Chemical Composition and Toxicity against Sitophilus zeamai s and Tribolium castaneum of the Essential Oil of Murraya exotica Aerial Parts

In our screening program for new agrochemicals from Chinese medicinal herbs, Murraya exotica was found to possess insecticidal activity against the maize weevil, Sitophilus zeamais and red flour beetle, Tribolium castaneum. The essential oil of aerial parts of M. exotica was obtained by hydrodistillation and investigated by GC and GC-MS. The main components of M. exotica essential oil were spathulenol (17.7%), α-pinene (13.3%), caryophyllene oxide (8.6%), and α-caryophyllene (7.3%). Essential oil of M. exotica possessed fumigant toxicity against S. zeamais and T. castaneum adults with LC50 values of 8.29 and 6.84 mg/L, respectively. The essential oils also show contact toxicity against S. zeamais and T. castaneum adults with LD50 values of 11.41 and 20.94 μg/adult, respectively.


Introduction
Sitophilus and Tribolium species are two of the major pests of stored grains and grain products in the tropics and subtropics [1]. Control of stored product insects relies heavily on the use of synthetic insecticides and fumigants, which has led to problems such as environmental disturbances, increasing costs of application, pest resurgence, pest resistance to pesticides and lethal effects on non-target organisms, in addition to direct toxicity to users [2]. Fumigation plays a very important role in insect pest elimination in stored products [3]. Plant essential oils and their components have been shown to possess potential for development as new fumigants and they may have advantages over conventional fumigants in terms of low mammalian toxicity, rapid degradation and local availability [4]. Essential oils derived from more than 75 plant species have been evaluated for fumigant toxicity against stored product insects so far [5].
Botanical pesticides have the advantage of providing novel modes of action against insects that can reduce the risk of cross-resistance as well as offering new leads for design of target-specific molecules [2,6]. During our screening program for new agrochemicals from local wild plants and Chinese medicinal herbs, the essential oil from aerial parts of Murraya exotica L. have been found to possess insecticidal activity towards the maize weevil, Sitophilus zeamais (Motsch) and red flour beetle, Tribolium castaneum Herbst.

Insecticidal activity
The essential oil of M. exotica showed contact toxicity against S. zeamais and T. castaneum adults with LD 50 values of 11.41 and 20.94 μg/adult, respectively (Table 2). Compared with the famous botanical insecticide, pyrethrum extract, the essential oil was 2.5 times less active against the maize weevils and 58 times less active against the red flour beetles because pyrethrum extract displayed LD 50 value of 4.29 and 0.36 μg/adult, respectively ( Table 2). The essential oil also possessed strong fumigant activity against S. zeamais and T. castaneum adults with LC 50 values of 8.29 and 6.84 mg/L, respectively ( Table 2). The currently used grain fumigant, methyl bromide (MeBr) was reported to have fumigant activity (24 h) against S. zeamais and T. castaneum adults with LC 50 values of 0.67 and 1.75 mg/L, respectively [33]. The essential oil was 12 times less toxic to the maize weevils and 4 times less toxic to the red flour beetles compared with the commercial fumigant MeBr. However, considering the currently used fumigants are synthetic insecticides, fumigant activity of the essential oil of M. exotica is quite promising and it shows potential to be developed as a possible natural fumigant for control of stored product insects.
In traditional Chinese medicine, M. exotica is used to treat many problems such as rheumatic fever, coughs, giddiness, hysteria and skin diseases [8]. It seems that this medicinal herb is quite safe to human consumption. However, no experimental data about the safety of this herb is available so far. In the previous studies, one of the main components of the essential oil (α-pinene) has been demonstrated contact and fumigant toxicity against several species of insects (cockroaches, head lice, cotton rootworm and stored product beetles and weevils) and mites [36][37][38][39][40]. However, no data on insecticidal activities of the other main components (spathulenol, caryophyllene oxide, α-caryophyllene, bicyclogermacrene) of the essential oil was available. The isolation and identification of the bioactive compounds in the essential oil of M. exotica aerial parts are of utmost importance so that their potential application in controlling stored-product pests can be fully exploited.

Plant material
Dried aerial parts (15 kg of leaves, young floriferous stems) of M. exotica were purchased from Puning Chinese Medicinal herbs Market (Guangdong 515300, China). The aerial parts were ground to a powder. The species was identified, and the voucher specimens (BNU-liuzhilong-2009-08-29-028) were deposited at the Herbarium (BNU) of College of Life Sciences, Beijing Normal University.

