Effect of selected pesticidal plants leaf powder on Callosobrunchus maculatus in stored cowpea (Vigna unguiculata (L.) Walp

1 Godifrey et al. Int. J. Biosci. 2020 RESEARCH PAPER OPEN ACCESS Effect of selected pesticidal plants leaf powder on Callosobrunchus maculatus in stored cowpea ( Vigna unguiculata (L.) Walp Juliana Godifrey1*, Ernest Mbega1, Patrick A. Ndakidemi1 1Department of Sustainable Agriculture, Biodiversity and Ecosystem Management, School of Life Science and Bio-Engineering, Tanzania 1The Nelson Mandela African Institution of Science and Technology (NM-AIST), P. Box 447, Arusha, Tanzania Key words: Actellic dust; Dysphania amdrosioides; Hyptis suaveolens; Ocimum suave; Oviposition. http://dx.doi.org/10.12692/ijb/16.6.1-10 Article published on June 16, 2020 Abstract Among the major insect storage pests of cowpea are cowpea weevils (Callosobruchus maculatus) which is a big threat causing huge postharvest loss. This study used double bagging experiment to test the pesticidal activity of plants leaf powders of Ocimum suave, Hyptis suaveolens and Dysphania ambrosioides on Callosobrunchus maculatus in stored cowpea. The experiment was arranged in Randomized Block Design (RBD) with 3 replications. The plant powders were applied at the rates of 30, 60 and 90 g 1.5 kg-1 of cowpea seeds spread on the surface of the inner cotton bag of 2 kg filled with 1.5 kg of cowpea seeds. The storage time was three months consecutive. The effectiveness was compared with the synthetic pesticide Actelic® dust (2 g) as the positive control and untreated cowpea seeds as a negative control. Based on the results, all pesticidal leaf powder showed a different level of pesticidal activities at the tested rates. Higher dosages of plant leaf powders at 60 and 90 g for H. suaveolens and D. ambrosioides and 90 g for O. suave significantly (P < 0.001) affected insects’ mortality, survivorship and reduced seed damage. Comparatively, H. suaveolens and D. ambrosioides at 90 g were more effective in inhibiting egg deposition by C. maculatus just as successful as the positive control. Therefore, this study demonstrates that, O. suave, H. suaveolens and D. ambrosioides leaf powder can effectively offer protection in stored cowpea seeds against C. maculatus when applied at high dosages. * Corresponding Author: Juliana Godifrey  julianashik[email protected]m International Journal of Biosciences | IJB | ISSN: 2220-6655 (Print), 2222-5234 (Online) http://www.innspub.net Vol. 16, No. 6, p. 1-10, 2020 2 Godifrey et al. Int. J. Biosci. 2020 Introduction Cowpea, Vigna unguiculata (L.) Walp, is a widely grown legume that belongs to the family Leguminosae, subfamily Papilionaceae (Mkenda et al., 2015). It is an important grain legume grown in most parts of Eastern and Western Africa but also in Brazil and India (Ileke et al., 2012). In Tanzania it is cultivated in semiarid areas like Shinyanga, Mtwara and Kigoma due to its ability to tolerate moisture stress (Mkenda et al., 2015). Nutritionally, cowpea has high protein content of about 23 to 25 %, thus forming a cheap alternative source of dietary protein particularly for low income people who are unable to afford the cost of animal proteins foods (Brisibe et al., 2011; Ileke and Olotuah, 2012). This helps in minimizing the risk of malnutrition to young children. In addition, cowpea contain high iron content with low fat content where the young green leaves are consumed as vegetables, thus providing additional nutrients including vitamins to consumers (Adedire et al., 2011). This crop is usually grown as monocrop or intercropped with other cereal crops (Myaka et al., 2002). Being a leguminous plant, it fixes nitrogen and thus, adds up soil fertility where it is grown (Brisibe et al., 2011). Cowpea production is constrained by high infestation of