Essential oil composition of different fractions of Piper guineense Schumach . et Thonn from Cameroon using gas chromatography-mass spectrometry and their insecticidal effect on Sitophilus oryzae ( L . )

Essential oil fractions from dried seed powder of Piper guineense were analyzed by gas chromatography-mass spectrometry (GC-MS) and evaluated for their insecticidal effects on Sitophilus oryzae L. The GC-MS analysis showed quantitative and qualitative differences between the oil fractions. Chromatographic results revealed chemical constituents like eugenol, piperanol, pinene, carene, copaene with insecticidal properties. New chemotypes were seen in the different fractions. Instead of βcaryophyllene reported in literature, α-caryophyllene was found in all the different fractions. Caryophyllene oxide, an oxygen-containing sesquiterpene was present in all fractions except n-hexane. In addition to α-phellandrene present in all, β-phellandrene, a monoterpene hydrocarbon was found in the n-Hexane fraction. Contact toxicity on wheat grains showed that all fractions caused significant (P < 0.001) mortality of the weevils. The oil fractions also showed variable contact toxicity on impregnated filter paper. All doses of the n-hexane fraction were very toxic to the test insect than the control, causing 100% mortality after five days of exposure. All the fractions produced a strong repellent activity against the test insect. These results suggest that P. guineense has potentials for development as an organic insecticide against S. oryzae and other pests of stored grains.

Author(s) agree that this article remains permanently open access under the terms of the Creative Commons Attribution License 4.0 International License stored grains constitute the most important food crop (Nukenine et al., 2010).Control of these insect pests is primarily dependent upon applications of synthetic insecticides (Bekele et al., 1996).Sometimes, insecticides may seem effective, but repeated application has disrupted biological control by natural enemies, leading to outbreaks of other pest species and also the development of resistance (Park et al., 2003).The problems caused by synthetic pesticides and their residues have increased the need for effective biodegradable pesticides of plant origin with greater selectivity and history of traditional use.Botanicals plants have been successful against a number of pests in Africa (Bekele et al., 1997;Ogendo et al., 2008).
Botanical products are based on powders, extracts or purified substances of plant origin and are promising alternatives to synthetic residual pesticides in Africa, where subsistence farming practice predominates.In many parts of Cameroon, farmers use different kinds of plant products for insect control (Tapandjou et al., 2000;Ngamo et al., 2007).Although, very little use is known by farmers in the South West region of Cameroon on plant products for insect pests" control.Plant products are known to have negligible effects on beneficial insects and lower environmental impacts.They are easily affordable, available and play useful role in integrated pest management IPM programs in developing countries.Since most developing nations suffer from high cost of synthetics, crop protection products with modest efficacy are preferred if they are readily available and less expensive than the conventional pesticides.Plants powders or extracts could be produced with limited skills and knowledge and their use entails little or no financial expenditure.
Of the 700 species of Piper, only three are recognized to be indigenous to Cameroon (Hutchinson and Dalziel, 1963).The most widely distributed, Piper guineense, is a native to tropical Africa, ranging from Guinea to Kenya and South Zambia.It is a forest liana with branchlets spiralling up to shrubs to about 10 m.The leaves are elliptical in shape and have a pleasant aroma when crushed.The spherical fruits (berries) are yellow becoming orange, red and finally black (Iwu, 1993).The fruits are commonly called "West African Black pepper" or "Poivrie" in French.The fruits are sold in the market as edible medicinal plant or food additives due to their strong, pungent aroma and flavour.In the South West region of Cameroon, the leaves (fresh or dried) are eaten as vegetables in "Soups", while the ground powder is used as additives in sauce.Powders obtained from ground seeds are used as stimulants (Sofowora, 1982).
P. guineense has demonstrated wide potentials against major noxious insect pests of arable crops (Ukeh et al., 2008;Ntonifor, 2011).A laboratory assessment of the repellent and anti feedant properties of both aqueous extracts and powders of P. guineense against Callosobruchus maculatus and Putella xylostella revealed potent repellent and anti feedant activities of this plant (Ajayi and Wintola, 2006;Ntonifor et al., 2010).Insecticidal activities of the essential oil have been tested (Asawalam et al., 2007).Several studies have been done on the essential oil composition of P. guineense seeds from different geographic origins (Amvam et al., 1998;Jirovtez et al., 2002).Many investigators have worked on whole fruit powder of P. guineense from Cameroon (Jirovtez et al., 2002;Tchoumgougnang et al., 2009).Their essential oil composition depended on the geographic locations and climatic conditions where the seeds were taken.Essential oil composition of different fractions of this plant by GC-MS was not reported.Therefore, the purpose of the present study was: (1) to identify the aroma compounds of different fractions of this plant responsible for the characteristic odour and taste, (2) to evaluate the insecticidal activity of the different fractions against Sitophilus oryzae, a pest of stored grains found in Cameroon.

