<65%) [87], the water adsorbs on the particle in the form of water vapor. So, the interparticle bond forces can be reduced as electrostatic forces are reduced. As the humidity increases exceeding the critical value, capillary condensation occurs at the contact points of the particles and liquid bridges form. Above

retained as NSA particles are (**Figure 5c**).

exposed to 125 ppm NSA. Format JEOL/EO, Version 1.0; Instrument JSM-6610.

As shown here and by different authors [55, 58, 60], insects possess bodily electric charges raised by walking or flying. In our experiments, the insects rubbing against flour on the 30° tilted ramp emulate their movement within a stored grain matrix where they charge themselves throughout friction (tribo-charging) and thereby enhance adherence of all particles bearing an opposite charge to their body.

As shown, the rate of insect tribo-charging at the start of the ramp was proportional to the saturation charge that decreases as the insects charge increases. This can be explained as follows: a sliding insect can be thought of as a conducting but electrically isolated object in motional contact with the ramp. The insect and ramp surface start with unequal electron affinities. The ramp surface has a high electron affinity so it takes electrons from the insect gaining negative charge and the insect gains a positive charge due to the loss of electrons [79].

The experimental results presented here (**Figure 2**) show that adults *S. oryzae* take up and retain a positive electrostatic charge on the cuticle, approximately proportional to the distance shifted on the experimental wheat flour ramp (d1.25cm = +0.766 (±0.254) pC/insect to d40.0cm = +2.56 (±0.221) pC/insect), which is consistent with the results obtained by Jackson and McGonigle [60] experiments. Thus, when *S. oryzae* was exposed to wheat kernels treated with NSA and/or DE dry powder, negatively charged particles became attracted to the positive tribo-charged insect body surface. However, bonding of DE particles on the insect body surface is 8.13 times weaker than NSA due to lower electric net charge (−11.554 (±2.342) pC/grain) of DE and its larger particle size (**Figure 4b**) and mass. Instead, bonding of NSA particles to the insect body surface is strong due the magnitude of its electric charge (−93.91 (±2.62) pC/ grain) and because particles are smaller and lighter (**Figure 4a**).

**4.3. Electrostatic charge in wheat kernels**

94 Insecticides - Agriculture and Toxicology

bearing an opposite charge to their body.

**5. Discussion**

tive, averaging −0.191 (± −7.15 × 10−2) pC/grain.

The electrostatic charge measured on wheat kernels var. Baguette NIDERA was weakly nega-

In principle, all life forms are immersed in an ionized environment. Ions bear an electric charge and thus an electric field may be influenced by another one. Thus, the electric fields from two bodies would interact in such a way that initially the ions would be driven or set into motion.

Our experiments showed that electrostatic charge in wheat seeds is weakly negative (−0.191 (±−7.15 × 10−2) pC/grain), the electrostatic charge of diatomaceous earth is slightly negative (−11.554 (±2.342) pC/grain) and nanostructured alumina bears a strong negative electrostatic charge (−93.91 (±2.62) pC/grain). These data indicate despite the negative charge of wheat kernels, other characteristics such as rugosity and hairs on wheat kernels' surface are determinant for the surface attachment of DE and NSA particles (**Figure 3a, b**). Thus, even the repelling force between like charged particles and wheat kernels, the low net charge density of these will not be relevant enough for particle detachment from the kernels and therefore,

As shown here and by different authors [55, 58, 60], insects possess bodily electric charges raised by walking or flying. In our experiments, the insects rubbing against flour on the 30° tilted ramp emulate their movement within a stored grain matrix where they charge themselves throughout friction (tribo-charging) and thereby enhance adherence of all particles

As shown, the rate of insect tribo-charging at the start of the ramp was proportional to the saturation charge that decreases as the insects charge increases. This can be explained as follows: a sliding insect can be thought of as a conducting but electrically isolated object in motional contact with the ramp. The insect and ramp surface start with unequal electron affinities. The ramp surface has a high electron affinity so it takes electrons from the insect gaining negative

The experimental results presented here (**Figure 2**) show that adults *S. oryzae* take up and retain a positive electrostatic charge on the cuticle, approximately proportional to the distance shifted on the experimental wheat flour ramp (d1.25cm = +0.766 (±0.254) pC/insect to d40.0cm = +2.56 (±0.221) pC/insect), which is consistent with the results obtained by Jackson and McGonigle [60] experiments. Thus, when *S. oryzae* was exposed to wheat kernels treated with NSA and/or DE dry powder, negatively charged particles became attracted to the positive tribo-charged insect body surface. However, bonding of DE particles on the insect body surface is 8.13 times weaker than NSA due to lower electric net charge (−11.554 (±2.342) pC/grain) of DE and its larger particle size (**Figure 4b**) and mass. Instead, bonding of NSA particles to the insect body surface is strong due the magnitude of its electric charge (−93.91 (±2.62) pC/

the smaller the particles the denser the wheat grain surface coverage (**Figure 3b**).

charge and the insect gains a positive charge due to the loss of electrons [79].

grain) and because particles are smaller and lighter (**Figure 4a**).

