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INTRODUCTION
The cowpea, Vigna unguiculata (L.) Walp., is a multifunctional crop that provides food for humans and livestock. It is a valuable and dependable revenue-generating commodity widely grown by farmers in all agro-ecological zones in Nigeria. The cowpea is a major staple food crop in sub-Saharan Africa and a major source of inexpensive dietary proteins that complement expensive animal sources of protein in many developing nations.
In most developing countries, small scale cowpea cultivation plays a substantial role in rural economies. Small scale farmers face severe post-harvest problems from bruchid infestations, particularly in grain storage. Research has shown that the bruchid developmental stages (Callosobruchus maculatus), under field and laboratory conditions, include: incubation period, duration of larvae stage evolution, pupae duration and longevity of new adults. The entire life cycle takes an optimum of 27 d at 30°C and 80% relative humidity (RH) (Kouninki et al., 2009).
Farmers who want to protect their produce from bruchid infestations use synthetic pesticides if they can afford them. These chemicals are frequently misused because the majority of small-scale farmers in many African countries are illiterate. The chemicals contaminate stored food, leaving behind harmful residues, especially when application dosages are not properly followed (Parugrug and Aurea, 2008). Synthetic pesticides are not only expensive but may also have harmful effects on the health of consumers. Moreover, some pests have developed some resistance to some synthetic pesticides, so alternative natural solutions to the bruchid problem in cowpea are needed.
The objective of this study was to compare the response of Callosobruchus maculatus to M. charantia and A. wilkesiana powders in stored cowpea.
MATERIALS AND METHODOLOGY
Location of the experiment
This study was carried out in the Department of Crop, Soil and Pest Management Laboratory of the Federal University of Technology, Akure (Ondo state, South-west Nigeria).
Insect culture (Callosobruchus maculatus)
The Callosobruchus maculatus adults used for the experiment were obtained from infested cowpea seeds from the Oja-Oba market, Owo, Ondo State. The cowpea seeds were put in a kilner jar covered with muslin, which prevented the insects from escaping and allowed for ventilation (Adesina et al., 2016). The jar was kept at room temperature (28°C and 75% RH) to allow the insects to breed and multiply; the emerged insects were used for this experiment.
Collection of cowpea seeds for bioassay
The drum variety cowpea seeds used in the bioassay were obtained from the same market. The seeds were properly hand-picked and sieved, ensuring that only whole, uninfected seeds were used. These seeds were kept in a deep freezer for a week to kill any immature stage insects (if any), followed by air drying in the laboratory for 24 h to prevent mustiness (Adesina and Mobolade-Adesina, 2020) and storage in a cool dry place (Iloba and Ekrakene, 2006).
Collection and preparation of plant materials
Fresh M. charantia and A. wilkesiana leaves were obtained from Rufus Giwa Polytechnic, Owo, Ondo state and identified at the Department of Forestry and Wood Technology at the institution. The plant leaves washed with clean water to remove dirt and air dried at room temperature for 15 d. Thereafter, the dried leaves were grounded into a powder using a hammer mill and sieved before storage in a tightly covered container.
Application of plant materials
Twenty grams of clean un-infested and disinfected cowpea seeds were weighed using a TS 400D digital balance (precision standard), which were placed in Petri-dishes with a 9 cm diameter. Thereafter, 0.2, 0.4, 0.6 and 0.8 g of each pulverized plant’s leaves were thoroughly mixed with the seeds inside each Petri-dish. Another Petri-dish containing 20 g of untreated cowpea seed was also prepared to serve as a control. Five pairs (10) of freshly emerged C. maculatus adults were introduced into each Petri-dish. Each treatment level was replicated three times in a completely randomized design on the laboratory workbenches.
