Introduction 

Maize (Zea mays) is an important agricultural produce in the group of cereals. It occupied the first position among the cereal grains produced in the world (Ashamo and Odeyemi, 2001) and it serves as one of the major source of energy for both man and animals because of its high content of digestible carbohydrate (Ling and Edmeades, 1997; Ofuya, 2003; Ashamo, 2007). Despite the importance of this valuable grain among Nigerians, its production and storage has been threatened by many insect pests including Rhizopertha dominica, Sitophilus zeamais, S. oryzae, S. granarious and many others (Ashamo, 2007). Because of field to store notorious activities of S. zeamais, it has remained the major pest of maize and its infestation do not only brought about diminution in both quantity and quality of this crop but also led to the declination of market price of this imperative cereal (Oni et al., 2011; Akinneye and Ogungbite, 2013).

Since early 1940s, synthetic chemical insecticides have been overwhelmingly relied upon as means of controlling many of these infamy stored product insect pests in order to safe guard farm produces including maize from attacks and to improve food security. Despite the success of synthetic chemical insecticides in the control of store product insect pests, the problems allied with their use have called for urgent alternative of insect control (Oni and Ileke, 2008; Ileke and Oni, 2011; Akinneye and Ogungbite, 2013; Ashamo et al., 2013; Ogungbite and Oyeniyi, 2014). In order to obviate the use of these perilous synthetic chemical insecticides, the world entomologist have focused on plants derivatives  as a new boulevard of insects control as many of them are medicinal, readily biodegradable and less toxic to mammals. Plants oils contain myriad of secondary metabolites that are believed to have toxic effects against insect pests (Zibaee, 2011).

Hitherto, even though botanical oils have proven their efficacy against wide range of insect pests, their insecticidal activities are yet to be comparable to many synthetic chemical insecticides and have commanded less than 1% of the insecticides market of the globe which could be due to adverse effect associated with them, for example change in taste (Begum et al., 2013). For that reason, there is still need to search for other plants species which could effectively vie with chemical insecticides and be more acceptable to local farmers and consumers. Moreso, tropical zones of the world including Nigeria are well endowed with medicinal botanicals which could have insecticidal properties. Capsicum spp. is one of these plants with high medicinal value and high daily consumption but despite its popularity throughout the world, the insecticidal prospective of its oils have not been investigated (Oni, 2011). Therefore, bearing in mind the importance of crop protection as well as safeness of the materials used for their protection, this present work investigated the effectiveness of cayenne pepper, sweet pepper and long cayenne pepper as protectant of maize grain against S. zeamais infestation. Moreso, dust of dried peppers have been used by West African farmers for decades even before the commercial success of synthetic chemical insecticides (Akinkurolere, 2007).

1Results

1.1 Toxicity of extracts of Capsicum spp. cultivars on mortality S. zeamais after initial, three and six months of storage

The percentage mortality of S. zeamais expose to different dosages of Capsicum spp. after initial, three and six months of storage was presented in Table 1. Percentage mortality of S. zeamais was recorded on maize grains treated with oil extracts of different cultivars of Capsicum spp. at 96 and 110h post treatment after initial (immediately after treatments), three and six months of storage. Mortality of S. zeamais varied with cultivar of Capsicum spp. used, dosages of oil extracts and period of exposure. Insect mortality increased with increase in extract dosages and period of exposure regardless the cultivar of Capsicum spp. used. All the dosages of cayenne and long-cayenne pepper were able to achieve complete beetle mortality within 96h of exposure after initial and 3months of storage except 0.5ml dosage of long-cayenne pepper which only achieved 91.25% mortality but its effect was not significantly (p>0.05) different from other dosages of the two cultivars. After six months of storage, only the extract cayenne pepper at 10.0ml dosage was able to achieve above 50% mortality of S. zeamais after 110h of exposure and its effect was significantly (p<0.05) different from the extracts of other plants which achieved low or no mortality after this storage period. Regardless the dosages of the extracts, the oil extract of sweet pepper achieved the lowest beetle mortality throughout the period of exposure except after six months of storage where long-cayenne pepper could not also achieve any insect mortality except at 10.0ml dosage where it achieved 18.75% mortality. However, the effectiveness of all the cultivars of capsicum spp. decreases as the period of storage after treatment increases.

