Background

Farmers store their products for several reasons which include (i) making food available at all times to man (ii) making food / diet available to livestock (iii) making raw materials available to industries. Storage of Agricultural products has been hampered drastically as a result of infestation by insect pests (Udo, 2011). As in field crops, a wide range of insect pests attacks stored products and the commonest being beetles and moths (Udo, 2011). The primary insect attacking cowpea seed during storage is Callosobruchus maculatus. The larvae bore into the seed which becomes unsuitable for human consumption and loose viability (Taylor, 1981). C. maculatus is known to cause up to 100 % loss of stored cowpea and estimates have shown that over 30 million U.S. dollar is lost as a result of cowpea damage in Nigeria (Udo and Epidi, 2009). Infestation usually starts in the field just before harvest and the insects’ developmental stages are carried into the stored where the population builds up rapidly (Ilelke et al., 2012).

Farmers still depend on the use of synthetic insecticides in the management of stored product especially under large scale production (Gbaye and Holloway, 2011). The continued and intensive usage of these insecticides has produced some undesirable toxic effects on non-target biotic components of the ecosystem (Ojo and Ogunleye, 2013). Other potential difficulties associated with the continuous use of these chemical insecticides include; high cost of procurement, pest resurgence and resistance, poisonous residue accumulation in foods and poor knowledge of application (Ileke and Oni, 2011). To solve these problems, entomologists all over the world are in search for botanical insecticides which are safe and environmentally friendly for the control of these insects’ pests (Isman, 2006). Currently, research efforts are focused on the use of botanical products as alternatives to synthetic insecticides. These plant products which include ashes, plant powders, extracts and oils have been confirmed to be cheaper, safe, and eco-friendly (Adedire et al., 2011; Ojo and Ogunleye, 2013). There is no doubt  that botanical insecticides are an interesting alternative to insect pest control, and on the other hand only a few of the more than 250,000 plant species on our planet have been properly evaluated for this purpose (Khalequzzaman and Osman Goni, 2009). Use of plant products for infestation control in stored grains therefore seems to offer desirable solutions, especially in developing tropical countries where plants are found in abundance everywhere throughout the year (Khalequzzaman and Osman Goni, 2009).

Securidaca longepedunculata (Fres.) is a semi deciduous shrub used as a traditional medicine in many parts of Africa against a number of invertebrate pests, including insects infesting stored grains (Burkill, 1997). In Africa (Ghana; Nigeria) the plant decoction is prescribed by Ghananian healers to treat asthma and other diseases associated with smooth muscle contraction (Rakuambo et al., 2006). In Nigeria, extracts from various parts of the plants has been reported to possess both gastrointestinal and trypanocidal effects (Olajide et al., 1999).

1 Materials and Methods

1.1 Preparations of Callosobruchus maculatus cultures

The culture of Callosobruchus maculatus was obtained from naturally infested cowpea seeds, Vigna unquiculata Walp bought from Oja-Oba Market in Ikole-Ekiti, Ekiti- State, Nigeria. The adult C. maculatus were cultured in the laboratory on clean uninfested Ife brown variety collected from Agricultural Development Project (ADP), Ikole Ekiti, Ekiti State, Nigeria. 750 g of the cowpea seeds was measured into a Kilner jar, thereafter, twenty (20) pairs of adult C. maculatus (10 male :10 female) were introduced into the Kilner jar covered with muslin cloth to let in air but prevent the escape of beetles (Jambere et al., 1995). The insects were allowed to lay eggs seven (7) days. After that all the insects both alive and dead were then removed to allow new generations to emerge and after 28 days of adult removal, the progeny that emerged were used for re-culturing. Subsequently, insects that emerged were used for the different experiments. Insect rearing and the experiment were carried out at the laboratory, Department of Crop Science and Horticulture, Federal University Oye Ekiti (FUOYE), Ekiti State, Nigeria at ambient temperature of 28±2⁰C and 75±5 % relative humidity.