Plant material and extract preparation
Dried fruits (berries) of P. guineense were purchased from a local market in Yaounde, Cameroon.The spices were authenticated by comparing with herbarium specimens at the Limbe Botanic Garden (Limbe, Cameroon).Samples were air-dried for 3 days at room temperature and crushed to a fine powder using a mechanical blender.The powder was then packed in air-tight bags and exported to Faculty of Agriculture and Environment (University of Sydney, Australia).Extracts of the spice were prepared in a sequential manner as follows: 100 g of seed powder was dissolved in 200 ml methanol, vortexed for 15 min and sonicated for 1 h under high frequency in an ultrasonic cell disruptor (Microson™).This process was to break and disrupt the plant cells for easy extraction.The process was repeated 3 times.The resulting solution was filtered with Whatman No. 1 filter paper using a vacuum pump.The filtered solution was evaporated in a BUCHI (R-114) rotavapor under reduced pressure at a temperature of 60°C to concentrate the samples.The sample was re-dissolved in 50 ml methanol and transferred into a separating funnel and eluted successively in hexane, chloroform, ethyl acetate and methanol.Each solvent was evaporated in a BUCHI rotavapor under reduced pressure at a temperature of 60°C.The resultant residues were re-dissolved in acetone, to give a stock volume of 10 ml for each fraction.The solutions were refrigerated prior to application and GC-MS analysis.

Gas chromatography-mass spectra analysis
The essential oil fractions of P. guineense were analyzed by GC-MS and identification of their constituents was achieved based on their retention indices determined with reference to standards and by comparing with those reported in the literature (Jirovtez et al., 2002;Adams, 2007;Tchoumbougnang et al., 2009).Analytes were tentatively identified by reference to the NIST 2008 mass spectral library.An Agilent 7890A series GC with 5975C inert MSD triple axis detector (Agilent Technologies, USA) was used for GC-MS analysis.A Markes Series 2 ultra-unity system (Markes International Ltd., UK) was used for automated thermal desorption.Extract (3 µL) was injected onto sampling tubes containing Tenax TA (Supelco Inc., Bellefonte, PA, USA) adsorbent resin via a stream of zero air (100 mL/min).The samples were desorbed by heating the sample tubes for 6 min at 300°C and focused onto a Tenax TA cold trap at -30°C for 6 min.The cold trap was then flash-heated to 300°C for 5 min and the sample injected onto a HB5 column (30 m × 250 µm × 0.25 µm ID) via a heated transfer line held at 260°C.The GC oven was initially held at 40°C for 2 min, heated to 160°C at 5°C min −1 , 320°C at 10°C min -1 , then held for 2 min.

Insect culture
S. oryzae adults were obtained from a stock maintained at the Faculty of Agriculture and Environment (University of Sydney) and reared on whole wheat grains in a constant temperature and humidity chamber (26°C, 65% RH) in darkness.

Bioassays
Bioassays were conducted using one month old adults of S. oryzae.Doses of 0, 0.5, 1 and 2 ml of the stock solution of each oil fraction were serially diluted in acetone to make up 10 ml, giving a series of dilutions 0, 0.05, 0.1 and 0.2 of each solution.

Grain contact toxicity
One millimeter from the new preparation was mixed with 40 g of wheat grains and stirred thoroughly for 5 min to allow even distribution over the grains.The control grains were treated with acetone only.Treated grains were kept for 20 min to allow the solvent evaporation.Twenty (20) adults of one month-old from the laboratory culture were introduced to the grains in a 250 ml glass vial with perforated lids.Each treatment and control was replicated four times.Mortality was evaluated after 24 h for up to five days.

