Contact and fumigant toxicity of hexane flower bud extract of Syzygium aromaticum and its compounds against Pediculus humanus capitis (Phthiraptera: Pediculidae)

Abstract

The head lice, Pediculus humanus capitis De Geer is an obligate ectoparasite of humans that causes pediculosis capitis, a nuisance for millions of people worldwide, with high prevalence in children. P. humanus capitis has been treated by methods that include the physical remotion of lice, various domestic treatments, and conventional insecticides. None of these methods render complete protection, and there is clear evidence for the evolution of resistance and cross-resistance to conventional insecticides. Non-toxic alternative options are hence needed for head lice treatment and/or prevention, and natural products from plants are good candidates for safer control agents that may provide good anti-lice activity. The plant extracts are good and safe alternatives due to their low toxicity to mammals and easy biodegradability. The present study carried out the pediculocidal activity using the hexane flower bud extract of Syzygium aromaticum (Myrtaceae) against P. humanus capitis examined by direct contact and fumigant toxicity (closed- and open-container methods) bioassay. The chemical composition of S. aromaticum flower bud hexane extract was analyzed by gas chromatography-mass spectrometry. The major chemical constituent (58.79%) of flower bud hexane extract S. aromaticum was identified as chavibetol (5-allyl-2-methoxyphenol) by comparison of mass spectral data and retention times. The hexane extract of S. aromaticum was subjected to gas chromatography analysis, and totally 47 compounds were detected, of which chavibetol was predominantly present. The other major constituents present in the hexane extract were eugenol acetate (phenol,2-methoxy-4-(2-propenyl)-,acetate (15.09%), caryophyllene-(I1) (2,6,10,10-tetramethyl bicyclo [7.2.0] undeca-1,6-diene (13.75%), caryophyllene oxide (3.04%), 2,6,6,9-tetramethyl-1,4,8-cycloundecatriene (1.67%), and copaene (1.33%). The filter paper contact bioassay study showed pronounced pediculicidal activity in the flower bud hexane extract of S. aromaticum. The toxic effect was determined for every five in an 80-min treatment. The result showed percent mortality of 40, 82, and 100 at 5, 10, and 20 min, and the median lethal time (LT₅₀) value was 5.83 (0.5 mg/cm²); 28, 82, and 100 at 5, 10, and 30 min. (LT₅₀ = 6.54; 0.25 mg/cm²); and 13, 22, 42, 80, and 100 at 5, 10, 20, 40, and 80 min (LT₅₀ = 18.68; 0.125 mg/cm²), respectively. The vapor phase toxicity was tested at 0.25 mg/cm². There was a significant difference in pediculicidal activity of S. aromaticum extract against P. humanus capitis between closed- and open-container methods. Adult mortalities were determined for every five in 60 min (closed method) and for every ten in 180 min (open method). The closed method showed the percent mortality was 45, 88, and 100 at 5, 10, and 15 min (LT₅₀ = 5.39), respectively. In the open-container method, the percent mortality was observed 5, 20, 47, 84, and 100 at 10, 20, 60, 120, and 180 min (LT₅₀ = 47.91), respectively. The mortality was more effective in the closed containers than in open ones, indicating that the effect of hexane extract was largely a result of action in the vapor phase exhibited fumigant toxicity. Studies of anti-lice activity of extract provide the basis for preliminary conclusions of structure activity relationships; although no clear patterns can yet be drawn. We here attempt to provide a concise compilation of the available information on anti-lice activity of plant extracts and plant-derived compounds.