2.2 Lethal concentration of A. auriculiformis required to achieve 50 and 95% mortality of C. maculatus within 96 h of exposure

The lethal dosage of A. auriculiformis oil extract required to achieve 50 and 95% mortality of the insect within 96 h post treatment (Table 2). The negative coefficient of the oil indicated that the higher the concentration of the oil, the higher will be the mortality rate of the insect. Also, chi square values that are greater than zero indicated the high level of relationship between the concentration of the oils and the mortality of the insect. There was a great significant relationship between the mortality of the insect and concentration of oil as the p-value of the calculated chi square is lesser than 0.05. Only 4.91 and 9.94% concentration of the plant oil is required to achieve 50 and 95% mortality of the beetle within 96 h post treatment.

 

Table 2 Lethal concentration of A. auriculiformis required to achieve 50 and 95% mortality of C. maculatus within 96 h of application Note: S. E: standard error; X2: Chi square; LC: lethal concentration; FL: fidiciual limit

 

The oviposition rate and adult emergence of C. maculatus exposed to different concentrations of A. auriculiformis oil extract (Figure 1). Significant statistical differences occurred between the treatments at F6, 14=280.779, p<0.0001 (oviposition) and F6, 14=849.974, p<0.0001 (adult emergence). The oil extract of the plant significantly reduced the oviposition and adult emergence of the beetle. The effect of the oil on oviposition adult emergence of the beetle increased with increase in the concentration of the oil. Regardless of the concentration used, the oil was unable to prevent the oviposition and adult emergence of the insect. 5% concentration of the oil extract recorded the lowest oviposition (47.00) and adult emergence (24.08%) and its effects were significantly (p<0.05) different from all other concentrations. The concentrations of the oil were significantly (p<0.05) different from the two controls in their effects on oviposition and adult emergence of the insect. However, the two controls were not significantly (p>0.05) different from each other.

2.4 Damage, weight loss and weevil perforation index of A. auriculiformis treated cowpea seed exposed to C. maculatus infestation

The percentage damage, weight loss and WPI of A. auriculiformis treated cowpea seed infested by C. maculatus (Figure 2). The seed damage, seed weight loss and WPI varied with the concentration of the A. auriculiformis oil extract. There were statistically significant differences between the treatments at F6, 14=285.489, p<0.0001 (Damage); F6, 14=139,756, p<0.0001 (Weight loss) and F6, 14=226.964, p<0.0001 (WPI). 5% concentration of the oil recorded the lowest seed damage, seed weight and WPI of 17.77, 10.45 and 20.93% respectively and its effects was significantly (p<0.05) different from all other concentrations of the oil extract except 4% concentration that recorded 22.26, 13.17 and 26.07% of seed damage, weight loss and WPI respectively. Moreover, there was no significant (p>0.05) different in the seed damage, weight loss and WPI of the two controls.

 

Figure 2 Percentage seed damage, weight loss and weevil perforation index (WPI) of protected cowpea seed

 

2.5 Correlation between mortality and oviposition as well as between adult emergence and weight loss

The correlation between adult mortality at 96 h of exposure and oviposition as well as adult emergence and weight loss of the protected cowpea seed (Table 3). The R value of 0.827 that tends to 1 reflected high correlation between the mortality of the insect and the oviposition rate. The R2 value showed that only 68.4% of the oviposition rate of the insect can be explained by the mortality of the insect. However, after the adjustment of the R2 value, only 66.7% of the insect oviposition rate can be determined by the mortality rate. The t-value of -6.410 that is lesser than -1.98 indicated that there was a great statistically significant relationship between the mortality and oviposition rate of the insect at F1, 19 = 41.088, p<0.0001. There was a great relationship between the adult emergence of the insect and the weight loss of the protected cowpea grains as reflected by their R value (0.978) which tends to 1. The R2 value showed that 95.7% of the seed weight loss of the cowpea can be determined by the emergence of the adult beetle. After the adjustment of the R2 value only 95.4% on the seed weight loss was being explained by the emergence of the adult beetle. The t-value (20.493) which is greater than 1.98 showed that the correlation between the adult emergence of the beetle and the weight loss of the cowpea was statistically significant at F1, 19=419.960, p<0.0001. It was noted that the correlation between the adult emergence and seed weight loss was greater that between the adult mortality and oviposition as reflected by their t-value.