Filter paper contact toxicity
Contact effect of extract fractions was evaluated on filter paper, 7 cm Whatman no.1.The filter paper was placed in Petri dishes and 0.5 ml of the serially diluted extract fractions was applied on the filter paper disc.The control filter papers were sprayed with acetone only.The acetone was allowed to evaporate for 20 min, after which, 20 unsexed one month-old adult insects were introduced at the center of each disc.This was kept under laboratory conditions (25±1°C, 60% RH).The treatments and control were replicated 4 times.Insect mortality was recorded after 24 h for up to five days.Percent mortality was calculated using Abbott correction formula for natural mortality in untreated controls (Abbott, 1925) (PT), as follows: PT = (PO-PC)/(100-PC) × 100 Where, PO = observed mortality of treated adults (%), PC = control mortality.

Repellency bioassay
A repellence effect was evaluated using the modified area preference method of McDonald et al. (1970) (Tapondjou et al., 2005).Test areas consisted of 7 cm Whatman no. 1 filter paper cut in halves.Test solutions were prepared by series of dilutions of the extract fractions in acetone as above.Each solution (0.5 ml) was uniformly applied to a half-filter paper disc using a micropipette.The other half disc was treated with acetone alone and served as control.Treated and untreated discs were air-dried for 10 min to evaporate the solvent completely.Full discs were remade by attaching treated halves to untreated halves with clear adhesive tape.Each remade disc was placed in a 7 cm Petri dish and 20 unsexed adult insects of one month-old were released at the center of the filter paper disc and the Petri dishes were covered.Each treatment and control was replicated 4 times.The number of insects present on the control (Nc) and treated (Nt) areas of the discs was recorded after 3 h.Percent repellency (PR) was calculated as follows: PR=

Statistical analysis
The results of the experiments were analysed by one-way analysis of variance (ANOVA) using the General Linear Model Procedure (GLM) of GenStat 13 th edition.Data were log-transformed before analysis.Duncan multiple range tests were used to compare the means.Percentage mortalities were calculated from the overall number of dead insects.Significant levels were set at 0.05.

Contact toxicity by grain treatment
Insect mortality by grain treatment of the different oil fractions differed significantly (P < 0.05) (Table 2).The nhexane fraction produced 100% mortality of S. oryzae after five days of grain treatment, while the aqueous methanol fraction produced less than 50% mortality after five days of treatment.The chloroform and ethyl acetate fractions showed very little mortality and were not significantly (P < 0.05) different from the control (Figure 1).All the different doses of n-Hexane fraction of the essential oil induced 100% mortality after five days of exposure.This mortality could have been due to the high percentages of sesquiterpenes hydrocarbons present in this fraction.Methanol fraction, on the other hand, had percentages of eugenol and piperanol which induced high mortality in the essential oil.The 0.2 ml/cm² of the methanol fraction induced more than 30% mortality different from the control.

Filter paper contact toxicity
The percentages of insect mortality recorded after five days of exposure to increasing concentrations of volatile oil on filter paper discs showed similar results.The n-Hexane fraction had a significantly (P < 0.05) higher mortality on S. oryzae and doses of 0.1 and 0.2 ml/cm² were able to induce 85 and 100% mortality, respectively.The Chloroform, Methanol and Ethyl acetate fractions were almost nil within the first three days, and were not significantly (P < 0.05) different from the control, causing less than 50% mortality after five days of exposure on impregnated filter paper disc (Figure 2).There was a significant correlation between treatments, duration and concentrations as seen below (Table 3).

Repellency testing
Repellency bioassay using the area preference test showed that P. guineense extract fractions significantly repelled S. oryzae with an overall repellency of 87.5% (Table 4).The effect of the volatile oil on repellency of S. oryzae after 3 h of exposure was not dose-dependent.
The n-Hexane fraction produced the highest repellency at different concentrations, while chloroform, methanol and ethyl acetate had 50% repellency at the highest concentrations.

