Chemical Composition and Insecticidal Activity of Essential Oil from Coriandrum sativum Seeds against Tribolium confusum and Callosobruchus maculatus

The biological activity of essential oil extracted from coriander, Coriandrum sativum L. (Apiaceae), seeds against adults of Tribolium confusum Duval (Coleoptera: Tenebrionidae) and Callosobruchus maculatus F. (Coleoptera: Bruchidae) was investigated in a series of laboratory experiments. Fumigant toxicity was assessed at 27 ± 1°C and 65 ± 5% R.H., in dark condition. Dry seeds of the plant were subject to hydrodistillation using a Clevenger-type apparatus. The composition of essential oil was analyzed by gas chromatography mass spectrometry. The predominant components in the oil were linalool (57.57%) and geranyl acetate (15.09%). The mortality of 1–7-day-old adults of the insect pests increased with concentration from 43 to 357 μL/L air and with exposure time from 3 to 24 h. In the probit analysis, LC50 values (lethal concentration for 50% mortality) showed that C. maculatus (LC50 = 1.34 μL/L air) was more susceptible than T. confusum (LC50 = 318.02 μL/L air) to seed essential oil of this plant. The essential oil of C. sativum can play an important role in stored grain protection and reduce the risks associated with the use of synthetic insecticides.


Introduction
The global pest-harvest grain losses by insect damage and other bioagents range from 10% to 40%. Methods used to control stored grain insect pest included physical, chemical, and biological treatments [1]. Chemicals largely used as pesticides in crop protection could have undesirable effects such as ozone depletion, environmental pollution, toxicity to nontarget organism, pest resistance, and pesticide residues, in addition to direct toxicity to users [1]. Therefore, the development of bio insecticides has been focused as viable pest control strategy in recent years [2,3].
Plants may provide potential alternative to currently used insect-control agents because they constitute a rich source of bioactive chemicals [4]. Aromatic plants are among the most efficient insecticides of botanical origin and essential oils often constitute the bioactive fraction of plant extracts [3].
Essential oils are secondary metabolites abundant in aromatic plants families such as Lamiaceae and Apiaceae which contain a large number of compounds such as monoterpens and sesquiterpenes. Essential oils are known to exhibit low toxicity to mammals, and the most terpenoids and phenols found in plant essential oil have minimal toxicity and have even been approved as flavoring agents in food [2,5]. The insecticidal [6,7], nematicidal [8], and antibacterial [9] effects of coriander essential oil have previously been reported.
Coriandrum sativum (Apiaceae) is a native of the Mediterranean region and is grown in North Africa, central Europe, and Asia as a culinary herb and medicament. The essential oil of C. sativum exhibited volatile toxicity to stored product insects. López et al. [6] fractionated the seeds of Coriander sativum by column chromatography and tested them in the laboratory for volatile toxicity against three stored rice pests, Sitophilus oryzae, Rhyzopertha dominica and Cryptolestes pusillus. Their experiment showed the active compound of coriander essential oil against S. oryzae was linalool, while the fractions that contained mixtures of linalool, camphor, and geranyl acetate were as active against 2 ISRN Pharmaceutics R. dominica, and C. pusillus as linalool alone. Also the results of Islam et al. [7] showed that the C. sativum essential oil had fumigant toxicity on Tribolium castaneum adults, larvae, and pupae.
The present study was carried out to determine the fumigant toxicity of essential oils extracted from seeds of C. sativum against adult stage of two other stored product insects, Tribolium confusum and Callosobruchus maculatus.

Materials and Methods
Callosobruchus maculatus and Tribolium confusum were reared in plastic containers (20 cm length, 14 cm diameter and 8 cm height) containing bean grain and wheat flour mixed with yeast (10 : 1, w/w), respectively, which were covered by a fine mesh cloth for ventilation. Adult insects, 1-7 days old, were used for fumigant toxicity. The culture was maintained in the dark in growth chamber set at 27 ± 1 • C and 65 ± 5 relative humidity. All experiments were carried out under the same environmental conditions. Seeds of C. sativum were collected in August, 2009, from Zabol region located in Sistan and Baluchestan province, Iran. The plant material was dried naturally on laboratory benches at room temperature (23-24 • C) for 7 days until crisp. The dried materials were stored at −24 • C and then hydrodistilled to extract its essential oil.
Essential oil was extracted from the plant samples using a Clevenger-type apparatus where the plant material is subjected to hydrodistillation. Conditions of extraction were fifty grams of seeds samples, 1 : 10 plant material/water volume ratio, and 4 h distillation. The C. sativum oil was dehydrated with anhydrous sodium sulphate and stored in airtight glassware in refrigerator at 4 • C until being used in the treatment.
The essential oil of the Coriandrum sativum seeds was analyzed on a gas chromatograph (Varian suturn 2200) mass spectrometer (varian cp 3800) (GC-mass). The GC column was DB-5 (30 m × 0.25 mm i.d, 0.25 μm film thickness). The GC conditions were as follows: injector temperature, isothermal at 50 • C for 1 min, then programmed to 200 • C at 4 • C/min. Helium was used as the carried gas at the rate of 0.8 mL/min. The effluent at the GC column was introduced directly in to the source of the MS. Spectra were obtained in the EI mode with 70 eV ionization energy. The sector mass analyzer was set to scan from 40 to 300 amu for 1s. Unknown essential oil was identified by comparing its GC retention time to that of known compounds and by comparison of its mass spectra, either with known compounds or published spectra.
To determine the fumigant toxicity of the C. sativum oil, filter paper (2 cm diameter) was impregnated with oil at doses calculated to give equivalent fumigant concentration 43-357 μL/L air. The impregnated filter papers were then attached to the screw tightly caps of glass with volumes of 70 mL. Caps were screwed tightly on the glass, each of which contained separately 10 adults (1-7 days old) of each species of insect. Each concentration and control was replicated four times. Mortality was determined 3 to 24 hours from commencement of exposure in serial time method. When no sign of leg or antennal movement was observed, insect was considered as dead.
Insect mortality percentage in another experiment was designed to assess 50% lethal doses (LC 50 ). A series of dilutions was prepared to evaluate mortality of insect after an initial dose-setting experiment for determination concentrations with about 10 to 90% mortality. Concentrations of the oil were tested at 0.4, 0.8, 1.2, and 2 μL/L air on C. maculatus and evaluated at 92.5, 185, 278, 370, and 462.5 μL/L air on T. confusum, respectively. Control insects were kept under the same conditions without any essential oil. Each dose was replicated five times. The number of dead and live insects in each bottle was counted 24 h after initial exposure to the essential oil. The mortality was determined as described in previous experiment. The treatment bottles were monitored for at least 48 h after recording the data and no affected insect recovered. Data obtained from each dose response bioassay were subjected to probit analysis. LC 50 values were estimated by probit analysis using SPSS 16.0.