1.2 Collection of plant materials and cowpea seeds (Vigna unquiculata)

The plants evaluated for insecticidal activity were Securidaca longepedunculata (leaf, stem-bark and root-bark)).These plant parts were collected from a farmland along Itapaji Ekiti, Ekiti State and taken into the laboratory. The root bark and stem bark were washed thoroughly with water and cut into small pieces. The plant parts are then air-dried in the laboratory for 30 days. Each plant part was pulverized separately into fine powders using Professional Blender (JTC OmniBlendV, Model TM-800). The powders were further sieved to pass through a 40 mesh sieve and kept in the refrigerator to retain its quality before use. The Ife brown variety of cowpea seeds used for the experiment was bought from the Agricultural Development Project (ADP), Ikole Ekiti, Ekiti State, Nigeria. The seeds were cleaned of foreign materials and disinfested by keeping in deep freezer at -5℃ for 168 h to kill all hidden infestations. This is done because all the life stages, especially the eggs are very susceptible to cold (Koehler, 2003). The disinfested cowpea seeds were then air dried in the laboratory to prevent mouldiness (Adedire et al., 2011).

1.3 Contact toxicity assay

To assess the toxic effects of the plant powders on C. maculatus, twenty grammes (20 g) of clean and disinfested cowpea seeds of susceptible Ife brown were weighed using Electronic balance (Model XY3000C) into 160 ml plastic cups. After this, 0.2 g, 0.4 g, 0.6 g, 0.8 g and 1.0 g of the pulverized plant powders was weighed and mixed with the cowpea seeds. Plastic cups were shaken thoroughly to ensure adequate mixing of cowpea seeds and plant powders. 20 newly emerged adults of C. maculatus (0-2 day old) were introduced into each of the treatments. Each treatment was replicated four times. The control experiment had only the cowpea seeds and beetles i.e no plant powder was involved in the control experiment. The mortality of the beetles was monitored daily for 4 days. Dead beetles were those who neither moved nor respond to pin probing. Percentage adult mortality was corrected using Abott’s formula (Lale, 2006) thus:

Where P_T = corrected mortality (%); P_O= observed mortality (%); P_C= control mortality (%)

1.4 Effect of plant powders on oviposition and adult emergence of C. maculatus

Twenty grammes (20 g) of clean and uninfested cowpea seeds of susceptible Ife brown variety was weighed into 160 ml plastic cups. Then 0.2 g, 0.4 g, 0.6 g, 0.8 g and 1.0 g of various plant powders were weighed and thoroughly mixed with the 20 g of disinfested seeds. The seeds in the control contained no plant powders. Five (5) pairs (5 male: 5 female) of adult C. maculatus (0-2 day old) sexed according to the methods described by Odeyemi and Daramola (2000) were introduced into the treated and untreated (control) cowpea seeds. Male C. maculatus have comparative shorter abdomen and the dorsal side of the terminal segment is sharply curved downward and inward. In contrast, the females have comparative longer abdomen and the dorsal side of the terminal segment is only slightly bent downward. The females also have two dark visible sports on their elytra (Odeyemi and Daramola, 2000). Four replicates of the treated and untreated control were laid out in Complete Randomized Block Design. The plastic cups were covered with muslin cloth to let in but prevent the escape of beetles (Jambere et al., 1995). The experiments were left for 7 days for the insects to oviposit. The total number of eggs laid by C. maculatus on cowpea seeds were counted and recorded after allowing the insects to lay eggs for 7 days. The Percentage adult emergence was calculated as described below:

1.5 Statistical analysis

Data obtained from all the parameters 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. In addition, data obtained from beetles’ mortality were subjected to regression analysis to calculate the LD₅₀ and LD₉₅ of the powders after 96 h of application using probit analysis.

2 Results

2.1 Effect of S. longepedunculata powders on mortality of C. maculatus

Beetle mortality of C. maculatus on cowpea seeds treated with the plant powders S. longepedunculata is presented in Table 1. Adult mortality in the treated cowpea was significantly (P<0.05) different from beetle mortality in the control. The percentage adult mortality remained 0% in the control experiment throughout the period of exposure. However, low insect mortality was observed in all the treatments after 72 h of application. The highest beetle mortality of 71.25% was recorded in cowpea seeds treated with the root bark at the highest dosage of 1.0 g within 96 h of application and it was significantly different from beetle mortality of 65.0% and 46.25% in cowpea seeds treated with the powders of S. longepedunculata stem bark and leaf at the same concentration and period of exposure.