**Figure 3.** SEM image (300×) of a wheat grain *Triticum sativum* var. Baguette NIDERA. a – exposed to 125 ppm DE; b – exposed to 125 ppm NSA. Format JEOL/EO, Version 1.0; Instrument JSM-6610.

These differences in attachment effectiveness are evidenced by the fact that insects exposed to surfaces treated with NSA became massively and uniformly coated with NSA particles (**Figure 5a**). In contrast, insects exposed to surfaces treated with DE showed a scant and diffuse distribution of particles on their body surface (**Figure 5b**) demonstrating that DE are not retained as NSA particles are (**Figure 5c**).

Insecticidal inert powders in general, attach to the insect cuticle surface (**Figure 1**) damaging the cuticle and producing a negative effect on insect water balance [36]; furthermore insecticidal efficacy decreases as ambient humidity increases and this may negatively impact the efficacy of inorganic powder insecticides [26, 32]. This decrease in efficacy in at higher relative ambient humidity of abrasive powders as DE can be explained by a delayed drying process [80] due to a slower rate of water loss through the damaged insect cuticle [40, 81–84]. The natural transpiration rate of an insect into the surrounding air is dependent on water vapor pressure. With increasing relative humidity the vapor pressure increases in the air and the water discharge from the insect body surface tends to decrease. These results are consistent with earlier findings for abrasive insecticide powders which suggested that toxicity of insecticide powders on arthropods is a consequence of the "cuticular water flux" [84]. On the other hand, the loss of insecticide efficacy in sorbtive insecticide powders such as NSA at higher relative humidity can be explained by analyzing the effect of moisture on the interaction of tribocharged insect body surface and the small but high electrically charged particles of NSA as follows: at constant temperature, a substance absorbs moisture from the air until the material and humidity are in equilibrium attaining the adsorption isotherm of the substance [85]. As shown, triboelectric charge is the main reason for insecticide powder adhesion to insect body surface. In general, the importance of the triboelectric effect increases with low humidities and with smaller particles. High relative humidity can influence the interparticle forces when certain quantity of water is condensed on particle surface reducing the electrostatic forces by electrostatic discharge [86]. Initially, at lower humidity (<65%) [87], the water adsorbs on the particle in the form of water vapor. So, the interparticle bond forces can be reduced as electrostatic forces are reduced. As the humidity increases exceeding the critical value, capillary condensation occurs at the contact points of the particles and liquid bridges form. Above

in addition to low sorbtive properties. So, DE works, in general, stochastically by damaging the insect body surface mechanically when it moves within a stored grain matrix. On the other hand, NSA's high insecticidal efficacy depends on its increased electrical affinity to the insect body surface (**Figure 4c**) in addition to having greater sorbtive properties. The whole mechanism of action consists of two steps in sequential order. First, there is a strong electrical binding between negatively charged NSA particles and the positive tribo-charged insect. Next, dehydration of the insect occurs due to strong sorptive action of NSA particles removing the insect cuticular waxes responsible for protecting insects against water loss. Hence, the mechanism of action of NSA does

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Nanostructured alumina (NSA) is a nano-engineered material which has insecticide properties. The current study investigated its mode of action and demonstrated that tribo-charging is a key aspect in the interaction of NSA and the insects' cuticle. In fact, triboelectric charge is the main reason for insecticide powder adhesion to the insect body surface, and could explain at least in part, the efficacy differences observed in previous studies between NSA and diatomaceous earth (DE). Insects exposed to surfaces treated with NSA became massively and uniformly coated with NSA particles while insects exposed to surfaces treated with DE showed a scant and diffuse distribution of particles on their body surface. This in turn, was accompanied by a difference in charge between both powders, where NSA has a greater intrinsic electric charge than DE. Moreover, NSA charges did not decay as a consequence of NSA low wettability. Thus, the current study supports previous studies showing that NSA has a greater affinity towards the insect cuticle and a greater insecticidal efficacy than other inert powders, and provides a reasonable explanation of its mechanism of action through triboelectric and sorbtive phenomena. Further research is necessary to contribute to the knowledge of the complex relationships between physical and chemical parameters of insects and powders, responsible for insecticide activity. Future studies should focus on determining the insect chemical and physical characteristics that are involved in toxicity of inert powders such as NSA to insects. Measuring the tribolectric charges of different insect species could shed light on the basis of these differences in toxicity observed among different insect species to NSA, which may be related to their chemical composition as well as their