1.2 Effect of oil extract of cultivars of Capsicum spp. on adult emergence of S. zeamais on treated maize grain after initial, three and six months storage

Table 2 presented the effect of oils of Capsicum spp. on adult emergence of S. zeamais on treated maize grain after initial, three and six months. All the oil extracts of capsicum significantly reduced the emergence of the adult beetle when compared to the control. At the initial storage of stored grains, only the oil extract of cayenne pepper at 5.0ml was able to prevent the adult emergence and its effect was significantly (p<0.05) different from all other Capsicum spp. regardless of their dosages. However, none of the pepper cultivar was able to prevent the emergence of the adult beetle in spite of the dosage of their oils after three and six months of storage of treated grains. It was also noted that oil from sweet pepper at 0.5ml dosage was less effective than the two remaining cultivars as it recorded highest number of adult emergence of 32.00, 36.00 and 71.50 at initial storage, after three and six months of storage respectively. The longer the period of storage the higher the number of adults that emerges in each treated maize grains.

1.3 % damage and weight loss of maize grains treated with oils of Capsicum spp. after storage

Table 3 and Table 4 showed the % damage and % weight loss of Capsicum spp. oil extracts treated maize grains after storage respectively. All the oil extracts at all levels of dosage significantly reduced the % grain damage and weight loss when compared to the control. Moreover, it was noted that only the oil extract of cayenne pepper at 5.0ml and dosage was able to prevent the damage and weight loss of treated maize grains at the initial storage and was significantly (p<0.05) different from other oil extracts at this level. None of the oil extract was able to prevent the damage and weight loss of stored grains after three and six months of storage. After six months of storage, the % grain damage and weight loss observed in maize grains treated with 0.5 and 1.0ml dosages of sweet pepper oil extract was as high as those observed in the control treatment.

1.4 Effect of oil extracts of Capsicum spp. on viability of treated grains

The percentage viability of treated maize grains was shown in Table 5. All the treated grains achieved above 90.00% viability at all levels of dosages except sweet pepper oil extract at 0.5 and 1.0% which recorded 80.33 and 84.00% germination. However, the % viability of the treated grains were not significantly (p>0.05) different from each other and from the control.

2 Discussion

Botanical base insecticides have not commanded more than 1% of the total global insecticides market despite the relative safety of these botanicals (Isman, 2000). This could be due to the fact that many of the botanicals involve in plant protection have bitter taste and can therefore change the taste of the commodity they are protecting. Therefore, there is need to search for plant species that could be more acceptable to consumers. Capsicum spp. are popular spice eating by most population of the world on daily basis (Oni, 2011); thus plant base insecticides could gain more popularity in the worldwide insecticides market if such plant species is tested against insect pests of farm produces.

The results obtained in this work showed that Capsicum spp. cultivars have high effect on the mortality and adult emergence of S. zeamais. The oil extract of cayenne pepper showed the greatest mortality effect among the three cultivars of Capsicum spp. used. This could be due to the strong pungent odour of this cultivar and could contain more active compounds than other cultivars. The ability of these oils to achieve high beetle mortality could be resulted from inability of the insects to breath due to the blocking of their spiracle by the oils, thus leading to asphyxiation and subsequent death of the adult insects (Ileke and Oni, 2011). The insect mortality may also be due to inability of the insects to feed on the maize grains that have been coated with the plant oils thereby leading to their starvation. These oils may also contain some active compounds that blend together to have venomous effect on the insect survival. The result of this work acquiesced with the work Akinneye and Ogungbite (2013) in which extract of Zanthoxylum zanthoxyloides found to cause high mortality of S. zeamais that other plant materials used. However, the ability of the extracts greatly reduced after three and six months of storage of treated grains. Similar result was obtained by Ashamo et al. (2013) in which the oils of Newbouldia laevis were found to reduce in their effectiveness against C. maculatus infestation on cowpea.

The result obtained on the adult emergence showed that all the cultivars of Capsicum spp. used showed greater insecticidal effect against adult emergence of this beetle. Nevertheless, the extract of cayenne pepper was more effective than other extracts as it prevented the emergence of adult S. zeamais at the initial storage and has doggedness effect as it has the lowest number of adult that emerged after three and six months of storage. The ability of these extract to prevent or reduce the emergence of adult S. zeamais could be due to the death of the insect larval which may occur due to inability of the larval to fully cast off their exoskeleton which remained linked to the posterior part of their abdomen (Oigiangbe et al., 2010). The prevention or reduction of number of adult S. zeamais in the initial storage (that is, introduction of insect immediately after treatment) could also be due to the high insect mortality which may reduce mating ability of the insects as well as rate of oviposition. The results of this study indicated that the extracts of these peppers may had obvious effect on post embryonic survival of this insect which resulted in prevention of adult emergence at different concentrations. The cultivars of Capsicum spp. used in this research have been reported to rich in flavonoid, alkaloid and saponin (Lee et al., 1995; Renault et al., 2003; Hallmann and Rembiałkowska, 2008).These compounds could be responsible for the inability of the adult insect to emerge as opined by Murdue-huntz and Nibet (2000) as well as Yang et al. (2006) that many of these compounds are found to disrupt growth and reduced larva survival as well as disruption of life cycle of insects. The result of this research agreed with the work of Oni et al. (2011) as well as Ashamo et al. (2013) which plant extracts were found to reduce or prevent adult emergence of C. maculatus. However, it was noticed that the number of adult S. zeamais that emerged from treated maize grains increased as the month storage increases. This could be associated with the fact that botanical extracts and powders are short live and lost their potency over time as suggested by Oruonye and Okrikata (2010). The result of this present work was in accordance with the research of Ashamo et al. (2013).