insect pest at storage which cause yearly economic loss of several billion dollars worldwide (Odeyemi et al., 2006). In Africa, loss due to storage insect pests goes up to 80 % yearly, contributing to food insecurity and low-income generation to farmers (Brisibe et al., 2011; Ileke and Olotuah, 2012). C. maculatus has been mention by several scholars as an important storage pest of cowpea (Adedire et al., 2011, Mkenda et al., 2015; Odeyemi et al., 2006). Infestation of cowpea seeds primarily happens in the field where insects are then carried into store and multiplication continues very rapidly (Adedire et al., 2011). For many years, chemical control has been the employed method in the management of cowpea weevils (Odeyemi, 2006; Ileke et al., 2013; Mkenda et al., 2015;). Although chemical control seems to be effective in controlling C. maculatus, the negative impacts associated with the use of chemicals are eminent such as pest’s resistance development, increased food toxicity, ecological consequences and sometimes lethal effects to non-target organisms have raised a concern to consumers and even researchers (Odeyemi, 2006; Ileke et al., 2013). In addition, even the conventional fumigation technology used in the developed countries, is currently being inspected for a number of reasons such as ozone destruction potential of methyl bromide and carcinogenic concerns with phosphine (Adedire, 2011). All these have resulted into a call for new methods to control stored products insect pests that are readily available, affordable and even harmless to human health and the environment. In response to this, locally available plants with pesticidal properties can be used as an alternative to synthetic pesticide in the storage of cowpea. Therefore, this study aimed at evaluating the effect of pesticidal leaf powders derived from three known pesticidal plants Ocimum suave, Hyptis suaveolens and Dysphania ambrosioides on cowpea weevils (Callosobruchus maculatus). Materials and methods Experimental site The experiment was carried out in a special storage room designed at Nelson Mandela African Institution of Science and Technology (NM-AIST), ArushaTanzania. The room was well ventilated with enough air circulation inside. Insects The Adult cowpea weevil (C. maculatus) used to establish a colony were obtained from highly infested cowpea grains bought from the local market in Moshi Tanzania. Insects were identified with a University Entomologist in the laboratory of Life Sciences and Engineering Department at the NM-AIST, Arusha, Tanzania. Rearing of cowpea weevil was done in ventilated 10 L plastic containers half filled with 5 kg of uninfected cowpea seeds, covered on top with 10 mm mesh sieve to allow free air movement while restricting weevils from escaping. Rearing was carried out at room temperature 25±3 °C and relative humidity (RH) 75±5 %. Adult weevils were kept for 3 Godifrey et al. Int. J. Biosci. 2020 20 days to allow their multiplication, after which they were harvested and used in the experiment. Insecticidal plant materials Plant materials used in this study were Ocimum suave, Hyptis suaveolens and Dysphania ambrosioides. O. suave and H. suaveolens were handpicked from farms at Kibosho village where as D. ambrosioides was collected at Lyamungo village both in Moshi Tanzania. These plant species were selected for this trial because they are traditionally used by farmers in the local areas as medicines and their readily available in the northern Tanzania (where this research was conducted). Matured plants leaves were dried under shed to reduce photolysis of bioactive compounds and then ground into a fine powder by using an electric mill, then packed into a plastic container with airtight lid to maintain the aroma and stored in the dark at ambient conditions of 25±3 °C and 75±5 % RH. Cowpea seeds Cowpea seeds used for this experiment were newly harvested by farmers from Tunduru District, in Mtwara –Tanzania. The seeds were free from insecticides. They were cleaned thoroughly by winnowing and mechanical sorting to remove infested and damaged grains. The clean seeds were sterilized by placing them into a freezer at 7 °C for 24 hours, then heated in an oven at 60 °C for 24 hours to kill any larvae and adult cowpea weevils that might remain in the process of cleaning. Testing the Effect of Leaf Powder to C. maculatus The dosages were set at different rates (0 g-negative control, 30 g, 60 g and 90 g 1.5 kg-1 of cowpea seeds) for each of the insecticidal plant powder used, so as to obtain the effective dose. To test the effect of leaf powder, 1.5 kg of healthy, fresh, clean, and unbroken cowpea grains were loaded into 2 kg sisal storage bags. Each bag with cowpea seeds was placed inside another bag of the same volume and the leaf powder of O. suave, H. suaveolens and D. ambrosioides at different rates (30, 60 and 90 g) were then spread on the outer surface of the inner bags (Double bagging experiment). Three treatments (O. suave, H. suaveolens and D. ambrosioides) leaf powder at rates 30, 60 and 90 g 1.5 kg-1 cowpea seeds plus the positive and negative control were arranged in Randomized Complete Block Design (RCBD) in 3 replicates. Experimental bags in each treatment (including the negative control) were arranged to surround a single plastic container containing heavily infested cowpea seeds left opened to allow movement of weevils to the surroundings. Data collection A subsample of 1000 cowpea seeds was drawn out from each bag for insect pest’s assessment. Counting of C. maculatus (live and dead) seeds with eggs on the surface, damaged seeds (seeds with holes and/or larval inside) was done after every 21 days. After assessment everything were taken back into the respective bag and the bags were sealed. The experiment ran for three consecutive months. Analysis of data Data collected were subjected STATISTICA (data analysis software system Version 8.0). to test for treatment effects over the study period. One-way analysis of variance (ANOVA) was used to analyze the collected data and Fishers Least Significant Difference (LSD) test was used to compare treatment means at 5 % confidence interval (P = 0.05). Results Effect of Pesticidal Leaf Powders on Live Adult C. maculatus The application of plant leaf powders at rates of 30, 60 and 90 g 1.5 kg-1 of cowpea seeds, showed varied pesticidal effects on the survival of adult C. maculatus and their differences were statistically significant (P ≤ 0.001) (Table1). The highest mean number of live C. maculatus were recorded in the bags with negative control all over 12 weeks of insect pest’s assessment. For the first three weeks all tested plants leaf powder at the rates of 30, 60 and 90 g 1.5 kg-1 of cowpea seeds effectively inhibited adult C. maculatus emergence 4 Godifrey et al. Int. J. Biosci. 2020 similar to the positive control (Actelic® dust) However, from 6 to 12 weeks, the effects were dose dependent whereby increased rate of leaf powder consequently increased pesticidal effect against C. maculatus on stored cowpea. O. suave at 90 g and H. suaveolens and D. ambrosioides, at 60 and 90 g significantly (P ≤ 0.001) inhibited the emergence of adult C. maculatus when compared with their respective lower rates. Table 1. Mean number of live C. maculatus in stored cowpea treated with three pesticidal leaf powder. Treatments Rates (g) 1.5 kg-1 3weeks 6weeks 9weeks 12weeks Control (-ve) 0 8.00±1.53a 10.67±1.20a 12.67±0.88a 85.67±5.21a O. suave 30 0.33±0.33b 5.00±1.53b 6.00±0.58bc 26.33±4.91b O. suave 60 0.33±033b 5.00±0.58b 5.00±0.58c 17.67±1.86b O. suave 90 0.33±0.33b 1. 