Introduction

The quality and availability of food in the past decades has been greatly affected by the gradual and daily transition in the global, environmental climatic conditions. This deplorable climatic condition results in the exponential increase in the amount of both beneficial and harmful insects. This is because increase in temperature equally increases the rate of insect growth, thereby aggravating the problem of food shortage (Odeyemi, 2000).

Cereal crops such as maize (Zea mays), are good sources of carbohydrate and protein; have been greatly affected by the activities of insect pest. These insects have caused a considerable reduction in the annual production/ yield of maize grains. Different methods have been employed in the control of maize weevil, S. zeamais: the major insect infesting maize grain, but today, attention is been shifted from synthetic chemical control to botanical control. This shift is due to the fact that most synthetic insecticides are too stable and the residual effects of these synthetic insecticides have posed much concern to the environmentalist, hence, the need for environmental friendly insecticides. Botanical extracts and secondary metabolites of plants are considered to be a potential alternative approach against insects such as S. zeamais (Shin-foon,1984; Mohan and Kumar, 2012).

In the control of S. zeamais, organic chemicals such as oils extracted from groundnut, alligator pepper and sweet orange have been reported to be effective in the control of S. zeamais (Ivbijaro, 1984; Ashamo and Odeyemi, 2001). Also, powder of alligator pepper has been reportedly used to control S. zeamais (Lale, 1992). Considering the facts that there are many medicinal benefits accrued to lemon grass, C. citratus (Swaminathan, 1992), and has been used as an active insecticide in the control of house fly Musca domestica (Zeneida et al., 2014), this research work is posed to evaluate the insecticidal and repellant activities of C. citratus methanol and ethanol extracts on adult maize weevil (S. zeamais).

1 Results and Discussion

Table 1 shows the repellant class and range in percentage of the number organisms (repelled) assigned to each of the repellant classes as described by Jilani and Su (1983).

Table 2 and Table 3 show the effect of high concentrations of C. citratus Methanol and Ethanol extracts on the Mortality of 0–24h old adult S. zeamias introduced. The results revealed that no mortality was recorded in all the high concentrations used; these indicate that the extracts were not effective botanicals that could cause the death of adult S. zeamias through contact toxicity.

The protective effects of both methanol and ethanol extracts of C. citratus are shown on Table 4 and Table 5 respectively. The result showed that as the concentration of the extracts used as protectant on the maize grains increases, the damages caused by the insects decrease. The highest percentage damages of 17.17 ± 0.60 and 22.5 ± 0.87% measured through weight loss were observed in the untreated controls of methanol and ethanol respectively. Five percentages (5%) concentrations of the extracts used as protectants on the maize grains recorded 3.83 ± 0.67 and 14.00 ± 0.87% damages respectively for methanol and ethanol extracts. The lowest percentages damages on the protected maize grains were recorded in the 25% concentrations of the extracts (0.67±0.17 and 1.33±0.44% methanol and ethanol extracts respectively). It was also observed that there was no significant difference in the percentage damages caused by S. zeamais on the grains protected by 5, 10, 15 and 20% concentrations of methanol oil extract of C. citratus.

Table 6 and Table 7 show the percentage repellant of the methanol and ethanol oil extracts of C.citratus leaves on 0-24 h old adult S. zeamais. The results showed that at concentrations 10% and above, repellant class of V (indicating 80.01-100% repellant range) was observed. This shows that the insects (S. zeamais) were effectively repelled by the extracts at concentration of 10% and above in both methanol and ethanol oil extracts

Lemon grass has been reported to have medicinal active ingredients and have been widely used in the cure of diseases such as malaria fever, flu, ringworm and cough in Nigeria (Odugbemi, 2006). The oil of C. citratus contains citronella; geraniol, lemonene and myrcene, chemicals which are believed to have insecticidal properties (Swaminathan, 1992; Zeneida et al., 2014). The research carried out on the oils of C. citratus shows that the oil of C. citratus leaves; extracted using methanol and ethanol as extracting solvents have less entomicidal/ insecticidal effects, through contact toxicity on the adult S. zeamais. This is contrary to the report of Zeneida et al. (2014), who widely reported the insecticidal activity of C. citratus oil against house fly M. domestica. Although, structurally, there is a wide range of difference between S. zeamais and M. domestica, this might contributed to the variation in the result.

