Acaricidal properties of 5-methylfurfural identified from Valeriana fauriei and its structural analogues against synanthropic mites and Asian longhorned tick with color alterations

Acaricidal activities and color alterations of 5-methylfurfural derived from Valeriana fauriei essential oil and its structural analogues against Dermatophagoides farinae, D. pteronyssinus, Haemaphysalis longicornis and Tyrophagus putrescentiae were evaluated in the present study. Based on the LD₅₀ values of 5-methylfurfural and its analogues, 4,5-dimethylfurfural showed the highest acaricidal activity (LD₅₀; 9.95, 9.91, and 7.12 μg/cm²), followed by 5-methylfurfural (11.87, 11.00, and 8.59 μg/cm²), furfural (12.94, 13.25, and 10.36 μg/cm²), and V. fauriei essential oil (15.15, 13.64, and 10.14 μg/cm²) against D. farinae, D. pteronyssinus and T. putrescentiae, respectively. However, all tested compounds did not show the acaricidal activities against H. longicornis. Interestingly, the color alterations of the mites and ticks were observed by furfural, 5-methylfurfural, and 4,5-dimethylfurfural from colorless to red brown during the acaricidal experiments. Furthermore, 4,5-dimethylfurfural which exhibited the highest acaricidal activity was formulated as nanoemulsion. The nanoemulsion of 4,5-dimethylfurfural showed higher acaricidal activity than it was emulsified in ethanol. The nanoemulsion was also found to show color changes of the mites and ticks from colorless to red brown. The results suggest that 5-methylfurfural and its analogues could be developed as an effective and easy-to-recognize acaricides to mites and ticks.