Background

One of the world major contests which have lasted for ages includes the contention between insects and human. Both humans and insects share almost the same source of surviving, that is food; and this has led to struggle between them. Because of the ease of reproduction and ability to attack both on the field and in store, insects have been able to threat the human existence in term of food security. Large amount of human agricultural produce have been lost to insects attack both on the field and in storage where they speedily multiply (Ogungbite and Oyeniyi, 2014). This in turn is causing over 800 million people of the world to suffer inadequate food supply (Lale, 1995). In fact, more than 5-10% of grain loss in the temperate regions and 20-30% grain loss in the tropical zones of the world was solely associated to the insects attack (Rajashekara and Shivanandappa, 2010) especially in resource poor country like Nigeria.

Rice being one of the major highly consumed cereal grain in every parts of the world has been suffering different attacks from different species of insects ranging from the beetles to weevils to moths (Ashamo, 2010; Ileke, 2013; Adeyemo et al., 2013; Ogungbite and Oyeniyi, 2014). This valuable cereal crop that feeds more than 50% of the world total population annually is being attacked almost incessantly between harvest and consumption by insects such as Ryzopertha dominica F., Sitotroga cerealella (Olivier), Sitophilus oryzae L., S. granarius L., as well as Scirpophaga innotata and Scirpophaga incertulas (Ashamo and Akinnawonu, 2012; Sarwar, 2012). S. cerealella is very popular among other insects that attack rice grains because of its malevolence did to other important grains like sorghum and wheat. In most developing countries including Nigeria where the level of literacy among the farmers is still low and government intervention in term of adequate storage facilities provision is minimal, the production and market price of this weighty cereal has dropped steadily over the years (Ashamo and Khanna, 2007; Sarwar, 2012).

In order to fight back this infamy insect and other stored product insects and to have position in the contest for survival, the world entomologist have come up with different strategies among which synthetic chemical control measure remained the most popular up to this present moment. But in spite of the popularity and achievements of synthetic chemical insecticides, their uses have dropped steadily because of many cons that attached with them (Ogungbite et al., 2014). In fact, many of these notorious insects have developed resistance to many of them and this has increased their chances of winning the battle and cause food insecurity among men. Nevertheless, before the discovery and the commercial success of all the chemical insecticides, use of wood ashes and plant powders was the major missile in the farmer’s armory (Forim et al., 2012). Medicinal plants are believed to contain myriad of chemicals that could be insecticidal. Therefore, researches have been shifted toward the use of botanicals as insecticides moreso that every parts of the world are rich in different species of medicinal plants (Akinkurolere et al., 2006; Akinkurolere, 2007).

Newbouldia laevis is one of those medicinal plants whose insecticidal potential has not been fully investigated as it was done for other plants like neem, clove, Nicotiana spp and other popular plants whose insecticidal potentials have been widely investigated. The insecticidal potential of this plant has been tested against beetles (Ashamo et al., 2013; Ogungbite and Oyeniyi, 2014; Ogungbite et al., 2014) but its powders as well as its extracts has not been tested against any moth. To join other entomologists of the world in their quest to fight against insects and hunger especially in the developing countries, this work investigated the insecticidal potency of N. laevis extracts against S. cerealella, an important pest of paddy rice, Oryza sativa.

1 Results

1.1 Effects of oil extracts on mortality of S. cerealella on paddy rice and the lethal concentration required to achieve 50% insect mortality

The effect of leaf, stem and root bark extracts of N. laevis on mortality of adult S. cerealella at different concentrations and periods are presented in Table 1. The percentage mortality in each treatment varied with the period of exposure, plant part extract used, and the concentration of the extract. Significant differences (p<0.05) existed among all the plant extracts. The extracts achieved high percentage mortality of the adult moth at higher concentrations (4 and 5%). The leaf, stem bark and root bark extracts were not significantly different (p>0.05) from each other. They all achieved 100% mortality within 96 hours at 4 and 5% oil concentration. At 4% concentration, root bark extract achieved 100% mortality within 72 hours of application. The percentage mortality from these extracts was significantly different from both the controls. This result indicates that root extract of N. laevis exhibited the greatest insecticidal activity as it required the lowest lethal concentration (1.59%) to achieve 50% mortality of the insect.