The percentage damage and weight loss of grains was increasing gradually as the months of storage increases. However, the oil extract of cayenne pepper showed more protection than other two oils as it was able to prevent damage and weight loss of the stored maize grains. The ability of these oils to prevent or reduce damage and weight loss of treated grains could be associated with the effect of the oils on the adult emergence. The results obtained on the percentage weight loss and damage relates to the findings of Oni et al. (2011), Ileke and Olotuah (2012) as well as Ashamo et al. (2013). The extracts of all the cultivars of Capsicum spp used in the research have no adverse effect on the viability of the treated maize grains as they all achieved above 80% seed germination. Therefore, the n-hexane oil extract of cayenne pepper and long cayenne pepper could be integrated into pest management strategy since they showed high efficacy on adult mortality, adult emergence as well as preventing seed weight and damage than sweet pepper oil extract. However, these oils could only be used for only short term storage as they loss their potency over time and dosage of the oils can be increase if they are to be used for long term storage since they have no adverse effect on the seed viability. Moreso, they are medicinal plant and generally eaten by consumers on daily basis.

3 Materials and methods

3.1 Insect culture

Parent stock of S. zeamais was obtained from naturally infested white maize grains from Oba market, Akure, Nigeria. The insect was reared on clean uninfested T-swam variety of maize grain collected from Agricultural Development Project (ADP) Akure, Nigeria and was placed inside 2L jars covered with muslin cloth. The jars were placed inside breeding cage and the culture was maintained by replacing devoured maize grains with new uninfested ones. The research was maintained at 28±2^oC and 75±5% relative humidity.

3.2 Preparation of extracts 

Fresh cayenne pepper, sweet pepper and long-cayenne pepper fruits were collected from a research farm in Akure, Nigeria. The fruits were separately washed, sliced into small bits and air dried in the laboratory. Fifty grammes of air dried pepper fruits were pulverized into fine powder using an electric Binatone blender (Model BLG-400) and the oils from these three cultivars of Capsicum spp. were extracted with soxhlet apparatus between 40-60^oC using n-hexane as solvent. After extraction, the solvent was evaporated under vacuum in a rotary evaporator in order to separate the solvent from the oil. From the main stock, dosage of 0.5, 1.0, 2.0, 5.0 and 10.0ml were made. The extracted oils were stored in Mc Cartney bottles which were kept in a refrigerator at 5^oC before use.

3.3 Bioassay

Twenty grammes of maize grains were placed inside 250ml plastic containers and were separately thoroughly mixed with 0.5, 1.0, 2.0, 5.0 and 10.0ml dosages of cayenne pepper, sweet pepper and long cayenne pepper. Twenty 0-24hour old adult S. zeamais were introduced into the containers with treated grains. Maize grains that were not treated with oil extracts were set as control. The experiment was setup in a complete randomized design with each treatment replicated four times. After exposure of insects to treated grains, mortality was observed after 96 and 110hrs and the setup was left 33 days to allow emergence of F1 generation as well as calculating the percentage seed damage and weight loss. This procedure was repeated after three and six months of which the maize grains have been treated with oils of different peppers. The number of adult that emerged was counted and the percentage seed damage and weight loss were calculated using the formulae below:

3.4 Viability test

 Viability of treated grains was conducted by randomly picking 40 grains after 90days post treatment. Treated and untreated seeds were separately planted on a moistened filter paper in labeled Petri-dishes moistened with distilled (deionized water). Germination was observed and recorded after seven days the seeds have been planted and the percentage viability was calculated by using the formula below;

3.5 Statistical analysis

 All the data obtained were subject to analysis of variance (ANOVA) and where significant differences existed, means were separated with Turkey’s test using SPSS Version 20.

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