Chemical Composition and Insecticidal Activity of Origanum syriacum L . Essential Oil Against Sitophilus oryzae and Rhyzopertha dominica

Origanum genus belonging to the Lamiaceae family is aromatic and medicinal plant. It has been used in many countries for medicinal and pharmaceutical purpose. Aerial part of the Origanum syriacum L. was dried at shade. The essential oil was generated by steam distillation and compounds were identified by GC-MS analysis. γ-terpinene (26.7%), thymol (26.6%) and carvacrol (22.9%) was detected as the mail constituents. The essential oil was tested for insecticidal activity against adult of Sitophilus oryzae (L.) (Coleoptera: Curculionidae) and Rhyzopertha dominica (F.) (Coleoptera: Bostrychidae) using fumigation method. Essential oil revealed the excellent fumigant effect on R. dominica adults with a median lethal concentration (LC50) value of 0.124 μL/L and 0.107 μL/L for 48 h and 72 h respectively. LC50 values of S. oryzae were found as 0.173 μL/L and 0.135 μL/L for 48 h and 72 h respectively. As a result, essential oil of O. syriacum has a potency to be a natural insecticide. ARTICLE HISTORY Received: 12 January 2018 Revised: 04 March 2018 Accepted: 11 March 2018


Introduction
Plants include numerous fascinating compounds revealing plenty of biological activities [1][2][3][4].Therefore, they play an important role in drug discovery and development process.Origanum genus belonging to Lamiaceae family, mainly distributed through the Mediterranean region and the Balkans has been represented almost 50 species through the world, fourteen of which are endemic for Turkey [5].Origanum plant consumed as herbal tea and has been used for traditional medicine since ancient times as stimulating, antirheumatic, antispasmodic, and has antibacterial effects to treat various illness such as revulsion, dyspepsia, muscle contraction, diarrhea and infection diseases [6].The secondary metabolites isolated from O. rotundifolium and O. majorana displayed the significant antiproliferative and antioxidant activities [7,8].Origanum species are well known for their essential oils which employ in flavoring agent mainly for meat, fish, soup and bottled food [9].Carvacrol, caryophyllene, terpinene and thymol are the dominant essential oil compounds of Origanum revealing the significant biological activities [10].Origanum demand has been increasing steadily in the world market, on accounts of the significance in food, pharmaceutical and cosmetic industries [11].
Insect pests have threatened the crop production all over the world.Synthetic insecticides such as methyl bromide and phosphine fumigants have been used extensively to destroy stored pets and to protect stored food, feedstuffs, and agricultural materials [12].However, widespread usage of fumigants has brought about the development of resistance in stored-product insects [13].Moreover, owing to the depleting of ozone layer, residue forming, and carcinogenicity, some synthetic insecticides have been prohibited [14,15].As a result, it is essential to find novel fumigants which should be harmless to the environment, ecosystem, and non-target organisms to combat stored product insects.Natural products, especially essential oils have gained the great interest to replace synthetic products in fighting pets recently [16].The rice weevil, Sitophilus oryzae and bean weevil, Rhyzopertha dominica are the most widespread and destructive insect pests of stored grains as well as stored products [17,18].
Essential oils (EOs) exhibiting a broad spectrum of biological activities are generated from various aromatic plants.EOs have been extensively used in food and pharmaceutical industry [19].Moreover, due to the their insecticidal effect, they have been employed in agricultural purpose [20].
Herein, the essential oil of O. syriacum was generated by steam distillation and identified by GC-MS.In addition, insecticidal activity test of essential oil was carried out against S. oryzae and R. dominica.The results revealed that the essential oil exhibited excellent insecticidal activity against these pets.

Plant Material
The aerial parts of cultivated Origanum syriacum were harvested from the Medicinal and Aromatic Plant Field of Gaziosmanpasa University, Tokat, Turkey and identified by Prof. Dr. Ozgur Eminagaoglu.A voucher specimen was deposited in the Herbarium of the Faculty of Forest Engineering (ARTH: 5256) Artvin Coruh University.