1.2 Effect of oil treatment on adult emergence of S. cerealella

There was no adult emergence from treated samples at 4% and 5% oil extract concentrations. Nevertheless there was adult emergence in the 1% (26.7%-33.3%), 2% (23.3%- 30%), 3% (16.7%- 20%) of all plant parts respectively. There was higher emergence in both treated control (76.67%) and untreated control (96.6%). The effect 4% concentration of leaf and root extracts was significantly (p<0.05) different from other concentrations and the controls (Table 2).

1.3 Effect of oil extract on ability of S. Cerealella to cause weight loss of paddy

Table 3 showed the effect of N. laevis extract on S. cerealella in causing weight loss. The extracts reduced the weight loss of the grains regardless of the concentration used. The effect of the extracts was significantly (p<0.05) different from the control which had 17.39% weight loss. However, there was 0% seed weight loss at 4% and 5% concentrations of leaf, stem bark and root bark extract of N. laevis.

1.4 Effect of oil treatment on water absorption capacity of seeds

The water absorption capacity of treated seeds varied with the type of extracts used, extract concentration and the period of submergence (Table 4). The result of the water absorption test was generally positive at all concentrations for all the extracts although at different rates. The water absorption capacity was not affected by the treatments sine the results obtained from the control seeds were similar to those of the treated seeds. However, there were some differences in the results obtained for the different extracts but not significant (p>0.05). High percentage of water absorption was recorded in the controls for all concentrations. Water absorption capacity after 1hr of submergence ranged from 25.71 to 28.71, and for 4 hours it was from 27.53 to 33.72 and for 24hours it ranged from 30.16 to 38.20 in the leaf extract. Also in the stem extract after 1hr of submergence ranged from 25.06 to 28.72, and for 4 hours it ranged from 27.27 to 30.38 and for 24hours it ranged from 29.53 to 33.37. In the root extract after 1 h submergence ranged from 25.06 to 28.72 and for 4 hours it ranged from 27.27 to 30.38 and for 24hours it ranged from 29.53 to 33.37.

1.5 Effect of oil treatment on seed viability

Table 5 presents the effect of the leaf, stem bark, and root bark extracts of N. laevis on viability of the seeds. The percentage viability of the extracts was not significantly different (p>0.05) from the control except at 2% leaf concentration. However, none of the extracts achieved 100% viability like that of the control. Viability of seed treated with leaf extract ranged from 75 to 95%, stem extract ranged from 76 to 86% and root extract ranged from 80 to 90%.

2 Discussion

The use of botanical origin insecticides is gaining more recognition among the populace of the world more so that governments of many of the developed countries are banning the use of chemical insecticides. Unlike 1990s when the total market growth of botanical insecticides is less than 2% (Isman, 2000) the use of botanical and natural enemies have gained more sales and reputation in the recent years probably because of the public awareness of the danger allied with chemical insecticides (Zibaee, 2011; Ileke and Ogungbite, 2014; Ogungbite et al., 2014). In order to increase the sales and market awareness of botanical insecticides, there is need to search for other plant that could be promising in insect control with low or no mammalian toxicity.