The number of adult weevils that died were observed and recorded at 24 h intervals for 120 h (5 d). The insects were jabbed using a blunt dissecting probe (Obeng-Oforie et al., 1997), and the insects that did not respond to three probes were considered dead. Thereafter, all insects were removed, and the experiment proceeded until adults (first filial generation) started to emerge. Oviposition was recorded at 48 h, along with the number of eggs laid on the seeds (usually whitish in color). The number of adults that emerged were counted and recorded after a period of 30-35 d; the newly emerged insects were removed after they were counted. The seed weight loss was determined after the experiment.
RESULTS
Comparative effect of M. charantia and A. wilkesiana powders on the adult mortality of C. maculatus
Table 1 shows the adult mortality of C. maculatus in the cowpea seeds treated with the plant powders (M. charantia and A. wilkesiana). At the lowest dosage of 0.2 g with 24 h of exposure, both treatments had the lowest mortality (3.3%); no mortality was recorded for the control. The treatments increased in the adult mortality of C. maculatus as the exposure period and dosage increased, with the highest values (90 and 100% for A. wilkesiana and M. charantia, respectively) observed with 120 h of exposure at the 0.8 g dosage level.
The M. charantia and A. wilkesiana powders were potent against C. maculatus because they recorded a higher percentage of beetle mortality (Tab. 1).
Table 1 Comparative effect of M. charantia and A. wilkesiana powders on C. maculatus adult mortality.
| Period of exposure (h) | |||||
|---|---|---|---|---|---|
| Treatment | 24 h | 48 h | 72 h | 96 h | 120 h |
| 0.0 g (Control) | 0.0±0.00 a | 0.0±0.00 a | 0.0±0.00 a | 0.0±0.00 a | 6.7±3.33 a |
| 0.2 g (A. wilkesiana) | 3.3±3.33 ab | 13.3±3.33 b | 23.3±3.33 b | 36.7±3.33 b | 53.3±3.33 b |
| 0.2 g (M. charantia) | 3.3±3.33 ab | 23.3±3.33 bc | 36.7±3.33 bc | 53.3±3.33 bc | 63.3±3.33 b |
| 0.4 g (A. wilkesiana) | 10.0±0.00 bc | 30.0±0.00 cde | 43.3±3.33 cd | 56.7±3.33 c | 66.7±3.33 bc |
| 0.4 g (M. charantia) | 13.3±3.33 c | 33.3±3.33 cde | 46.7±3.33 cde | 66.7±3.33 c | 6.7±3.33 bcd |
| 0.6 g (A. wilkesiana) | 13.3±3.33 c | 33.3±3.33 cde | 56.7±3.33 cde | 70.0±5.77 cd | 0.0±0.00 bcd |
| 0.6 g (M. charantia) | 16.7±3.33 c | 40.0±0.00 de | 66.7±3.33 fg | 83.3±3.33 d | 90.0±5.77 cd |
| 0.8 g (A. wilkesiana) | 20.0±0.00 c | 40.0±0.00 de | 60.0±0.00 ef | 83.3±3.33 d | 90.0±0.00 cd |
| 0.8 g (M. charantia) | 20.0±0.00 c | 46.7±3.33 e | 76.7±3.33 g | 100.0±0.00 e | 100.0±0.00 d |
Each value is the mean ± standard error of the three replicates. Values in the same column with same letter(s) do not differ significantly at P>0.05 using Tukey’s honest significance test.
Comparative effect of M. charantia and A. wilkesiana powders on oviposition suppression in C. maculatus
Table 2 shows the oviposition suppression activity of M. charantia and A. wilkesiana powders in stored cowpea seeds. The untreated seeds (control) had the highest oviposition (175.0). The treatments had significant effects on the oviposition of the insects (C. maculatus), as compared with the control. At the 0.2 g dosage, the Acalypha wilkesiana treated seeds had 107.7 eggs laid by the insects. M. charantia had 105.7 eggs laid by the insects. At the 0.4 g dosage level, A. wilkesiana and M. charantia recorded 97.0 and 88.3 eggs, respectively. At the 0.6 g dosage level, Acalypha. Wilkesiana recorded 84.7 eggs laid on the stored cowpea seeds, and M. charantia recorded 70.3 eggs. At the 0.8 g dosage level, the A. wilkesiana treated seeds had 66.3 eggs laid by the insects, while M. charantia had 63.0 eggs.
