Insectpollinating And Antiparasitic Activity

Origanum taxa, especially those that are rich in essential oils, have been extensively studied for their insect-pollinating (Ricciurdelli d'Albore, 1983; Beker et al, 1989) or nectar yielding (Kucherov and Siraeva, 1981; Jovancevic etal., 1984; Jablonski, 1986) effects. Although scientifically poorly understood, the traditional knowledge on attracting effects Origanum spp. for pollinating insects, especially honeybee (Apis mellifera), has been practically exploited since 1877, when the idea of culturing the bee forages with additional but non-marketable values was born (Ayers and Ayers, 1997). Interesting findings, that reveal the very complex mechanism underlying the communication between insects and attracting plants, were reported by Beker et al. (1989). They have observed that honeybees are capable of discriminating between different blends of odours and behave selectively to different parts (leaves, inflorescence) or chemotype (thymol, carvacrol) of O. syriacum due to perceiving distinct olfactory stimuli. It was assumed that the aroma blend from the whole plant serves as a long-distance olfactory cue, while the final short-range orientation is dependent on floral odour signals.

Observations from studies on Origanum benefit effects in parasite-control in pollinating insects show promising results (Abou Zaid etal., 1987; Mazeed, 1987; Kraus etal., 1994; Gal, 1997; Long etal., 1997), although some authors were sceptical towards the practical significance of essential oils in treatment of parasite infestation (Koeniger, 1991). In laboratory tests Origanum oils showed a high acaricidal effect (80—90 per cent mortality) on Varroa jacobsoni. Under the subtropical climatic conditions of Israel, a high mortality (85 per cent and 91 per cent) of Varroa mites was observed after spring treatment with 20 per cent and 33 per cent oregano oil impregnated in cardboard, respectively. Origanum treatment in autumn as well as the use of pure origanum oil during summer was harmful to the bee colonies (Gal et al., 1992; Lensky et al., 1996). The use of essential oil of O. majorana in treatment of V. jacobsoni has attracted much practical attention from several authors. A combination of formic acid and of essential oil of marjoram has been shown to be very effective in treatment of Varroa mites both in laboratory trials and in field experiments (Long etal, 1997). In field experiments, that were carried out under tropical (Vietnam) and temperate (Germany) climatic conditions, formic acid was applied to a tray covered by gauze and placed on the bottom board of the hive while O. majorana essential oil was applied to two wood pieces (1.5 ml per piece), that were placed on the top bars of the combs. The combination of essential oil and formic acid, applied at 15 per cent concentration, resulted in 96.24—99.68 per cent mite mortality in tropical climate and in 97.56—99.92 per cent in temperate climatic conditions. Due to the relatively low concentrations of formic acid this combination did not affect bee mortality and was proposed as a promising practical method in the control of V. jacobsoni. The highly significant repellent activity and antiparasitic effects of essential oil of marjoram were observed towards Varroa mites, which were exposed to test wax tubes with incorporated essential oil at 0.1 per cent and 1 per cent (Kraus etal., 1994). These concentrations of O. majorana essential oil were not noxious to honeybees.

Effective antiparasitic activity was observed also when essential oil of O. majorana was sprayed onto bees in colonies infested with V. jacobsoni at concentration (100 ppm), that was found non toxic to bees (Fathy and Fouly, 1997). Origanum majorana essential oil has been shown as a potent acaricidal agent against Acarapis woodi (Renie), the acarine disease-causing parasite that invades the tracheal system of the honeybees during winter and early spring. Infestation percentage in the bee colonies, treated with O. majorana essential oil (10 drops of oil per piece of cotton wool in a Petri dish, that was put under the combs of infested colonies) was significantly reduced already after 15 days of treatment, and after 30 days of treatment no infestation was found among the tested bees (Abou Zaid etal, 1987; Mazeed, 1987).

In respect of the control of human parasites or parasite-related diseases both in vitro and in vivo studies were conducted. O. vulgare essential oil was studied for its in vitro antimalarial activity on Plasmodium falciparum (Milhau et al, 1997). It displayed only moderate (IC50 = 516 pg/ml after 24 h and 355 pg/ml after 72 h) antiparasitic effects against chloroquine resistant strains of Plasmodium falciparum when compared to the more active oils of Rosmarinus officinalis or Myrtus communis (IC50 = 2 67 pg/ml after 24 h and 149 pg/ml after 72 h). However, the observed efficacy of the tested oils against both chloroquine-resistant and -sensitive strains allowed it to be deduced that the oils could interfere with P. falciparum growth by different mechanisms than chloroquine. This preliminary screening of activity together with concomitant analysis of essential oils set the direction toward selection of major components, like carvacrol, that were proposed for future investigations for antimalarial potential.

A clinical study, done by Force et al. (2000), showed that emulsified O. vulgare diet (600 mg daily) for 6 weeks considerably affected the enteric parasites (Blastocystis hominis, Entamoeba hartmanni, Endolimax nana) and significantly improved the gastrointestinal symptoms in seven of 11 patients, who were positive for Blastocystis hominis.

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