Bio-larvicidal potential of green–synthesized nanoparticles and aqueous extract of Moringa oleifera seed extract against the malaria vector, Anopheles gambiae ss (Diptera: Culicidae)

Malaria constitutes a major public health challenge across the globe. In the context of the current COVID-19 pandemic, it is a significant threat to human health, as its symptomology is very similar to the Covid-19 infection and it is also a comorbidity of the infection. In the face of dwindling resource allocations to fight other diseases of public health importance such as malaria, HIV and TB, it therefore becomes important to innovate and develop sustainable interventions in the fight against the malaria challenge. Interventions that seek to control mosquito populations can limit disease spread considerably in endemic communities. The challenge of insecticide resistance and environmental pollution also prompt the search for novel products such as green-synthesized nanoparticles and plant- derived mosquitocidal products. In this study, the bio-efficacy of two products- Moringa-synthesized silver nanoparticle (MS-AgNPs) and Aqueous extract of Moringa oleifera seed (AEMOS) were tested at five different concentrations on Anopheles gambiae ss larval instar stages (L1-L4). Twenty mosquitoes per stage were exposed to concentrations of the different treatments with five replications for 5 days. Control medium was distilled water and mortality was recorded daily. The MS-AgNPs was characterized using UV-vis spectroscopy and Transmission Electron Microscopy (TEM). Data were evaluated using Descriptive and Probit analyses. The UV-visible absorption spectra showed broad SPR band with one peak at 384.30 nm, illustrating the presence of a homogeneous distribution of hydrosol Ag-NPs at 24-hours post reaction time. The TEM analysis revealed that the MS-AgNPs were spherical in shape with an average diameter size of 25±2nm, these observed features are characteristic of silver NPs. The results from the mosquito bioassays showed that the mosquito life stages were susceptible to the MS-AgNPs and AEMOS within 24hours. The larval mortality recorded across the stages had a dose-dependent relationship with the treatments administered with significant difference among the treatment groups (p< 0.05). Mortality ranges among the larvae in the MS-AgNPs treatment group was >75% at 24-hours post-exposure while in the AEMOS group, the mortality recorded across the laral stages was >60% at 24-hours post-exposure. In terms of effectiveness of product, the MS-AgNPs treatments were highly effective against the mosquito larvae at lower doses {with LC50 and LC95 values ranging from 9.27 (16.69) ppm - 11.29 (20.32) ppm} compared to the AEMOs treatment with LC50 and LC95 values ranging from 203.99 (367.18) ppm -122.82 (221.07) ppm. Furthermore, mortality occurred more in the larval stages exposed to the MS-AgNPs compared to those exposed to AEMOS. A study is ongoing to determine the MS-AgNPs effect on growth parameters and adult emergence with respect to its biology and possible genetic implications for interventions. The Moringa synthesized-AgNPs (MS-AgNPs) and aqueous extract (AEMOS) had larvicidal activity on the exposed Anopheles mosquitos with different level of effectiveness for both products. This study highlights the potential of green-synthesized AgNPs in the search for green, low-cost and innovative interventions for the control of malaria in the ongoing COVID-19 pandemic. Further study on the mode and mechanism of action of the nanoparticles on the mosquito is recommended.