2.2 Lethal dose (LD₅₀ and LD₉₅) of plant powders required to achieve 50% and 95% mortality in C. maculatus after 96 h post treatment

The lethal dose (LD₅₀) and (LD₉₅) of the plant powders is presented in Table 2. The result obtained indicated that to achieve 50% and 95% mortality in the beetle, the plant powders must be applied at higher dosage except the powder obtained from the root bark in which low dosage is required for 50% (2.56-4.01) and 95% (5.10-6.45). The powder obtained from leaf appeared to be the least effective as indicated by their amount of dosages required to cause 50% (3.53-7.01) and 95% (8.34-11.14) mortality.

2.3 Effect of S. longepedunculata powders on oviposition and adult emergence by C. maculatus

All the plant powders of S. logepedunculata tested in this study reduced the number of eggs laid by C. maculatus (Figure 1). Oviposition by C. maculatus was significantly lower (P<0.05) in powder-treated seeds than in untreated (control) seeds. The highest number of eggs (162.25) was laid on control (untreated) seeds. The least eggs (54.50, 58.75 and 70.75) were laid on cowpea seeds treated with root bark , stem bark and leaf at the highest dosage of 5.0% w/w and it was significantly (P<0.05) different from other dosages of plant powders.

The highest percentage adult emergence (88.59%) was obtained on the control (untreated) seeds (Figure 2). The least percentage adult emergence (27.52% and 29.78%) were obtained on cowpea seeds treated with the powders of root bark and stem bark of S. longepedunculata at the treatment level of 5.0% w/w and it was significantly different from other dosages of the plant powders.

3 Discussion

The use of plant powders in the control of stored products insects is an ancient practice (Ileke and Olotuah, 2012; Ojo and Ogunleye, 2013). In this study, the insecticidal activities of the leaf, stem bark and root bark of S. longepedunculata as contact insecticides against C. macualtus infesting cowpea seeds were evaluated. The results obtained from the experiment showed that the powder of S. longepedunculata particularly the root bark was very potent against C. maculatus causing 71.25% mortality at the rate of 1.0 g / 20 g within 96 h of exposure. Plant powders have been used to suppress the population of storage pests (Ogunleye et al., 2004; Ojo and Ogunleye, 2013). In this study, the observed high mortality recorded on cowpea seeds treated with the root bark may be due to the strong choky odours it produced which could asphyxiate insects by blocking the spiracles (Amusan and Okorie, 2002). Most insects breathe by means of trachea which usually open at the surface of the body through spiracles, the plant powders that were mixed with the seeds might have blocked these spiracles thereby leading to suffocation and death of the insect (Obembe and Kayode, 2013). Insecticidal property of any plant material would depend on the active constituents of the plant material (Asawalam et al., 2007). Efual et al. (2016) reported that the root powder of S. logepedunculata contains 2-hydroxy-benzoic acid methyl ester (methyl salicylate, 1) which is responsible for its biocide effect against the stored grain insects. The result from this investigation is similar to the observation of Efual et al. (2016) who reported the insecticidal, anti-ovipositant, ovicidal and repellent properties of S. longepedunculata against S. zeamais and C. maculatus.