Isolation of the Essential Oils
Aerial parts of plant material (50 g) were diluted with distillated water (250 ml) then subjected to hydrodistillation for 4 h, using a Clevenger apparatus.The oil was dried over sodium sulphate and stored in the freeze (4 °C) until to be analyzed.

GC and GC-MS Analysis
GC analyses were performed on a Perkin-Elmer Clarus 500 Series, in divided mode, 50:1, equipped with a flame ionization detector (FID) (Perkin-Elmer Clarus 500) and a mass spectrometer-equipped (Perkin-Elmer Clarus 500) BPX-5 capillary column (30 m × 0.25 mm, 0.25 um i.d., SGE Analytical science-Trajan scientific and medicinal).The temperature of injection was steady and FID was performed at 250 ºC.Helium was the carrier gas at a rate of 1.0 ml/min.The oven temperature was 50 ºC at the beginning then was raised to 220 ºC with a rate of 8 ºC/min.In the mass spectrometer, ionization energy was 70 eV and the temperature of transfer line was at 250 ºC.The standard components were used for the majority of the essential oil constituents and Kovats retention indices (RIs) were determined for all the sample components using the Van den Dool and Kratz equation according to homolog n-alkane series retention times.

Insect
The cultures of S. oryzae and R. dominica were supplied from Department of Plant Protection, Bozok University.One third of 5 L glasswares were filled with clean wheat, and then adult males and females were added to lay eggs to get a single aged population.After the adults were removed, cultures were incubated at 27 ± 2 ºC in a dark climate chamber with 60 ± 5% r.h.(relative humidity) for 48 h.The new generation of adults came into view by 45 day and 3-4 week-old adults were used in tests [21].

Dose-Response Bio-Assay
Insecticidal activity of essential oil was determined by fumigation method.Conducted dose effect on S. oryzae and R. dominica of O. syriacum was carried out.In the experiments, 5, 10, 15 and 20% (v/v) (essential oil/acetone) concentrations of the essential oil were used.Compressed rubber-capped glass tubes (20 ml) were used.Small paper discs were cut and then they are fixed to the cover and 10 adult specimens were situated in the each glass.10 μl volume of the oil mixture was saturated to the filter papers by pipettes.After this process, it was waited for 5 minutes to volatilize the acetone.Then the caps were covered on the glasses and they were waited in the incubator at 25ºC in dark conditions for 24-48 and 72 hours.The dead specimens were counted and recorded for each 24 hours interval.The randomized block design was used for the study.All experiments were repeated 3 times and each repetition consists of 3 replications [22].

Statistical Analysis
Dose-mortality response data from the bioassays were tested by POLO-PC probit analysis to determine the LC50, LC90, confidence level.The differences of LC50, LC90 were executed by comparison of confidence levels [23].