Results and Discussion
Chemical composition of essential oil from the seed of coriander, Coriandrum sativum, is given in Table 1. The major components in the essential oil from C. sativum seeds were found: linalool (57.57%); geranyl acetate (15.9%); βcaryophyllene (3.26%), camphor (3.02%), and p-cymene (2.5%). Chemical analysis indicated clearly that linalool was the main component of coriander essential oil.
The result of our analysis was in agreement with the other literatures that reported linalool as major constituent in the essential oil of coriander [6,10,11]. It has been reported that major compounds in the seed coriander oil in Bangladesh were linalool (37.7%), geranyl acetate (17.6%), and γ-terpinene (14.4%) [11]. However, the difference in the oil composition was related to the relative proportion of the constituents and not to the presence/absence of a particular component. These variations may be attributed mainly to the plant part, the season (temperature, photoperiod, and hygrometry), the method of harvesting, the geographical zone, pedological condition, and the method used to isolate the plant product [12][13][14].
In all cases, considerable differences in mortality of insect to essential oil vapor were observed in different concentrations and exposure times. The mortality increased with rising concentrations from 43 to 357 μL/L air and with exposure time from 3 to 24 h.
Results indicated that the oil was relatively more toxic against C. maculatus than T. confusum (Figure 1). The lowest concentration (43 μL/L air) of the oil yielded 100% mortality of C. maculatus after 24 h exposure but the mortality of T. confusum at the same concentration was 25% after 24 h.
The toxicity of C. sativum oil on C. maculatus and T. confusum was significantly different, as inferred by the confidence intervals of LC 50 (Table 2). Probit analysis showed that C. maculatus was more susceptible (LC 50 = 1.34 μL/L air) to C. sativum oil than T. confusum (LC 50 = 318.02 μL/L air) ( Table 2). A difference response of the insect species to the essential oils has previously been reported for stored product insects [15][16][17].
The insecticidal property of coriander essential oil was reported in previous studies and its toxicity generally was attributed to linalool [6,7], the volatile compound which was found to be the primary component of seed oil in our chemical analysis. Moreover, the high toxicity of linalool and camphor was reported against stored products pests [16,18,19].
Insecticidal effects of C. sativum seed essential oil on adults of Tribolium castaneum and Sitophilus oryzae, Rhyzopertha dominica, and Cryptolestes pusillus have been shown before [6,7]. Islam et al. [7] reported that fumigant LC 50 value of T. castaneum at 24 exposure for adults insect was 0.011 μg/mL acetone in glass vials (25 × 50 mm) or in other words nearly 448 μg/L air. This value was more than the LC 50 value calculated for T. confusum species (318 μL/L air) in our study. More recently, it is shown that the seed oil of the Coriandrum sativum with linalool as major component (55.1%) has significant toxic effects against the larvae of Aedes aegypti with an LC 50 value of 21.5 ppm and could play an important role as immunotoxicity on the insect [20]. The observed difference seems to be reasonable because of different insect species and methodology of oil extraction, as shown in similar experiments with various stored product pests and essential oil vapors [21,22].
Studies on the mode of action of the natural insecticide have shown that treatments the insects with natural compounds such as essential oils or pure compounds may cause symptoms that indicate neurotoxic activity including hyperactivity, seizures, and tremors followed by paralysis (knock down), which are very similar to those produced by the insecticides pyrethroids [23]. It has been recognized that essential oils are potent neurotoxins and could affect through acetyl cholinesterase enzyme inhibition in the central nervous system [24].
Coriander oil and its major constituent, linalool, have low acute oral and dermal toxicity in laboratory animals. The acute lethality studies suggest that the toxicity of the oil is from its constituent, linalool. The acute oral LD 50 of the major constituent of coriander oil, linalool, in Osborne Mendel rats was reported to be greater than 2.79 g/kg [25]. Collectively, these studies indicate that coriander oil and its major constituent, linalool, are of slight acute toxic potential [26]. As coriander oil contains approximately 70% linalool, the oral LD 50 studies suggest that the lethality caused by the oil is from its constituent, linalool.

Conclusions
Results of the study show that the essential oil of C. sativum seeds can play an important role in stored grain protection and reduce the risks associated with the use of synthetic insecticides. The mode of action of C. sativum oil is of special interest. Further work should focus on its penetration into insect cuticle and grain, metabolic target in the insect body and, its effects on mammals fed on treated material. Therefore essential oil from C. sativum can become an interesting alternative to conventional chemical control strategies.