The results obtained from this study showed that the three plant powders significantly reduced oviposition and adult emergence of C. maculatus on cowpea treated seeds. The effect of the powders on oviposition could be due to respiratory impairment which probably affects the process of metabolism and consequently other systems of the body of the insects (Adedire et al., 2011; Ileke et al., 2014). The beetles were unable to move freely within the treated cowpea seeds due to the presence of the powders and this might have affected mating activities and sexual communication (Ojo and Ogunleye, 2013). The resultant reduced adult emergence could be to the fact the C. macualtus lay eggs on the seed coat thus bringing the eggs and larvae in close contact with the plant powders as reported by Adedire and Lajide (2001). It is evident in this research work that root bark powder of S. longepedunculata is effective in controlling the population of C. maculatus. The powder could be mixed with stored cowpea seeds in order to protect them against C. maculatus. The root bark powder could therefore serve as alternatives to synthetic insecticides for use by resource-poor farmers who store small quantities of the seeds for their consumption, sales and planting since the powder is readily available.

Acknowledgements

The authors would like to thank the technologists at the Department of Crop Science and Horticulture, Federal University, Oye Ekiti, Ekiti State, Nigeria for their assistance during the preparation of plant powders.

References

Adedire C.O., Lajide L., 2001, Efficacy of powders of some tropical plants in the control of the pulse beetle, Callosobruchus maculatus (F.) (Coleoptera: Bruchidae), Applied Tropical Agriculture 6, 11-15

Adedire C.O., Obembe O.M., Akinkurolere R.O., and Oduleye O., 2011, Response of Callosobruchus maculatus (Coleoptera; Chysomelidae: Bruchidae) to extracts of cashew kernels, Journal of Plant Diseases and Protection, 118(2): 75-79

https://doi.org/10.1007/BF03356385

Adonu C.C., Ugwu O.P.C., Esimone C.O., Ossai E.C., Bawa A., Nwaka A.C., and Okorie C.U., 2013, Phytochemical Analysis of the Menthanol, Hot water and N-Hexane Extracts of the Aerial parts of Cassytha filiformis (Linn) and leaves of Cleistopholis patens (Benth), Research Journal of Pharmaceutical, Biological and Chemical Sciences, 4(2), 1143-1149

Amusan A.S.S., and Okorie T.G., 2002, The use of Piper guineense fruit oil (PFO) as protectant of dried fish against Dermestes maculatus (Degeer) infestation, Global Journal of Pure and Applied Science 8, 197-201

https://doi.org/10.4314/gjpas.v8i2.16032

Asalawam E.F., Emosairue Ekelem F., and Wokocha R.C., 2007, Insecticidal effects of powdered parts of eight Nigerian plant species against maize weevil,Sitophilus zeamais Motschulsky, (Coleoptera:Curculionidae), Journal of Entomology and Agricultural Food Chemistry 6(11), 2526-2533

Burkill H.M., 1997, The useful plants of tropical West Africa, Vol 3 (3^rd Ed.) Royal Botanical Garden Kew, pp 857

Efual N.G., Akosua A.Q., and Abedi G.Y., 2016, Biological activity of S. longepedunculata against S. zeamais and C. maculatus, Advances in Agriculture and Agricultural Sciences 2(2): 033-041

Gbaye O.J., and Holloway G.J., 2011, Varietal effects of cowpea, Vigna unquiculata on tolerance to malathion in Callosobruchus maculatus. Journal of Stored Product Research, 47, 365-371

https://doi.org/10.1016/j.jspr.2011.06.003

Ileke K.D., and Oni M.O., 2011, Toxicity of some plant powders to maize weevil, Sitophilus zeamais (Motschulsky){ Coleoptera : Curculionidae} on the stored grains (Triticum aestivum), African Journal of Agricultural Research, 6(13), 3040-3042

Ileke K.D., and Olotuah O.F., 2012, Bioactivity Anarcadium occidentale (L.) and Allium sativum (L.) powders and oil Extracts against cowpea Bruchid, Callosobruchus maculatus (Fab.) (Coleoptera: Chrysomelidae), International Journal of Biology, 4(1):96-103

Ileke K.D., Odeyemi O.O., and Ashamo M.O., 2012, Insecticidal activity of Alstonia boonei De Wild powderagainst cowpea bruchid, Callosobruchus maculatus (Fab.) {Coleoptera:Chrysomelidae} in stored cowpea seeds, International Journal of Biology, 4(2): 125-131

https://doi.org/10.5539/ijb.v4n2p125

Ileke K.D., Ogungbite O.C., and Olayinka-Olagunju J.O., 2014, Powders and Extracts of Syzgium aromaticum and Anarcadium occidentale as Entomocides against the Infestation of Sitophilus oryzae (L.){Coleoptera: Curculionidae} on Stored Sorghum Grains. African Crop Science Journal, 22(4):267-273

