*3.6. Larvicidal Activity*

The diseases caused by mosquitos are considered key challenges to public health worldwide and many efforts have been established to control these diseases. The conventional methods (either biological or chemical methods) have been utilized to control the spread of mosquitos, but these methods have drawbacks with effects such as toxicity to consumers and an increase in mosquito resistance to these compounds [97]. Recently, due to the unique characteristics of NPs, they have been used as an alternative source to control mosquito-borne diseases instead of conventional methods. To date, this is the first report to investigate the efficacy of myco-synthesized MgO-NPs against *C. Pipiens.*

The mosquitocidal mechanism of MgO-NPs may be related to two mechanisms: the production of ROS and lipid peroxidation and the leakage of internal cellular contents due to cell membrane damage [73,98]. Once MgO-NPs is sprayed over one stage of mosquito life cycles such as egg, larvae, pupa, or adult, it is dissociated into Mg2+ and O2– ions in the surrounding environment. The high concentration of O2– ions forms ROS, which ultimately leads to oxidative stress and lipid peroxidation. Moreover, the small size of MgO- NPs can react with nucleic acid and deform it, then inhibit the growth of mosquitos [99]. Moreover, the high concentration of Mg2+ leads to the destabilization or damage of cellular equilibrium, causing more stress, leakage of cellular components, and finally mosquito cell death [75] (Figure 9A).

**Figure 9.** Mosquitocidal activity of MgO-NPs against *C. Pipiens.* (**A**) Proposal mechanism of MgO-NPs as mosquitocidal; (**B**) larvicidal activity of MgO-NPs against *C. Pipiens* at different concentrations (2, 4, 6, 8, and 10 µg mL−<sup>1</sup> ) after different contact times (24, 48, and 72 h); (**C**) repellence activity of different concentrations of MgO-NPs as compared to a positive control (DEET).

In the current study, data analysis showed that the mortality percentages differed based on MgO-NPs concentrations. More than 50% mortality was observed in all concentrations after 72 h. The lowest mortality percentages (52%) were recorded after 3 days of treatment with 2 µg mL–1 of MgO-NPs, whereas the maximum mortality percentages (96%) were obtained at 10 µg mL–1. At MgO-NPs concentrations of 4, 6, and 8 µg mL–1, the mortality percentages reached 64%, 72%, and 84%, respectively, after 72 h (Figure 9B). The LC<sup>50</sup> (concentration of MgO-NPs that inhibit 50% of the population) and LC<sup>90</sup> (concentration of MgO-NPs that inhibit 90% of the population) were 2.21 µg mL–1 and 10.71 µg mL–1 , respectively. Recently, several studies have reported the efficacy of different metal and metal oxides NPs to control or inhibit the mosquito populations [15,37,39]. The rod-shaped MgO/ hydroxyapatite nanoparticles showed potentiality to inhibit *Aedes aegypti, Anopheles stephensi,* and *Culex quinquefasciatus* through the accumulation of Mg2+, which destroys the mosquito cells [100].