DISCUSSION
The composition of different fractions of P. guineense essential fruit oil in this study was remarkably different from those reported earlier in Nigeria (Oyedeji et al., 2005;Owolabi et al., 2013) and Cameroon (Jirovetz et al., 2002;Tchoumgougnang et al., 2009 In all the different fractions, α-caryophyllene was present in high amounts instead of β-caryophyllene reported in literature.Such high content has not been found in P. guineense essential oil from Cameroon until now.Eugenol was abundant only in the Methanol fraction.These and others represented a number of new chemotypes revealed from the GC-MS analysis.The presence of constituents such as aromadendrene, piperazine and piperidine, though in low amount in the n-Hexane may have accounted for its   to have 5 to 8% Piperine which gives them their "heat" (Oparaeke, 2006).It also contains about 31-monosesquiterpenoids that have high insecticidal properties due to their pungent nature (Taponjou et al., 2005).The variability of the different constituents of P. guineense is important in light of its use as flavouring and medicinal agent.Therefore the efficacy varies depending on the chemotypes used.In the present study, the essential oil fractions of P. guineense exhibited different levels of toxic and repellent effects on S. oryzae at various doses.These effects may be due to factors such as the chemical composition of the plant as well as the insect susceptibility.The different doses of the n-Hexane fraction caused 100% mortality of S. oryzae on filter paper fumigation toxicity and grain contact after five days of exposure.The insects appeared to avoid the treated areas.The mortality of weevils on treated grains and filter paper varied with dosage of essential oil fractions.High mortality rates were recorded with the n-Hexane fraction at all doses compared to the other fractions.The toxicity of volatile oil from P. guineense is generally attributed to the presence of the alkaloids, piperine, chavicine and piperidine which are reported to be the major active components in P. guineense seeds (Lale and Alaga, 2000).This suggests that the toxicity of P. guineense was not due to ingestion of treated grains, but due to suffocation.The oil fractions evoked a very high repellency against the insect, suggesting that the oils could have contained a very pungent substance that caused high repellency of the insects.n-Hexane again demonstrated the highest level of repellency probably due to presence of constituents like Piperazine and Piperidine, though in very low quantity.Many other researchers have demonstrated the toxic and repellent effects of constituents of this oil such as 1S-α-Pinene, Copaene, β-caryophyllene, Eugenol, Piperanol and many others (Amvam Zollo et al., 1998;Owolabi et al., 2013).They are similar to those reported by Jirovetz et al. (2002) and Tchoumbougnang et al. (2009), in P. guineense obtained from Cameroon and Nigeria.
The results demonstrated a scientific rationale for the traditional incorporation of the dried seeds oils of P. guineense into grain protection practices in rural communities of Cameroon.However, there is need for further investigations into such practices to improve their efficacy and reliability in rural communities.With the right dosage and proper formulations this essential oil could be exploited at the small scale farmers" level against insect infestations in the fields and in storage.Most of the oils are more effective, less cumbersome and are not particularly dangerous to consumers because they are used in pharmaceutical preparations (Bauer et al., 1990).They are also less expensive, safe to the environment and harmless to humans and other mammals.This study concludes that all of the fractions from the seeds of P. guineense demonstrated toxic and repellent effect on S. oryzae, a storage pest in Cameroon.The study also showed that the n-Hexane fraction of the essential oil was more toxic to S. oryzae.

Figure 1 .
Figure 1.Percentage mortality of Sitophilus oryzae exposed to different fractions of Piper guineense essential oil coated on wheat grains (A-Hexane fraction; B-chloroform fraction; C-ethyl acetate fraction and D-methanol fraction).

Figure 2 .
Figure 2. Percentage mortality of Sitophilus oryzae exposed to different fractions of Piper guineense essential oil impregnated on filter paper discs (A-Hexane fraction; B-Chloroform fraction; C-Ethyl acetate fraction and D-Methanol fraction).

Table 1 .
Comparative percentage composition of essential oil major fractions from dried seeds of Piper guineense bought from Yaounde, Cameroon.

Table 2 .
Summary of analysis of contact toxicity by grain treatment.

Table 3 .
Correlation parameters for contact toxicity by grain treatment and impregnated filter paper assay.

Table 4 .
Repellent effects of different fractions of essential oil from Piper guineense on Sitophylus oryzae.
"heat" and aroma.P. guineense have been demonstrated