00±0.58c 0.67±0.67d 3.00±0.58c H. suaveolens 30 1.67±0.33b 6.33±0.88b 7.33±0.88b 20.33±1.45b H. suaveolens 60 0.00±0.00b 0.67±0.33c 0.67±0.33d 1.00±0.58c H. suaveolens 90 0.67±0.67b 0.33±0.33c 0.33±0.33d 0.67±0.33c D. ambrosieides 30 1.00±0.58b 6.33±1.20b 7.67±0.88b 22.00±3.21b D. ambrosieides 60 0.00±0.00b 0.33±0.33c 0.67±0.67d 1.00±0.58c D. ambrosieides 90 0.67±0.00b 0.67±0.33c 0.33±0.33d 0.33±0.33c Actellic dust (+ve) 2 0.00±0.00b 0.00±0.00c 0.00±0.00d 0.00±0.00c One Way ANOVA (F-statistics) 12.71*** 30.15*** 36.05*** 54.65*** Control (-ve) = negative control, +ve = positive control; *** significant at P ≤ 0.001 and means within the same column followed by the same letter (s) are not significantly different at (P = 0.05) from each other using Fisher’s Least Significant Difference (LSD) test. The Effect of Pesticidal Leaf Powder on the Mortality of Adult C. maculatus Different effects were observed by supplying 30, 60 and 90 g of plant leaf powder on the mortality of adult C. maculatus. The results showed significant difference (P ≤ 0.001) in the mean number of dead adult C. maculatus across treatments all over 12 weeks (Table 2). The highest mortality was observed outside the bags treated with Actellic dust (2 g) throughout the study period. The result of the first week observation for H. suaveolens and D. ambrosioides at 90 g was statistically similar to positive control followed by 90 g for O.suave and 60 g for H. suaveolens and D. ambrosioides. There were increased numbers of dead insects in treatments involving H. suaveolens and D. ambrosioides plant leaf powder at the rates of 60 and 90 g and O. suave at 90 g 1.5 kg-1 of cowpea seeds after the first 3 weeks up to 9th weeks of treatment indicating high effectiveness of these plants materials in reducing the number of adult C. maculatus at high rates of application. However, mortality started to decrease after 12 weeks of treatment contrary to Actelic® dust 2 g that maintained its effectiveness. This showed that, mortality of adult C. maculatus treated with plant leaf powder was affected by increased rate of plant material and its exposure time to cowpea seeds. There were no dead insects found in untreated bags (negative control) throughout 12 weeks of assessment. In this case, the lowest mortality was observed in a negative control while the highest number was observed in the bags treated with Actelic® dust (positive control) where dead insects were observed outside the bags. Additionally, 30 g of all plant leaf powder had a low pesticidal effect on the mortality of C. maculatus. 5 Godifrey et al. Int. J. Biosci. 2020 Table 2. Mean number of dead C. maculatus in stored cowpea treated with three pesticidal leaf powder. Treatments Rates (g) 1.5 kg-1 Week 3 Week 6 Week 9 Week 12 Control (-ve) 0 0.00±0.00e 0.00±0.00d 0.00±0.00f 0.00±0.00f O. suave 30 6.67±1.20d 8.67±1.20c 12.00±1.15e 11.67±2.33e O. suave 60 9.00±0.58cd 13.00±2.08bc 17.67±0.88cde 12.33±1.76de O. suave 90 15.67±0.88b 17.00±1.53ab 24.33±2.03ab 17.33±0.88bcd H. suaveolens 30 9.33±0.88c 12.00±2.52bc 12.00±1.73e 13.00±2.08cde H. suaveolens 60 16.00±1.00b 20.00±1.53a 21.67±2.19abcd 17.33±1.20bcd H. suaveolens 90 17.67±0.88ab 21.00±1.73a 23.33±5.36abc 19.33±0.33b D. ambrosieides 30 7.67±0.88cd 10.67±1.45 c 16.00±1.53de 13.33±1.85cde D. ambrosieides 60 15.67±1.45b 16.67±3.38ab 19.33±2.03bcd 18.33±1.45bc D. ambrosieides 90 17.00±1.15ab 19.67±2.33a 21.33±1.20abcd 18.33±0.88bc Actellic dust (+ve) 2 19.00±1.15a 21.67±2.73a 26.33±2.91a 29.67±4.98a One-Way ANOVA (F-statistics) 61.54*** 18.01*** 19.96*** 21.54*** Control (-ve) = negative control, +ve = positive control; *** significant at P ≤ 0.001 and Means within the same column followed by the same letter (s) are not significantly different at (P = 0.05) from each other using Fisher’s Least Significance Difference (LSD) test. The Effect of Pesticidal Leaf Powder on the Oviposition of C. maculatus Plants leaf powder supplied at rates of 30, 60 and 90 g 1.5 kg-1 of cowpea seeds displayed different effects on oviposition of C. maculatus. Although, the effectiveness depends on the dosage of plant leaf powder and its exposure time to C. maculatus yet, different rates of pesticidal leaf powder were statistically significant (P ≤ 0.001) when compared with negative control which had highest mean number of cowpeas with eggs on the surface throughout the study (Table 3). After the first 6 weeks of treatment, all pesticidal leaf powder at 60, and 90 g effectively inhibited eggs laying capacity of C. maculatus almost in a similar manner to the positive control. Generally, H. suaveolens and D. ambrosioides at the highest rate of 90 g tested in this study maintained their effectiveness up to 12 weeks of treatment. Table 3. Mean number on oviposition by C. maculatus in stored cowpea treated with three pesticidal leaf powder. Treatments Rates (g) 1.5 kg-1 Week 3 Week 6 Week 9 Week 12 Control (-ve) 0 50.33±4.91a 98.67±7.22a 234.33±3.53a 536.33±9.39a O. suave 30 20.00±1.53b 47.00±4.16b 139.00±23.12b 246.67±41.91b O. suave 60 6.67±1.45cd 12.67±3.18d 70.67±6.44c 124.00±22.54de O. suave 90 2.33±0.33d 3.00±1.00de 64.67±23.47c 60.67±24.34f H. suaveolens 30 18.33±6.36bc 31.67±1.86c 88.33±5.78c 210.00±25.32bc H. suaveolens 60 6.67±3.18cd 3.33±1.20de 6.00±3.46d 56.33±4.33fg H. suaveolens 90 1.33±1.33d 0.67±0.67de 5.33±1.76d 3.33±1.45h D. ambrosieides 30 20.33±6.06b 31.33±3.48c 59.33±20.30c 172.00±20.26cd D. ambrosieides 60 6.67±2.40cd 1.67±0.88de 5.67±1.20d 106.00±5.13ef D. ambrosieides 90 3.3±2.40d 0.67±0.67de 4.67±1.20d 4.67±0.88gh Actellic dust (+ve) 2 0.00±0.00d 0.00±0.00e 0.33±0.33d 2.00±0.58h One-Way ANOVA (F-statistics) 19.38*** 87.87*** 49.82*** 129.58*** Control (-ve) = negative control, +ve = positive control; *** significant at P ≤ 0.001 and Means within the same column followed by the same letter (s) are not significantly different at (P = 0.05) from each other using Fisher’s Least Significance Difference (LSD) test. 6 Godifrey et al. Int. J. Biosci. 2020 The trend was different from O. suave which showed less pesticidal activity after 9 weeks of treatment. All pesticidal leaf powder at 30 g displayed the least effectiveness in affecting egg laying capacity by C. maculatus. Actelic® dust 2 g showed the greatest capacity to inhibit oviposition by C. maculatus than the rest of the treatments. The Effect of Pesticidal Leaf Powder on Cowpea Damage by C. Maculatus Supplying plant leaf powder at rates 30, 60 and 90 g 1.5 kg-1 of cowpea seeds, displayed varied pesticidal effects on protecting cowpea grains against C. maculatus damage. Actelic dust at 2 g showed high effectiveness in controlling cowpea damage by C. maculatus (Table 4). Table 4. Mean number of damaged seeds by C. maculatus in stored cowpea treated with three pesticidal leaf powder. Treatments Rates (g) 1.5 kg-1 Week 3 Week 6 Week 9 Week 12 Control (-ve) 0 27.67±4.98a 33.00±3.06a 107.67±13.13a 132.67±16.60a O. suave 30 12.00±1.73b 18.33±1.45b 39.33±1.86b 77.00±6.25b O. suave 60 10.67±0.88b 12.00±1.53bc 15.00±2.00c 68.67±6.57b O. suave 90 7.00±1.15bc 8.67±0.88bcd 10.00±1.53c 9.67±0.67c H. suaveolens 30 10.33±1.45b 11.00±1.15bcd 36.00±6.03b 70.00±7.77b H. suaveolens 60 9.33±0.88b 7.33±1.20bcd 7.00±2.65c 7.67±0.88c H. suaveolens 90 8.67±2.03b 6.67±1.20cd 7.00±2.08c 5.00±1.15c D. ambrosieides 30 11.00±1.00b 12.67±2.73bc 33.33±8.37b 60.00±4.36b D. ambrosieides 60 8.67±1.20b 8.00±1.53bcd 10.67±0.88c 8.33±0.67c D. ambrosieides 90 6.33±2.85bc 7.33±2.03bcd 5.00±0.58c 5.00±0.58c Actellic dust (+ve) 2 0.00±0.00c 0.33±0.33d 1.00±0.58c 1.33±0.33c One-Way ANOVA (F-statistics) 15.77*** 9.66*** 49.01*** 47.84*** Control (-ve) = negative control, +ve = positive control; *** significant at P ≤ 0.001 and Means within the same column followed by the same letter (s) are not significantly different at (P = 0.05) from each other using Fisher’s Least Significance Difference (LSD) test. The highest grain damage was observed in a negative control throughout the study period. All plants leaf powder at rates 30, 60 and 90 g 1.5 kg-1 of cowpea seeds displayed similar effect in reducing cowpea damage in the first three weeks of treatment. However, from 6 to 12 weeks, their differences were statistically significant (P ≤ 0.001) and was dose dependent. Treatments with O. suave at 90 g, H. suaveolens and D. ambrosioides at rates 60 and 90 g showed promising results in protecting the grains against C. maculatus damage similar to synthetic chemical. Discussion Performance of O. suave on C. maculatus Ocimum suave contains phtytochemical eugenol as a major bio-active compound that excite insecticidal activities such as repellency, fumigant, contact toxicity, mortality and deterrent effect against storage insect pest of different crop including Sitophilus zeamais, Prostephanus truncatus (Obeng-Ofori and Reichmuth, 1997; Obeng-Ofori et al., 2000). In the present study, O. suave exhibited pesticidal activities against C. maculatus at high dosage (60 and 90 g) of plant leaf powder that effectively reduced oviposition and the number of adult C. maculatus emergence up to 6 weeks of their storage (Table 1 & 3). ObengOforri and Reichmuth, (1997) reported on toxicity of Eugenol against four species of stored-product Coleoptera when the store time increased. In this regards, O. Suave can be used as a grain protectant in a short-term storage. Thus, there should be periodic 7 Godifrey et al. Int. J. Biosci. 2020 reapplication of this plant materials if they are to be used for a long-term storage (Muzemu et al., 2013). However, other studies have shown that mixing O. suave with other pesticidal materials such as Cymbopogon citratus effectively prolong the storage time of cowpea seeds against C. maculatus by inhibiting oviposition and number of adult emergence (Ojianwuna, 2011). At higher dose of 90 g, plant leaf powder of O. suave showed effectiveness in increasing adult mortality of C. maculatus. This result is similar to the work of Bekele et al., (1999) in which O. suave ground dry leaves worked best at the higher dosage and evoked 100% mortality of three stored insect pests Sitophilus zeamais, Rhyzopertha dominica and Sitotroga cerealella. Ocimum species are known to have repellent and toxicant effect against various stored insect pests (Bekele et al., 1999; Hassanali et al., 1990) which might be the reason for high mortality and reduced oviposition caused by this plant materials to C. maculatus. The protection of grains against insect damage by the leaves of O. suave form the basis for their use by small scale farmers as traditional grain protectant in short -term storage. Performance of H. suaveolens on C. maculatus Hyptis suaveolens is known to contain sabinene (41.0 %), terpinen-4-ol (12.31 %), β-pinene (10.0 %) and βcaryophyllene (8.0 %) as four major compounds with biological activities against several field and storage insect pests (Tripathi and Upadhway, 2009; Sharma and Tripathi, 2008; Chi and Apiah, 2012; Hassan et al., 2018). In this study, leaf powder of H. suaveolens showed high effectiveness on reducing oviposition, adult emergence, seed damage and increasing mortality of C. maculatus at the higher dose of 60 and 90 g over the period of 12 weeks (Table 2). The repellency property of H. suaveolens contributed to the reduced oviposition which consequently resulted into inhibition of larval development that could grow into pupa and hence low number of adult cowpea emergence. Hassan et al., (2018) reported 100 % of eggs’ mortality when leaf powder of H. suaveolens were tested on C. maculatus eggs to test the ovicidal activity. Other scholars have also reported on similar effect of the H. suaveolens plant materials on controlling cowpea weevils (Tripathi and Upadhway, 2009; Adebayo and Eyo, 2014) and maize weevils (Asawalam et al., 2007). In this regards, H. suaveolens seems to have protectants ability against storage insect pests for a longer period than O. suave and hence can be used as an alternative to synthetic pesticide. Performance of D. ambrosioides on C. maculatus The chemical screening of D. ambrosioides identified the plant to have several chemical compounds including ascaridole (54 %), isoascaridole, (28 %) and p-cymene (8 %) (Dembitsky et al., 2008; Mkenda et al., 2015). It has been used for treating diseases and preserving postharvest grains from weevil attack (Tapondjou et al., 2002; Dembitsky et al., 2008; Salimena et al., 2015; Mkenda et al., 2015). In the present study, D. ambrosioides at the rates of 60 and 90 g, exhibited high effectiveness on reducing adult mortality, reduced seed damage and inhibition of adult emergences. The effectiveness of D. ambrosioides could be as a result of biological activity of ascaridole compound that contains insecticidal activity against insects’ pests. Chu et al. (2011) reported ascaridole to be the active constituent against the storage weevil Sitophilus oryzae. Similar to other plant materials used in this study, D. ambrosioides was very effective at higher dosage of plant leaf powder. Mkenda et al., (2015) reported a complete seed protection, higher mortality of C. maculatus and lower number of holes per cowpea seed using D. ambrosioides powder. Similar to results obtained in this study, the effectiveness of D. ambrosioides plant materials against several insect pests have been widely reported (Tapondjou et al., 2002; Chiasson et al., 2004; Denloye et al., 2010; Mkenda et al., 2015). Conclusion Generally, the three pesticidal leaf powder evaluated, provide protection to cowpea grains from C. maculatus at the high rates of plant leaf powder which may be as a result of ovicidal and larvicidal 8 Godifrey et al. Int. J. Biosci. 2020 properties possessed by the tested plant materials that killed few eggs laid while preventing the few once that hatched to grow into larva. Furthermore, high mortality caused by plant leaf powder might also be due to contact toxicity of the tested plant powders which affect insect’s survival thus kill them. Plants leaf powder showed decreased trend of insect mortality after the 12 weeks suggesting fast degradable nature of bioactive compounds present in these plants’ material and hence safety for human health as well as little or no residue to environment. Among the pesticidal leaf powder used, H. suaveolens and D. ambrosioides at 90 g effectively reduced the oviposition in stored cowpea similar to Actelic® dust. In this regards, small holder farmers residing in areas where these materials are locally available, can utilize these plant materials as grain protectants in stored cowpea. By considering that, adult cowpea weevils do not feed on stored cowpea seeds but only deposit their eggs, having good control to inhibit or in somewhat reduce eggs deposition could prolong the storage time for the grains. Therefore, based on the results obtained from this study, pesticidal plants leaf powder are good protectants of stored cowpea product as they can effectively protect the grains against C. maculatus however, its application can only be certain for high dose. Acknowledgement The authors are thankful to African Development Bank AfDB (Grant No. 2100155032816) for financial support provided to accomplish this study. References Adebayo RA, Eyo ME. 2014. 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