Despite the fact that the oils of C. citratus did not show any contact toxicity against adult S. zeamais, they however shows some level of protectant on the maize grains just as other botanicals used as protectant on grains against insect damage (Ashamo and Odeyemi, 2001). A close study of the S. zeamais shows that the adult S. zeamais introduced into the Petri-dish containing treated maize grains took time to adapt to the treated grains before feeding. The repellant effect of the oil extracts was so pronounced on the S. zeamais, this corresponds with the findings of Jilani and Su (1983) that some botanicals have repellant effect on some insects. This might be due to the aromatic scent produced by the extracts.

In conclusion, oil extracts of C. citratus leaves are biologically active as temporary repellant against S. zeamais and could protect maize grains over a short range of time. Furthermore, the insecticidal action of the extracts was very low against S. zeamais.

2 Materials and Methods

The weevil, S. zeamais used for this study were obtained from Crop, Soil and Pest management department of the Federal University of Technology, Akure (FUTA). A large quantity of the weevils was cultured in the Research laboratory of Biology department, FUTA. Clean seeds of white maize were also obtained and disinfested in deep freezer at the temperature of -2°C for 72 h. The leaves of lemon grass (C. citratus) used were harvested in Gbongan area of Osun state Nigeria. The leaves of the botanicals (C.citratus) were dried, pulverized and its oils were extracted using Soxhlet apparatus, using methanol and ethanol (separately) as extracting solvents. The resulting extracts contained both extracting solvents and the oils. After which the oils were exposed to air to remove traces of the volatile extracting solvents.

2.1 Bio assay with C. citratus methanol and ethanol oil extracts

The modified method adopted for this experiment was from Bankole (2004). Different low concentrations of 0.5, 1.0, 1.5, 2.0 and 2.5 ml of the oil extracts per 100 cm³ of the extracting solvents (methanol and ethanol) were prepared to produce 0.5, 1.0, 1.5, 2.0 and 2.5% concentrations of the oil extracts. Five milliliters of each of these concentrations were used as protectants on 20 g of white disinfested maize grains. Different high concentrations (5.0, 10.0, 15.0, 20.0 and 25.0 ml of oil extracts per 100 cm³ of the extracting solvents to produce 5, 10, 15, 20 and 25% concentrations of the oil extracts) were also prepared and each used as protectant on 20 g of white disinfested maize grains in plastic container of volume 10 cl. Two sets of control experiments one with extracting solvent and the other without any solvent were also set up. All the set up were in triplicate. Twenty adults S. zeamais (10 males and 10 females) of 0-24 h old were introduced into the plastic containers containing both treated (protected) and untreated maize grains (control experiment). The adult mortality was counted for 7 days and the cumulative percentage mortality was determined. The initial and final weights of the grains were also measured to estimate the percentage damage on the maize grains.

2.2 Repellant test

Melted wax were poured into six Petri-dishes and allowed to solidify. Six holes of the same radius were carefully made close to the circumference of the Petri-dishes. Five disinfested whole grains of maize of same sizes were treated with 5% concentration of methanol extract of the C. citratus. Other fives disinfested whole grains of maize of same sizes were treated with 10, 15, 20 and 25% concentration of methanol extract of the C. citratus. The untreated control of five disinfested whole grains of maize of same sizes was also prepared. The treatments were placed separately in the holes provided in the Petri-dishes (Jilani and Su, 1983).

The above set up was replicated in triplicate and also repeated for ethanol oil extract of C. citratus. At the centre of each of the Petri-dishes ten (10) adults S. zeamais (5 males and 5 females) of 0-24 h old were introduced. The set up were left and examined every day for three days to understand the repellant effect of the extracts at different concentrations. The overall average of repellency of each concentration was calculated and assigned a repellency class, using the scale range in Table 1 as described by Jilani and Su (1983).

All data were analysed using Analysis of Variance at 95% confidence level to see if there is any significant difference(s) among the different concentrations/treatments. New Duncan Multiple Range Post Hoc Test was used to separate the means.

Authors’ contributions

Salami, Olufemi Samson fine tuned the planed project, executed the project, collected data, analysed the data and drafted the manuscript. Olufemi-salami, Folasade Kemisola helped in data collection and editing of the drafted manuscript.

Acknowledgments

My sincere gratitude goes to Late Dr. Akinkuolere Oluwafemi Rotimi who helped in the planning and supervision of the project.

References

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