 

Table 3 Correlation between insect mortality at 96 hours of application and oviposition rate and correlation between adult emergence and seed weight loss Note: Where AD) = adjusted R square; K = constant; RC = regression coefficient; RE = regression equation; E= error; M4= mortality at day 4; O= oviposition and W = weight loss

 

3 Discussion

The result obtained in this work showed that A. auriculiformis oil extract have insecticidal properties as it significantly affected the survival rate, oviposition and adult emergence of C. maculatus and as well affected the ability of the insect to cause damage and weight loss of the protected cowpea seeds. The ability of the oil to effect high mortality of the insect, low oviposition rate and low adult emergence varied with the concentration of the A. auriculiformis oil extract. The high mortality of C. maculatus recorded by the oil of this plant could be due to in ability of the insect to feed on the protected cowpea seed. Despite adult C. maculatus does not eat they can live for longer period if provided with honey or sugary materials. Therefore, the mortality of the insect recorded by the oil indicated that the oil had no sugary substance that can serve as food for the insect and thereby lead to the starvation of the insect. Also, botanical base insecticides have been noted to have negative effect on respiratory organ of insects leading to hyperactivity and convulsion and total knockdown of insects (Schmutter, 2002; Zibaee, 2011; Rajashekar et al., 2014). Therefore, the mortality of cowpea beetle recorded in this work could be due to ability of the oil extract of A. auriculiformis to block the spiracle of the insect which resulted to asphyxiation and subsequent death of the insect. However, it was noted that the oil extract of the plant was required in high concentration to achieve 50 and 95% death of the insect as reflected by the probity analysis. The result obtained in this research agreed with the findings of Leatemia and Isman (2004) Adebiyi and Tedela (2012) as well as Ogungbite et al., (2014) in which the oils of botanicals were find to effect high mortality of C. maculatus and other stored product insect pest. The result obtained on the effect of oil extract of A. auriculiformis on the oviposition of C. maculatus indicated that the oil had significant effect on the oviposition rate and adult emergence of the insect. There was low rate of oviposition and adult emergence of the insect when compared to the controls. The low rate of oviposition of the insect indicated that the oil extract must have caused low mating communication between the male and female C. maculatus. Also, the low oviposition rate could be due to high mortality of the insect which resulted in low mating period that subsequently lead to low number of egg laid (Yusuf, 2009; Ashamo et al., 2013). Zibaee (2011) and Isman (2006) stated that botanical insecticides apart from having antifeedant effect on insects could also cause incomplete ecdysis in young insects and sterility in adult female insects. Therefore, the low oviposition rate implies that the oil must have caused sterility of the adult female C. maculatus. Furthermore, the low rate adult emergence could be due to inability egg laid by the insect to hatch into the larvae stage. The few larvae of the insect emerged may unable to drop off their exoskelecton that remained connected to the posterior part of their abdomen (Oigiangbe et al., 2010; Adeyemo et al., 2013). In addition, the low oviposition rate recorded by the insect could also have caused the low rate of adult emergence. Ogungbite (2015) as well as Obembe and Ogungbite (2016) suggested that botanical oils do block the chorion of insect’s egg and thereby prevents the emergence of adult insects or lead to deformation of the insect larvae. The result of this study showed that the oviposition rate of the insect was dependent on mortality rate of the insect while weight loss of the seed was also dependent on adult emergence of the beetle. The result of this research acquiesced with the findings of Yusuf (2009), Ileke and Olotuah (2012), Ashamo et al., (2013), Oni (2014), Oni et al., (2016) in which botanical oils and powders were found to significantly reduced or prevented the emergence of adult C. maculatus. Low grain damage, weight loss and weevil perforation index was recorded in this work. This implies low feeding habit of the insect larvae which could have caused high seed damage, weight loss and subsequent high weevil perforation index. This agreed with the report of Akinneye and Oyeniyi (2016) in which the powder of some botanicals was found to affect the ability of Sitotroga cerealella to cause damage and weight loss of paddy rice. Kaur et al., (2014) reported the presence of flavonoids, alkaloids and tannins in the extract of A. auriculiformis. These phytochemicals have been reported of displaying different insecticidal efficacy against wide range of insect pests. They are being reported of disrupting life cycle of insects by causing high mortality rate of insect (Yang et al., 2006; Adonu et al., 2013; Ogungbite and Oyeniyi, 2014). Therefore, the high mortality rate, low oviposition rate, low adult emergence, low seed damage, seed weight loss and low WPI could be due to the presence of the phytochemicals present in the oil of A. auriculiformis.

 

Since the world of insect pest control is tending toward the total rejection of any materials that could have adverse effect on human and environmental health, botanicals of different species have been suggested as promising tools in insect control. However, there is still a wide gap in the effectiveness of botanical insecticides compared with synthetic chemicals insecticides that are considered dangerous (Isman, 2006; Begum et al., 2013). Therefore, there is need for searching new botanicals with insecticidal potential.

