Introduction

Cowpea Vigna unguiculata (L) walp is widely grown in the tropics and subtropics for human consumption as well as animal feeds. Nigeria, Brazil and Niger are among the major producers and account for over 70% of the world’s cowpea production16. Cowpea is also known as blackeye pea, southern pea and crowder pea which is a legume of African origin that is useful as a rational cover-crop and for the production of high quality hay or silage, when mixed with crops such as corn or sorghum, or it can be used for rational grazing17. The seed can be consumed directly, can also be used to make flour, sprouts and wearing food for young children thus, reducing manourishment and stunted growth15. The tender green pods and leaves serve as a vegetable while the crop residue serves as fodder for animals9.

A major problem in storage of cowpea seeds in the tropics is infestation by bruchids especially Callosobruchus maculatus fabricus10. The cowpea weevil mutlplies very fast in storage, giving rise to a new generation every month2. The cowpea weevil Callosobruchus maculatus (F) belongs to the family Bruchidae members of which are generally associated with the plant family fabaceae, which is capable of causing up to 50% infestion of stored cowpea16. Farm storage for six months can lead to (a 70% grain infestation and 30% weight loss which renders the grains virtually unfit for consumption9. Cowpea grains losses are a serious problem in Africa and as much as 20 – 50% of grain is lost because of infestation due to this pest. It has also been estimated that about 40% or 30,000 tones valued at over 30 million dollars is lost annually to C. maculates (F) (Caswell and Akibu,1980).

The uses of synthesized chemicals like insecticides and fumigants are of great contribution to fight against insect pests13. Owing to the problems of synthetic organic chemical there is renewed interest on plants as alternative materials for use as stored grain protectants because they have been found to have broad spectrum insecticidal properties with reduced persistence compared to the organochlorines and organophosphates, carbamates and pyrethroids. They are also cheap to purchase and have no negative impact on the environment because they are easily biodegradable11. The use of serve natural compounds in the protection of grains requires them to be applied in high concentrations, which often exceeds the threshold of acceptable flavour to the consumer8. The current re-newed interest in reducing interest in reducing environment contamination and global warming have served as an added impetus for the re-evaluation and intensification of environment mentally friendly and cost-effective pest management technologies such as the use of traditional botanical pest control agents9. The objective of this study therefore was to evaluate the Toxicity of Ocimum suave (Wild basil) oil extract on Callosobruchus maculatus (F) cowpea weevil and the seed’s viability in storage.

Materials and Methods

Insect Stock Culture

Adult bruchids were obtained from already infested cowpea and identified as Callosobruchus maculatus by the assistance of the Nigerian Stored Product Research Institute (NSPRI) Sapele, Delta State, Nigeria. The C. maculatus cowpea (Vigna unguiculata) L. seed of kano white variety and maintained in large specimen containers with fine mesh gauze covering the opened end, for them to mate and oviposit under laboratory conditions. Adult emergence was checked daily and was observed on the third week which was then used for the experiment11.

Experimental Cowpea

Undamaged and clean cowpea seeds that were used were purchased from Abraka market, Delta State. Each seed was examined under microscope to make sure there were no damages, eggs laid and exit holes on them. They were kept in containers in the laboratory.

Preparation Of Insecticidal Plant Powder

The research plant identified as Ocimum suave by Botany Department of Delta State University was used for the experiment. The plant was obtained from Isele-Azagba (Aniocha North) Local Government Area of Delta State in April 2011. Fresh leaves from the plant was slowly dried for 3 weeks in an open wooden cabinet (1.0 x 0.5 x 1.0m) under room temperature before pulverization in the motorized high speed grinder. The powder were put in a air tight container to prevent active components from evaporating. This method was adopted by Ojianwuna and Umoru11.

Extraction Of Crude Oil

The O. suave plant powder was dissolved in methanol and extracted using a soxhlet extractor. The methanol was then recovered using a rotary evaporator. The extracted oil was then put in a dark air tight container to prevent the active ingredients from escaping then kept in the refrigerator until needed.

Insecticidal Bioassay

Contact toxicity of O. suave extracts

Contact toxicity assay was done according to Kim et al.,6 with some modifications. 20g of cowpea grains in Petri dishes were coated with 0.02, 0.04, 0.06 and 0.08mg plant extracts dissolved in 20ml of water respectively. Ten unsexed insect pairs were then introduced into each Petri dish and exposed to treatments. Controls contained untreated cowpea grains and unsexed insect pairs. The treatment was laid out on a completely randomized design (CRD) with 4 replicates per treatment. Numbers of dead insects were recorded 24, 48, 72 and 96 hours after setup to determine LC50 and LT50.

Germination Test ( Seeds’ Viability)

The effect of crude oil of O. suave on the germinative power of the cowpea grains was assessed using the methodology described by Perez et al.,14. Groups of 10 seeds were randomly selected from each of the concentrations and placed separately in Petri dishes containing permanently moistened filter paper on the bottom. The experiment was kept at room temperature for 7 days. Subsequently, the percentage of the germination was determined in comparison to the control which was clean cowpea without treatment from

Statistical Analysis

All data collected were subjected to Probit analysis3, Analysis of Variance (ANOVA) and multiple comparisons using Tukey multiple comparison Test and LSD.

