Facile synthesis of silver nanoparticles using harmala alkaloids and their insecticidal and growth inhibitory activities against the khapra beetle

Abstract

In order to find new, less toxic and eco-friendly pest control agents, natural products are considered to be alternative options of potentially active compounds. In this context, the aim of the present study was the isolation and structural elucidation of the β-carboline and quinazoline alkaloids of Peganum harmala L. seeds. Silver nanoparticles (AgNPs) were prepared using alkaloids through a green synthesis procedure. Harmala alkaloids and their AgNPs showed considerable insecticidal and growth inhibitory activities against khapra beetle, Trogoderma granarium (Everts) (Coleoptera: Dermestidae). On a toxicity bioassay using treated filter papers, the total harmala alkaloids (THAs) were the most toxic followed by harmaline, harmine and harmalol, where LC50’s ranged between (30.6–61.7 µg/cm2) and (24.4–46.1 µg/cm2) 24 h post-treatment against larvae and adults, respectively. The two quinazolines, vasicine and vasicinone, showed moderate to weak activities. At 48 h post-exposure, efficacy of all alkaloids was increased. When tested as AgNPs (size 22.5–66.2 nm diameter), toxicity of alkaloids, especially the β-carbolines, was strongly increased, where LC50 values ranged between (4.7–11.4 µg/cm2) and (4.1–10.2 µg/cm2) 48 h post-treatment against larvae and adults, respectively. Feeding the 2nd instar larvae AgNPs-treated grains at sublethal concentrations led to significant drawbacks on the normal growth and development of the insect. These effects appeared as a high percentage of malformed larvae and pupae, a prolonged life span and a significant reduction in adult emergence. Results suggest the potential of using harmala alkaloids and their AgNPs as a natural approach for controlling T. granarium.