target the water balance of the insect and dehydration is the leading cause of death.

physical structure, leading to electric charges of different sign and magnitude.

With regards to toxicity research studies should aim to determine the extent of absorption, systemic availability, accumulation and excretion of nanomaterials after inhalation and oral exposure, as well as genotoxicity. However, the necessary in vitro studies should be integrated into any toxicological studies to avoid unnecessary animal experiments. The influence of modifications in the NSA synthesis on the kinetic parameters as well as on the toxicological properties of the nanomaterials should also be examined. Finally, oxidative stress and the formation of reactive oxygen species (ROS) are fundamental key mechanisms of cellular defense

The current study, investigating the mode of action of NSA, supports previous studies demonstrating that NSA is more effective than other insecticide powders and has good potential

**6. Conclusions**

after particle capture.

**Figure 4.** Scanning electron microscopy image of: a – nanostructured alumina (NSA) particles; b – diatomaceous earth (DiatomiD®). Format JEOL/EO, Version 1.0. Instrument JSM-661; AccelVolt 10. Mag 400; Signal SEI; Spot\_Size 35. Vac Mode HV.

**Figure 5.** Prosternum of *S. oryzae*: a – SEM image of *S. oryzae* exposed to untreated wheat kernels (400×); b – exposed to wheat kernels treated with 125 ppm DE, silicon (*Si*) counts from Energy Dispersive Spectroscopy (EDS); c – exposed to wheat kernels treated with 125 ppm NSA, aluminum (*Al*) counts from EDS. Format JEOL/EO, Version 1.0; Instrument JSM-6610; Acc. Volt 10; Mag 400; Spot Size 35; Vac Mode HV. c – Aluminum (*Al*) counts from Energy Dispersive Spectroscopy (EDS) – Filter Fit χ2 value: 31.161; Errors: ±2; Sigma Correction Method: Proza (Phi-Rho-Z); Acc. Voltage: 12.0 kV; Take off angle: 35.9°.

the critical value, the capillary forces are the predominant forces [87]. Due to the different contact angles, hydrophilic substances as DE [88, 89] are more exposed to the influence of moisture than hydrophobic materials (synthesized Al<sup>2</sup> O3 ; [90]). In addition, water adsorption also affects the surface energy of the particles [91]. Similar to liquids, solids have an imbalance in the surface forces. However, in solids the molecules are much more strongly bonded to one another and the surface energy is not evenly distributed on the particle surface.

Differences in insecticidal efficacy between DE an NSA arise from structural and physical differences between these two products. DE combines high abrasive and low sorbtive properties due to sharp angular structure and large particle size (1 to about 150 μm [92]), (**Figure 3b**) and a relatively low specific surface area (ca. 4 m<sup>2</sup> /g) [93]. In contrast, NSA particles (**Figure 3a**) are small aggregates (≈1.5 μm; [32]) assembled by coarse accumulations of nanoparticles (40–60 nm) which increase the overall specific surface area of the powder (ca.14 m<sup>2</sup> /g [94]). Thus, DE insecticidal efficacy is lower than that of NSA due to its small electric affinity to the insect body surface (**Figure 4b**) in addition to low sorbtive properties. So, DE works, in general, stochastically by damaging the insect body surface mechanically when it moves within a stored grain matrix. On the other hand, NSA's high insecticidal efficacy depends on its increased electrical affinity to the insect body surface (**Figure 4c**) in addition to having greater sorbtive properties. The whole mechanism of action consists of two steps in sequential order. First, there is a strong electrical binding between negatively charged NSA particles and the positive tribo-charged insect. Next, dehydration of the insect occurs due to strong sorptive action of NSA particles removing the insect cuticular waxes responsible for protecting insects against water loss. Hence, the mechanism of action of NSA does target the water balance of the insect and dehydration is the leading cause of death.