 

3.1 Conclusion

The oil of A. auriculiformis used in this work has proven insecticidal against cowpea beetle, C. maculatus. Nevertheless, the insecticidal potential of this plant oil depended on the concentration of the oil and the period of application. The result showed that the oil had more effects on the oviposition and adult emergence of the insect as well as the ability of the insect to cause seed damage, weight loss and WPI. Hence, the oil could be used as protectant rather for the control of the insect. More so that it recorded low WPI at higher concentrations. Ileke and Olotuah (2012) opined that WPI less than 50% indicated high protected ability of any materials used for the protection. Nonetheless, there is still need for investigating the mode of action of the oil, its effect on detoxifying enzymes and its bioactive compound.

 

4 Materials and Methods

4.1 Insect culture

The initial culture of C. maculatus used was obtained from an already infested cowpea seeds. The insects were reared on the IT-86-KD cowpea variety to ensure the removal of effect of maternally inherited dietary of previous food eaten by the insect. The insects were cultured at temperature of 28±2ºC and relative humidity of 75±5% inside plastic container covered with muslin cloth to disallow the escape of the insect and as well disallow the entry of intruding insects that may act as parasitoid. The culture was maintained by replacing the consumed seeds with new uninfested seeds.

 

4.2 Collection of cowpea and plant materials

The cowpea variety (IT 89-KD) used was collected from the National Seed Service, Ibadan, Oyo State, Nigeria. The cowpea was disinfested before use by placing them inside freezer at -7^oC for eight weeks and the seeds were exposed to air in the laboratory to avoid mouldiness. The leaf of A. auriculiformis used was collected fresh in an open field around sport complex, Federal University of Technology, Akure. The collected leaves were air dried under shade and milled into fine powder using an electric blender. The powder of the plant was kept inside air-tight container for subsequent use.

 

4.3 Preparation of plant oil extract

Oil from the leaves of A. auriculiformis was extracted by cold extraction method using absolute ethanol as solvent. 100 g of the plant powder was soaked in 0.5 litre of ethanol and was thoroughly mixed together to ensure the powder was completely soaked with solvent. The mixture was left for four days being stirred every morning. The mixture was sieved with muslin cloth to remove the shaft. The oil was separated from the solvent using rotary evaporator. The resulting extract was exposed to the air to remove traces of the solvent. The extracted oil was kept in bottles with lids and stored in a refrigerator until needed.

 

4.4 Effect of A. auriculiformis crude oil extract against C. maculatus

Twenty grammes of the cowpea variety (IT-89-KD) was weighed into Petri-dishes of equal sizes and 2 ml of the crude oil extract applied topically to the grains at 1, 2, 3, 4 and 5% concentrations while the controls were cowpea treated with 2 ml of ethanol and those that were not treated with neither oil extract nor ethanol. Ten pairs of 0-24 old adult C. maculatus were introduced separately into those treated cowpea seeds at different concentration as in above. The setup was left inside breeding cage in the laboratory. Beetle mortality was assessed at 24, 48, 72 and 96 h of treatment. Dead and live insects were removed on the fifth day and oviposition was recorded. The percentage beetle mortality was calculated using the formula below:

 

 

Percentage adult emergence, seed damage, weight loss and weevil perforation index were calculated after 42 d using the formula below:

 

 

 

 

 

4.5 Statistical analysis

Data obtained were first transformed to normalize them and were later subjected to one-way analysis of variance. Means were separated with New Duncan’s Multiple Range Test. Also, the data obtained on adult mortality were subjected to regression analysis using probit to determine the LC₅₀ and LC₉₅ of the oil (Finney, 1971). In addition, linear regresion analysis was carried out to determine the relationship between adult mortality and oviposition as well as between adult emergence and seed weight loss.

 

5 Conclusion

The present study provides empirical primary ethnomedicinal data on the use of traditional knowledge to treat epilepsy and can contribute in preserving indigenous knowledge of tribes in Eastern Ghats of Andhra Pradesh. It is anticipated that these primary data will open new avenues to identify novel drugs that can help to alleviate sufferings of mankind.

 

Author’s contribution

Tedela P.O. and Ogungbite O.C. designed the work. Ogungbite O.C. carried out the research and as well carried out the anaylsis of data obtained. Tedela P.O., Ogungbite O.C. and Obembe O.M. prepared the manuscript while Tedela P.O. and Obembe O.M. proofread the manuscript.

 

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