Insects
The maize weevil, S. zeamais and red flour beetle, T. castaneum were obtained from laboratory cultures maintained in the dark in incubators at 29-30 ºC and 70-80% relative humidity. The red flour beetle, T. castaneum was reared on wheat flour mixed with yeast (10:1, w/w) while S. zeamais was reared on whole wheat at 12-13% moisture content. Unsexed adult weevils/beetles used in all the experiments were about 2 weeks old.

Essential oil distillation
The ground powder of M. exotica was subjected to hydrodistillation using a modified Clevengertype apparatus for 6 h and extracted with n-hexane. Anhydrous sodium sulphate was used to remove water after extraction. Essential oil was stored in airtight containers in a refrigerator at 4 ºC. The oil yield of M. paniculata was 0.15% v/w.

Gas chromatography and mass spectrometry
Gas chromatographic analysis was performed on an Agilent 6890N instrument equipped with a flame ionization detector and HP-5MS (30 m × 0.25 mm × 0.25 μm) capillary column, while the essential oil components were identified on an Agilent Technologies 5973N mass spectrometer. The GC settings were as follows: the initial oven temperature was held at 60 ºC for 1 min and ramped at 10 ºC min −1 to 180 ºC for 1 min, and then ramped at 20 ºC min −1 to 280 ºC for 15 min. The injector temperature was maintained at 270 ºC. The samples (1 μL) were injected neat, with a split ratio of 1:10. The carrier gas was helium at flow rate of 1.0 mL min −1 . Spectra were scanned from 20 to 550 m/z at 2 scans s -1 . Most constituents were identified by gas chromatography by comparison of their retention indices with those of the literature or with those of authentic compounds available in our laboratories. The retention indices were determined in relation to a homologous series of n-alkanes (C 8 -C 24 ) under the same operating conditions. Further identification was made by comparison of their mass spectra on both columns with those stored in NIST 05 and Wiley 275 libraries or with mass spectra from literature [34]. Component relative percentages were calculated based on GC peak areas without using correction factors.

Fumigant toxicity
The fumigant activity of the essential oil against S. zeamais and T. castaneum adults was tested as described by Liu and Ho [1]. A serial dilution of the essential oil (six concentrations) was prepared in n-hexane. A Whatman filter paper (diameter 2.0 cm) were each impregnated with 20 μL dilution, and then placed on the underside of the screw cap of a glass vial (diameter 2.5 cm, height 5.5 cm, volume 25 mL). The solvent was allowed to evaporate for 30 s before the cap was placed tightly on the glass vial, each of which contained 10 insects inside to form a sealed chamber. Preliminary experiments demonstrated that 30 s was sufficient for the evaporation of solvents. n-Hexane was used as a control. Five replicates were carried out for all treatments and controls, and they were incubated for 24 h. The insects were then transferred to clean vials with some culture media and returned to the incubator and observed daily for determination of end-point mortality, which was reached after one week. The experiments were repeated in three times. The LC 50 values were calculated by using Probit analysis [35].

Contact toxicity
The contact toxicity of the essential oil against S. zeamais and T. castaneum adults was measured as described by Liu and Ho [1]. A serial dilution of the essential oil (five concentrations) was prepared in n-hexane. Aliquots of 0.5 μL of the dilutions were applied topically to the dorsal thorax of the insects. Controls were determined using n-hexane. Both treated and control insects were then transferred to glass vials (10 insects/vial) with culture media and kept in incubators. Mortality of insects was observed daily until end-point mortality was reached one week after treatment. The experiments were repeated in three times. The LD 50 values were calculated by using Probit analysis [35]. Pyrethrum extract (25% pyrethrine I and pyrethrine II) was purchased from Fluka Chemika GmbH and used as a positive control.

Conclusions
Based on mass screening, essential oil of M. exotica was examined for their insecticidal activity against the two grain storage insects. The essential oil possessed strong fumigant toxicity against adults of the two grain storage insects, although it was 10 times and 4 times less toxic to the maize weevil and red flour beetle, respectively compared to commercial fumigant MeBr. The essential oil also showed contact toxicity against adults of the two grain storage insects. These findings, considered together, suggest that the essential oil shows potential for development as a natural fumigant for stored products.