Higher plants, especially medicinal and aromatic plants (MAP), are a potential source of new insecticides, and many research groups are trying to prove their activity against noxious pests. Some natural compounds, isolated from these MAP (such as rotenon, pyretrins, and azadirachtin) are already commercially available on the market. A range of active compounds, including terpenoids, flavonoids, tannins, essential oils or their components (like carvacrol), that are present in O. vulgare in relative high amounts, were considered as a potential source of natural biocides (Duke, 1992). Investigations on the activity of aromatic plants against stored product- or plant-noxious pests gave diverse results when considering different insect species, their developmental stages (eggs and adults) or the way of application (fumigant, contact) (Regnault-Roger and Hamraoui, 1993a; Shaaya etal., 1993; Regnault-Roger and Hamraoui, 1995; Kalinovic etal, 1997; Mateeva etal., 1997; Rakowski and Ignatowicz, 1997; Baricevic etal., 2001). Generally, plants or their essential oils showed more potent activities when applied directly to the insect surface than after fumigant application. Among different aromatic species, the plant essential oils from the Lamiaceae family have the best insec-ticidal effects against bean weevil A. canthoscelides obtectus (Regnault-Roger and Hamraoui, 1993a). Oregano (O. vulgare L.) is one of the plants, used traditionally in southern France, to control bean weevil (A. obtectus Say) in stored kidney beans (Phaseolus vulgaris L.) (Regnault and Hamraoui, 1993b; Bernath and Badulosi, 1997). A high carvacrol containing O. vulgare ssp. hirtum essential oil showed both fumigant and contact toxicities to bean weevil (A. obtectus Say) in laboratory trials (Baricevic et al., 2001). When considering fumigant toxicity, insecticidal effect (mortality rate 82.5 per cent) was observed 6 days after application of high concentrations of oregano essential oil (150 pl per 55 g of beans). When considering contact toxicity, both oregano drug plant and essential oil at all tested concentrations significantly increased the bean weevils' mortality rates in comparison to the controls. Essential oils (5 pl, 15 pl and 30 pl per 55 g of beans) induced 100 per cent mortality of the bean weevil population when applied directly to the surface of the beans (55 g) in Petri dishes. Also, egg laying and hatching was inhibited after treatment of bean weevil with powdered drug plant (0.33 g, 0.66 g, 1.0 g and 2.0 g) or with essential oil at all tested concentrations.
O. vulgare L. susbsp. hirtum essential oil with high carvacrol content showed insecticidal activity also against Drosophila melanogaster (Karpouhtsis et al, 1998) and a strong ovicidal activity against the eggs of stored product insects Tribolium confusum and Ephestia cautella (Shaaya etal, 1993) already at low concentrations (2 pl/l or 4 pl/l air), but showed very low fumigant toxicity against adult insects (Tribolium confusum, Tribolium castaneum, Ephestia cautella, Sitophilus oryzae) (Shaaya et al, 1993; Shaaya et al., 1997). The exposure to vapours of essential oil from O. syriacum var. bevanii Ietswaart resulted in 77 per cent and 89 per cent mortality of the eggs of the confused flour beetle (Tribolium confusum) and the Mediterranean flour moth (Ephestia kuehniella), respectively (Tunc et al, 2000). This oil (1 pl/l air) also showed high fumigant toxicity against females of two greenhouse pests, i.e. carmine spider mite (Tetranychus cinnabarinus) and cotton aphid (Aphis gossypii) (100 per cent mortality after 48 h and 96 h exposure, respectively) (Tung and Sahinkaya, 1998). By contrast, only limited insecticidal potential of essential oils of Origanum creticum, which showed only moderate contact toxicity (LD>90 = 100 pg/larva) against tobacco cutworm (Spodoptera litura), was reported by Isman et al. (2001).
Origanum vulgare essential oil (15 per cent) was also tested for its repellent activity against Culicoides imicola Kieffer, the vector of the African horse sickness virus. When applied at concentration of 4 mg/m2 on farm animals (horses), essential oil showed only nonsignificant repellency for 2 h and was far less effective than synthetic repellent di-ethyl toluamide (DEET), and was not recommended for its use in order to prevent the spread of Culicoides-borne pathogens (Braverman and Chizov-Ginzburg, 1997, 1998).
Toxicity and resistancy toward nematodes count as important attributes of aromatic plants, that offer new applications in the field of plant health care programmes of sensitive crops, especially when nematicides or resistant cultivars are not available. A high level of resistancy against infestation with root-knot nematode (Meloidogyne inconita Chitwood) was observed in O. vulgare and O. majorana plants, which were free of root galls even after exposure to initial nematode populations of 15 eggs/cm3 of soil medium in greenhouse experimental conditions (Walker, 1995). However, the root-knot nematodes caused a significant decrease in dry weight of O. vulgare but not that of O. majorana. Essential oil of O. majorana, with terpinen-4-ol (41.6 per cent) as the major compound, affected the soil stages of phytonematodes (Rotylenchulus reniformis, Criconemella spp., Hoplolaimus spp.) and inhibited more than 80 per cent of Meloidogyne incognita juvenile hatching compared to about 3.5 per cent at the control (Abd-Elgawad and Omer, 1995). Laboratory trials carried out with O. vulgare, O. majorana and O. syriacum leaf extracts or essential oils showed, that these plants considerably affected the spread of Meloidogyne nematodes, either by inhibition of egg hatching (Ramraj et al, 1991; Oka et al, 2000) or by immobilisation and exhibiting toxicity to nematode juveniles (Hashim et al, 1999; Oka et al, 2000). The toxicity increased with increasing concentration and exposure time. Origanum extracts or essential oils showed protective effects against root galling also when applied to nematode-sensitive crops. Alagumalai etal. (1997) observed that water extracts of O. vulgare dose-dependently diminished the population of M. incognita around chickpeas. Oka et al. (2000), who studied the nematicidal effects against Meloidogyne javanica in vitro and in pot experiments, found that O. vulgare and O. syriacum essential oils, when mixed in sandy soil at concentration of 200 mg/kg, reduced the root galling of cucumber seedlings. Similar effects were obtained by carvacrol and thymol at concentration of 150 mg/kg soil.
A strong molluscicidal effect of O. compactum ethyl acetate extracts (LC90 = 2.00 mg/l) against the schistosomiasis-transmitting snail Bulinus truncatus was attributed to the content of flavonoids and terpenoids, that are known to have molluscicidal potential (Hmamouchi et al, 2000). Interesting findings were observed by Vokou et al. (1998), who studied the effects of two subspecies of O. vulgare (ssp. hirtum and ssp. vulgare) on the behaviour of three snail species, native in Greece (Helix lucorum, H. aspersa, and Eobania vermiculata) during the different stages of the foraging cycle. O. vulgare ssp. hirtum, which contained much higher amounts of essential and was rich in phenolic compounds, considerably affected the snail feeding behaviour, while no significant effects were observed in ssp. vulgare. During the encounter stage, a repellent activity of
O. vulgare ssp. hirtum was observed. During the acceptance stage, all snail species tended to reject food types that contained high concentrations of subsp. hirtum essential oil, but at the feeding stage, subsp. hirtum essential oil caused a reduction of daily consumption rates. This is in agreement with Barone and Frank (1999), who found that polar (methanol) extracts of O. vulgare showed only scarce repellent effects on slug (Arion lusi-tanicus) feeding on rape.
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