Isman M.B., 2006, Botanical Insecticides, deterrents and repellents in Modern Agriculture and an increasingly regulated World, Annual Review of Entomology, 51, 45-66

https://doi.org/10.1146/annurev.ento.51.110104.151146

Jambere B., Obeng- Ofon D., and Hassanah A., 1995, Products derived from the leaves of Occimum kilomansharicum as infection of three major stored product pests, Bullettin of Entomological Research, 85: 351-367

Khalequzzaman M., and Osman Goni S.H.M., 2009.  Toxic potentials of some plant powders on survival and development of Callosobruchus maculatus (F.) and Callosobruchus chinensis L. Journal of Life Earth Science, Volume 3-4: 1-6

Koehler P.G., 2003. Biopesticides Data sheet volume 2, Entomology and nematology Department, Cooperative extension service, Institute of food and Agricultural Science, University of Florida, Gainesuilla, 326pp

Lale N.E.S., 2006, Dictionary of Entomology and Acarology (2^nd ed.), Mole Publication, Port Harcourt, Nigeria, Pp.1

Obembe O.M., and Kayode J., 2013, Insecticidal Activity of the aqueous Extracts of Four Under-utilized Tropical Plants as Protectant of cowpea from Callosobruchus maculatus Infestation, Parkistan Journal of Biological Sciences, 16 (4):175-179

https://doi.org/10.3923/pjbs.2013.175.179

Odeyemi O.O., and Daramola A.M., 2000, Storage practices in the tropics, Food Storage and Pest Problem (1^st edition), Nigeria, Dave Collins Publication, pp. 64-65

Ogunleye R.F, Adedire C.O., and Adesuyi S.A., 2004, Toxicity of some underutilized plants to the storage pests of maize, Sitophilus zeamais (Mots.) (Coleoptera; Curculionidae).Journal of Biological and Physical Sci 2:22-27

Ojo D.O., and Ogunleye R.F., 2013, Comparative Effectiveness of the Powders of Some Underutilized Botanicals for the control of Callosobruchus maculatus (Coleoptera: Bruchidae), Journal of Plant Diseases and Protection, 120 (5/6): 227-232

https://doi.org/10.1007/BF03356479

Olajid O.A., Ajayi F.F., Ekhelar A.I., Awe S.O., Makinde J.M., and Alada A.R.A., 1999, Gastrointestinal tract effect of Securidaca logepedunculata root extract, Pharm. Biol, 37: 134-137

https://doi.org/10.1076/phbi.37.2.134.6078

Rakuambo N.C., Meyer J.J.J., Hussein A., Huyser C., Mdlalose S.P., and Raidani T.G., 2006, In Vitro effect of medicinal plants used to treat erectile dysfunction on smooth muscle relaxation and human sperm, Journal of Ethnopharcology, 105, 84-88

https://doi.org/10.1016/j.jep.2005.10.008

Taylor T.A., 1981, Distribution, ecology and Importance of bruchids attacking grain legume and pulses in Africa, In: The Ecology of Bruchids attacking legumes (pulses), Series Entomologia 19, Laberyic V. (ed.) Springer Netherlands, 199-203

https://doi.org/10.1007/978-94-017-3286-4_20

Udo I.O., 2011, Potentials of Zanthoxylum xanthoxyloides (Lam.) for the control of stored product insect pests, Journal of Stored Products, Postharvest Research, 2(3): 40-44

Udo I.O., and Epidi T.T., 2009, Biological effect of ethanolic extract fractions of Ricinodendron heudelotii (Baill) Pierre ex Pax against Sitophilus zeamais and Callosobruchus maculatus Fabricius on stored grains, African Journal of Agricultural Research, 4(10): 1080-1085