Results and Discussion
Origanum species are well known with their rich essential oils contents which are clear red color, a pleasant and slightly spicy odor.In this study, EO analysis was executed by GC-MS and 14 components were identified with a rate of higher than 0.1%, representing 99.8% of the oil (Table 1).γ-Terpinene, thymol were the main compounds, at 26.7% and 26.6% respectively; carvacrol, p-cymene, monoterpenes which are the second most abundant compounds, were present at 22.9% and 13.3% respectively.In this work, fumigation assay was executed to investigate the toxicity of O. syriacum essential oil against adults of S. oryzae and R. dominica at four different concentrations.EO revealed the most fumigant effect on R. dominica adults with a median lethal concentration (LC50) value of 0.124 µL/L at the confidence level of 0.100-0.140for 48 h.Fumigation effect of EO increased with the time-dependence.LC50 value of R. dominica was 0.107 µL/L which was the best activity among the trials for 72 h.However, LC50 values of S. oryzae adults were calculated as 0.173 µL/L and 0.135 µL/L for 48 h and 72 h respectively.While comparing LC90 for R. dominica adults and S. oryzae adults, significant differences were detected.LC90 values of R. dominica adults and S. oryzae adults were calculated as 0.198 µL/L and 0.272 µL/L for 48 h respectively.Moreover, these values (LC90) were found as 0.192 µL/L and 0.222 µL/L for R. dominica and S. oryzae for 72 h respectively (Table 2).These results revealed a few differences regarding to the compounds and quantities with the literatures.For instance, O. syriacum collected from August at altitudes of 250-1900 m consisted of carvacrol and p-cymene at 60.5% and 7.4% respectively.In the same work, it was reported that O. onites essential oil constituted carvacrol (69.0%) as a chief product.Furthermore, carvacrol (%39.1),thymol (22.2%), and p-cymene (9.3%) were found in EO of O. vulgare [24].These results emphasized the important variability of chemical composition of Origanum oils through their components.These differences could be on account of various criterions such as part of the plant, harvest period, location, altitude, climate, environmental conditions.Indeed, the synthesis and secretion of oils are influenced by these factors [25].
Insecticidal activity of Origanum species EOs was executed against various insects that EOs displayed the considerable activity.For instance, O. majorana EO displayed the good fumigant activity against Spodoptera littoralis which was the serious pest of some crops such as cotton, chili, and tobacco [26].The major compounds of O. majorana EO were detected as thymol (38.8%), carvacrol (32.9%), p-cymene (7.9%), and γ-terpinene (5.1%).The chemical compounds of O. majorana have been similar with O. syriacum.Therefore, the insecticidal activity of O. syriacum may be due to the major constituents of essential oil components.Another work carried out in Algeria revealed that O.glandulosum has highly toxic potential power against Rhyzopertha dominica by both contact and fumigant assays [27].
Insecticidal components of most essential oils are usually monoterpene [28].Because of the high volatility, essential oils have fumigant and gaseous action that may be significance for stored product insecticides.The carvracrol, a monoterpene has extensive insecticidal and acaricidal activity against agricultural stored product, and medicinal pests and act as a fumigant [29].
A good natural insecticide has been accepted that it influences a limited range of pest insects, has no harmful effects on non-target organisms and the environment, act in many ways on various types of pest complex [30].Most plant extract and essential oils have been known to possess repellent and insecticidal activity against various stored product insects [31].Some essential oils such as mustard oil, cinnamon oil and horseradish oil were reported to have strong effect against S. oryzae insect.In addition, essential oils generated from Pinus longifolia, Eucalyptus obliqua and Coriandrum sativum were tested for contact and fumigant activities against rice weevil, Sitophilus oryzae, Callosobruchus chinensis and Corcyra cephalonica and the results varied with the test material, insect species and exposure time.In fumigation assay, Corcyra cephalonica and Eucalyptus obliqua oils at 130 µg/cm 2 , led to 100% toxicity to all the species within the 24 h of the treatment [32].

Conclusion
O. syriacum essential oil has a potential for applications for stored-grain pests due to its high volatility and fumigant activity.In addition, EO of O. syriacum might be applicable for managing S. oryzae and R. dominica insects in enclosed spaces such as storage bins, glasshouses, or buildings due to their fumigant action.These results suggested that the insecticidal action of the oils might be due to the fumigant action.In other word, the oils could be toxic by penetrating the insect body via the respiratory system.The significant insecticidal activity of O. syriacum could be caused from γ-terpinene, thymol and carvacrol which were the major compounds of EO content.The major three compounds have methyl and isopropyl moiety.Thymol and carvacrol are constitutional isomers.In other words, both compounds have same molecular formula, C10H14O but different structural formulas.Methyl, isopropyl and hydroxyl connected to the phenyl group in thymol and carvacrol.Hence, methyl, isopropyl and hydroxyl bonded to the phenyl may be effective for the activity.

Table 1 .
Chemical composition of O. syriacum essential oil analyzed by GC-MS

Table 2 .
Fumigant effect of O. syriacum essential oil on R. dominica and S. oryzae