Result

Acute Toxicity Test

The result of the acute toxicity test carried out in this experiment are shown in Table 1, 2 and 3. The insecticidal effects of the crude oil extract (O. suave) on the insects shows significant difference on the motality (p<0.05) as the concentration increased from 0.02 – 0.08mg/20ml of water. The median lethal concentration LC50 was Antilog -2.08=0.0083 conc. /20ml as shown in Fig 1.

Table 1: Mortality,Mean Mortality and Percentage Mortality caused by different concentrations of O. suave crude oil extract on Insects for within 24 to 96hrs.

Table 2: Mortality, % mortality and % probit kill caused by O. suave oil extract.

Mean of the same superscript letter do not differ significantly (p<0.05) using Tukey multiple comparison test.

Table 3: Germination Test.

Fig. 1: Log Concentration of the oil extract (toxicant) and percentage Probit kill of C. maculates.

LC50 = -2.08, = 0.0083mg.

However as the period of exposure of the crude oil extract (O. suave) on the adult bruchids increased from 24 to 96hrs,there was significant difference (p<0.05) on the mortality of the adult weevil. The survivorships of the insects also significantly (p<0.05) reduced as the exposure period increased from 24hrs to 96hrs as shown in Fig 3.

The median lethal time LT50 was 30hours as shown in Fig 2. The seed germination test showed that seeds with O. suave crude oil extract showed no significant difference (p>0.05) among the treatment when compared to the control as shown in table 3.

Fig. 2: Lethal Time of the oil extract (toxicant) and percentage Probit kill of C. maculates.

Fig. 3: Cumulative survival in relation to exposure periods for C. maculatus adults exposed to the toxic effects of O. suave in four different concentrations.

Discussion and Conclusion

The aim of this research is to evaluate the Toxicity of Ocimum suave (wild basil) crude oil extract against cowpea weevil (Callosobruchus maculatus) by ascertaining the LC50, LT50 and the seed’s viability.

The results of this present study showed significant difference (p<0.05) level of toxicity of the crude oil extract to C. maculatus at different concentration and time.

According to the results as the concentration of O. suave crude oil increased from 0.02 to 0.08mg/20ml of water, there was significant difference (p<0.05) as the percentage mortality of the weevil increased. The median lethal concentration LC50 was Antilog -2.08 = 0.0083mg/20ml of water. The LC50 even occurred at a low concentration indicating that the plant oil may still be effective at lower concentration. The toxicity of the O. suave crude oil may be due to its constituent which include eugenol and mono and sesquiterpenoids. These notable compounds show insecticidal activities which act as stomach and contact poison (insecticide) to C. maculatus4.

Ojianwuna and Umoru s11 also observed the Ocimum suave (Wild basil) powder caused mortality of adult C. maculatus in cowpea grain at 1 – 3g plant material per 20g cowpea. This protected the grains after 96h post treatment. The LC50 was (Antilog) 3.0g/20g cowpea seeds, this may be attributed to the fact that enough content of the volatile compounds has reached the respiratory tracts of the tested organism. Ilondu et al.,5 observed that Ocimum suave (wild basil) caused mortality of adult Rhizopertha dominica in grain cereals at 2-3g plant materials per 50g cereals (Rice, Maize and Sorghum), which protected the cereals after 90 days post treatment.

The result clearly showed that the material tested (O.suave) had distinct effect on survival of C. maculatus. The result shows that the median lethal time LT50 was 30 hours. As the treatment of cowpea with various concentration of the crude oil increased, the survival of the insects decreased, this was similar to the result of Abdullahi and Muhammad1 who assessed the toxic potentials of dried leaf powders of Guinea senegalensis, Piliostigma reticulatum and dried fruit powder of Piper guineense on the survival and development of C. muculatus during storage. Statistical analysis showed that there was significant difference (p<0.05) in concentration and hours of exposure. This observation was also noted by Law-Ogboma and Egharevba7, who used vegetable oils (rubber, seed oil, palm oil and palm kernel oil) against C. maculatus show a significant difference (p<0.05) mortality of the adult weevils.

The high percentage (80-100%) seed germination recorded in this study showed that there was no significant difference (p>0.05) compared to the control. This may be that the plant oil has no effect on the seeds viability. This agrees with the work reported by Umar16 who reported that the cowpea seeds remained viable which was treated with Jatropha curcas powder.

In conclusion, this study has shown that the crude oil extract of O. suave was effective in the protection of cowpea (Vigna unguiculata) against cowpea bruchid Callosobruchus maculatus. The seeds remained viable. Therefore, the use of O. suave and other plant materials in post–harvest protection should be encouraged because it is an environmental friendly plant and can easily be propagated.