The evaluation of N. laevis leaf, stem bark and root bark powders and extracts in this work showed that the three plant parts have momentous effect on the survival of S. cerealella. Regardless of the N. laevis oil extracts used, the plant achieved above 60% moth mortality at 4 and 5% concentration. This high effectiveness of the extracts in term of mortality could be related to ability of the extracts to encumber normal respiratory activities of the insect. Also, the high mortality recorded by these extracts could be due to inability of the insects to feed on the oil treated paddy rice or they may have blocked the breathing channels (spiracle) of the insects. The result obtained in this research compared with the findings of Ashamo et al. (2013), Ogungbite and Oyeniyi (2014) as well as Ogungbite et al. (2014) in which N. laevis was able to cause high mortality of Callosobruchus maculates, Sitophilus oryza and S. zeamais, there could be a possibility that these plant parts extracts of N. laevis may have inhibitory effect on the detoxifying enzyme of S. cerealella moreso that botanical extracts such as neem and Nicontiana tabacum had been noticed to have significant effect on wide range of insect’s detoxifying enzymes (Martins et al., 2012; Begum et al., 2013). The result obtained in this work acquiesced with the research of Adeyemo et al. (2013) as well as Ileke (2013) in which plant materials were found to cause high mortality of S. cerealella.

All the three plant parts of N. laevis extract significantly reduced the adult emergence of adult moth and their effects was proportional to the concentration used. Nonetheless, the leaf and root extract of this botanical at higher concentrations showed more insecticidal potential than the stem bark extract of the same plant. The ability of these plant parts extracts to reduce or prevent the emergence of the adult moth could be due to inability of the insect eggs to develop because botanical oils have been noticed to block the chorion which is the breathing channel of eggs (Ashamo, 2007; Adedire et al., 2011). Also, the early stage death of the insect larvae which was unable to fully cast off their old exoskeleton which typically remained linked to the posterior part of the abdomen (Oigiangbe et al., 2010). Alkaloids, tannins, flavonoids, saponins, cyanogenetic glycosides, cardiac glycosides and phenylpropanoids are the phytochemicals present in N. laevis extracts (Germann et al., 2006; Akerele et al., 2011). These chemicals are capable of disrupting growth and reduced larva survival as well as disruption of life cycle of insects (Yang et al., 2006). Therefore, the presence of these chemicals in N. laevis could be the reason for the low or no adult emergence of S. cerealella.

The ability of the insect to cause seed weight loss was greatly reduced or prevented by the plant extracts and this could be associated with low or no adult emergence of the insect. This result is in accordance with the facts of Ashamo et al. (2013) in which the extracts of N. laevis was able to prevent seed weight loss of cowpea, Vigna unguiculata. The result obtained in this work also showed that the extracts of this plant did not affect the water absorption capacity and viability of the treated seeds. There were no significant differences in the water absorption capacity of the control samples compared with oil extracts-treated samples. All the treated seeds and controls had viability of between 75-100% indicating that the oils did not hamper the germinability of the protected paddy rice. This research agreed with the work of Ashamo et al. (2011) in which plant powders and extracts does not affect seed viability. Base on the findings of this work, N. laevis could be a promising instrument for use in the combat between man and insects and could be incorporated into pest management techniques since its insecticidal fortitude against beetle and moth had been proven. Moreso, the extracts of this botanical have no effect on water absorption capacity and viability of the protected paddy. In conclusion, to achieve high protection of paddy rice from S. cerealella, root extract of N. laevis could be more promising since it appeared to be most effective among the plant part used. Nevertheless, the actual mode of action of this promising botanical as well as it effects on detoxifying enzymes of S. cerealella needs further investigation. Also, more research of the potency of N. laevis extracts is needed to be tested against wide range of insect pests as this could go a long way in protecting the food security from the hands of most of these perilous insects pest threatening human existence.

3 Materials and Methods

3.1 Insect culture

Adults S. cerealella used for the experiment were obtained from existing culture in the Food Storage Laboratory of the Department of Biology, Federal University of Technology, Akure, Nigeria. The moths were reared on clean disinfested local paddy obtained from Agricultural Development Project (ADP), Akure, Ondo State. The moths were reared in muslin cloth covered plastic jars which were kept inside insect breeding cage. The moth cultures were maintained by continuously replacing the devoured infested paddy with fresh clean disinfested grains. Both adult males and females were introduced into the culture jar with the aid of an aspirator. The culture and experiments were kept at temperature of 28±2°C and relative humidity of 75±5%.

