Laboratory and field tests of the effectiveness of the lemon-eucalyptus extract, Citridiol, as a repellent against land leeches of the genus Haemadipsa (Haemadipsidae)

Citridiol is an extract of the leaves of Corymbia citriodora (Myrtaceae), the lemon eucalyptus, and mostly consists of p-menthane-3,8-diol isomers. The effectiveness of this extract as a repellent against land leeches of the genus Haemadipsa (Haemadipsidae), primarily H. sylvestris, was tested in the laboratory and field, in Peninsular Malaysia. The formulation tested, Mosi-guard Natural spray, contained 40% (w/w) Citridiol in a base of ethanol, water and isopropanol. In the laboratory test, specimens of H. sylvestris that were placed within moist, untreated arenas enclosed by treated paper rings made numerous attempts to cross the rings but were prevented or delayed from crossing over, in a dose-dependent manner. Mortality was high among the leeches that attempted to cross over the paper rings that had been sprayed to saturation point but low among the leeches that attempted to cross over paper rings that had only been partially treated, with a droplet-spray. The field study was carried out using indices that were formulated to reflect the severity of leech attack and the degree of repellency. Heavy or moderate spraying of footwear and trouser legs (tucked into socks) not only gave complete protection against bites by H. sylvestris and H. picta but also provided high enough repellency to keep the treated footwear virtually free of leeches. Even a light spray greatly reduced the numbers of leeches on footwear and delayed their progression toward biting the test subjects, although it failed to prevent bites completely. There was no decline in the repellency of the Citridiol when hourly assessments were made over a 6-h test period in the field. The results of the study show that Citridiol is highly repellent as well as toxic to leeches, and can be effectively used to prevent leech bites in the field.