Evaluating the Toxicity of Synthetic Hydroxyapatite Nanoparticles (HAPNPs) against Pulse Beetle, Callosobruchus maculatus (Insecta: Coleoptera)

The pulse beetle Callosobruchus maculatus is a serious insect pest of stored legumes. Therefore, the management of such pests has become a necessity as it causes great economic loss to its plant host. Unfortunately, pest management programs against C. maculatus encounter several obstacles, such as the generation of insecticide resistance and environmental hazards of traditional insecticides. The current study was designed to overcome these obstacles by using synthetic nanoparticles as alternative insecticides. In this study, a synthetic form of eggshell-based hydroxyapatite nanoparticles (HAPNPs) was used as a control agent against C. maculatus. This material was selected because of its environmental safety, which is ensured due to its wide spectrum of applications in our daily activities. HAPNPs originating from eggshells were characterized by XRD, FTIR, and TEM. The obtained results revealed a lack of impurities in the synthesized particles and that the average plate size is ∼62.8 nm, while the rod structure has a length and width of ∼91 nm and ∼22.7 nm, respectively. A comparative study on the toxicity of HAPNPs and Malathion insecticide against C. maculatus showed a significant impact of NPs originating from eggshells than the positive control insecticide. Based on this finding, further analyses were performed to understand its subsequent effects. Eggshell-based HAPNPs disrupted C. maculatus fecundity and adult emergence rate. In the meantime, it highly reduced the negative effects of C. maculatus on cowpea seeds. Scanning electron microscopy showed clear disruption of the insect integument wax layer and the aggregation of HAPNPs on the beetle’s spiracles, leading to respiratory failure and hence the death of the insects. Interestingly, there was no impact of HAPNP application on total antioxidants and H₂O₂ levels in C. maculatus. These results introduce a novel management tool using a safer nanopesticide against the cowpea beetle.