3.2 Preparation of the plant extracts

The leaf, stem bark and root bark of N. laevis used were collected fresh and air dried under shade before they were separately grounded into fine powder using electric blender. The powders were further sieved to pass through 1mm² perforations before they were stored in separate plastic containers with tight lids for subsequent use. The extracts of these plant parts were prepared using soxhlet apparatus and methanol as solvent. The resulted extract of each plant part contained both the oil and the solvent and were separated from each other using rotary evaporation. From the oil of each plant parts, 1, 2, 3, 4 and 5% concentrations were made.

3.3 Effect of powders and extract of N. laevis on mortality of adult S. cerealella

Twenty grammes of the paddy grains were weighed into 250 ml plastic containers and oil extracts of these plant parts were separately mixed with the grains at 1, 2, 3, 4 and 5% concentrations while two controls were also set up (untreated and 2 ml methanol treated controls). Each treatment was replicated five times and the whole experiment was set up in a complete randomized design. Ten pairs of 0-24 h old adult of S. cerealella were introduced into treated samples. The setup was left inside breeding cage in the laboratory. Moth mortality was assessed at 24, 48, 72, and 96 hours after treatment in both the powders and the extracts. After twenty days the set up was observed for adult emergence. The weight loss caused by the S. cerealella was recorded after which no insect emerged for five consecutive days and the formula below used to calculate the percentage weight loss.

3.4 Effect of extracts on adult emergence of eggs of S. cerealella

Twenty grammes of plant part oil extract-treated paddy rice of different concentration were separately put in small plastic containers and control treatments were set up as in the case of mortality experiment. The set up for oil extracts was left for about 30 minutes so that the solvent can vaporize leaving only the oil extract on the seeds. Ten 0-24 h old eggs were introduced into the treated grains. An air space was created on the plastic cover by cutting a portion of the cover and replacing it with muslin cloth. Replication was made five times for each concentration of the leaf, stem and root powder and extracts. Adult emergence was observed after 20 days upward in each plastic container and recorded. The record was taken every 24 hour.

3.5 Effect of oil extracts on water absorption capacity of paddy grains

Twenty grammes of paddy rice were weighed into each plastic container. 2 ml of each oil concentration was added and mixed thoroughly. Two control treatments were set up as it was done for mortality experiment. The controls and oil-treated seeds were later submerged in water for 1 h then removed dried and reweighed and then submerged again for 4 h, then the seeds were removed dried and reweighed and finally submerged for 24 hours. The seeds were dried with multiple layers of filter paper on each occasion and reweighed after each interval.

3.6 Effect of oil extracts on viability of paddy grains

Twenty grammes of rice grains were put into each Petri dish and 2ml of each oil concentration was added. The grains in the dishes were mixed thoroughly with a glass rod for uniformity. The grains were left in the laboratory for 40 days. Ten wholesome grains were randomly picked from each Petri dish and planted in moist sawdust in three replicates. The sawdust served as a medium of growth and the set up was placed in the laboratory where they were exposed to sunlight for 5-7 days to allow for germination. A control experiment without extract was also planted. Another similar setup of grains was treated with methanol which serves as the second control experiment. Percentage viability of treated and untreated grains was recorded.

3.7 Statistical analysis

All the data obtained were subjected to one-way analysis of variance at 5% significant level and means were separated with New Duncan’s Multiple Range Tests using SPSS version 17. Also data, obtained from weevil’s mortality, were subjected to regression analysis to calculate the LC₅₀ of the extracts after 96 h of application using probit analysis (Finney, 1971).

Authors’ contributions

The three authors designed the research and carried out the research while author 2 analyzed the data obtained. The three authors wrote and proofread the manuscript before final submission.

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