Unveiling toxicological, microanatomical and in-silico assessment of beta-carboline alkaloids as potential acaricide against Haemaphysalis darjeeling ticks

Beta-carboline alkaloids have been reported to show insecticidal and acaricidal activities. In the present study, Haemaphysalis darjeeling ticks were exposed to beta-carboline alkaloids, Harmaline, Harmalol, Harmine and Harmane, at concentrations of 10, 20, 30, and 40 mg/ml for 6, 12, 18, and 24 h to evaluate their potential acaricidal effect and mode of action. Although all four alkaloids induced substantial mortality in both larvae and adults, Harmaline followed by Harmalol showed a tendency to achieve higher toxicity compared to Harmine and Harmane with prolonged exposure times. Microanatomical study further revealed severe body surface damage in Harmaline and Harmalol-exposed adults. Although MM/GBSA analysis indicates slightly stronger binding for Harmalol with Chitin Synthase, binding energy alone does not fully explain inhibitory potential. Molecular Dynamics (MD) results show that Harmaline maintains more stable interactions, lower residue fluctuations, and better structural compactness within the binding pocket. Together with the experimental toxicity, microanatomical damages and MD results suggest that Harmaline is more effective in inhibiting chitin biosynthesis. This study suggests that all the beta-carboline compounds, especially Harmaline, showed potential acaricidal efficacy targeting Chitin Synthase, and thus can be used as potential alternatives and eco-friendly acaricides to eradicate tick infestation.