Table 2 Comparative effect of M. charantia and A. wilkesiana powders on C. maculatus oviposition.
| Plants powders | ||
|---|---|---|
| Dosage level | A. wilkesiana | M. charantia |
| 0.0 g | 162.3±0.10 a | 175.0±2.40 a |
| 0.2 g | 107.7±1.20 b | 105.7±0.13 bc |
| 0.4 g | 97.0±0.09 de | 88.3±2.65 cd |
| 0.6 g | 84.7±1.86 f | 70.3±0.20 e |
| 0.8 g | 66.3±0.00 g | 63.0±0.00 g |
Each value is the mean ± standard error of the three replicates. Values with same letter(s) do not differ significantly at P>0.05 using Tukey’s honest significance test.
Comparative effect of M. charantia and A. wilkesiana powders on adult emergence in C. maculatus
Table 3 shows the activity of the plants powders (M. charantia and A. wilkesiana) on adult emergence in C. maculatus on stored cowpea seeds.
The result showed that there was a significant effect from the plant powders on the adult emergence of weevils in the stored seeds, as compared with the control. The control recorded the highest number of C. maculatus adult emergence (114.3). The M. charantia powder at the 0.8 g dosage level resulted in the lowest number of adult emergences (40.0). At the 0.2, 0.4 and 0.6 g dosage levels, there was no significant difference between the two plant powders. At the 0.8 g dosage level, there was a significant difference between the M. charantia (40.0) and A. wilkesiana powders (48.7).
Table 3 Comparative effect of M. charantia and A. wilkesiana powders on adult emergence in C. maculatus.
| Plants powders | ||
|---|---|---|
| Dosage level | A. wilkesiana | M. charantia |
| 0.0 g | 114.3±0.18 a | 113.7±3.53 a |
| 0.2 g | 79.0 ± 0.33 b | 67.0 ± 1.15 b |
| 0.4 g | 66.3 ± 1.20 c | 60.7 ± 0.33 c |
| 0.6 g | 57.0 ± 0.33 de | 51.0 ± 0.33 de |
| 0.8 g | 48.7 ± 0.00 d | 40.0 ± 0.00 e |
Each value is the mean ± standard error of the three replicates. Values with the same letter(s) do not differ significantly at P>0.05 using Tukey’s honest significance test.
Comparative effect of Momordica charantia and Acalypha wilkesiana powders on C. maculatus weight loss activity in stored cowpea seeds
Table 4 shows the effect of the M. charantia and A. wilkesiana powders on weight loss in the stored cowpea seeds as caused by cowpea weevils. The results showed that the seeds without powder (control) had the highest weight loss percentage (17.2%), which resulted from the insect infestation, followed by the seeds treated with the 0.2 g dosage level of M. charantia and A. wilkesiana (12.3%), the 0.4 g dosage level in seeds treated with M. charantia (10.8%) and A. wilkesiana (10.0%), the 0.6g dosage level in seeds treated with the two plant powders (8.0%), and the 0.8 g dosage level in seeds treated with the two plant powders, which had the lowest seed weight loss percentage (6.7%).
Table 4 Comparative effect of M. charantia and A. wilkesiana powders on C. maculatus weight loss activity in stored cowpea seeds.
| Plant powders | ||
|---|---|---|
| Dosage level | A. wilkesiana (%) | M. charantia (%) |
| 0.0 g | 17.2 ± 0.05 d | 17.2 ± 0.00 d |
| 0.2 g | 12.3 ±0.3 d | 12.3 ± 0.02 d |
| 0.4 g | 10.0 ± 0.05 bc | 10.8 ± 0.05 bc |
| 0.6 g | 8.0 ± 0.32 b | 8.0 ± 0.08 b |
| 0.8 g | 6.7 ± 0.13 a | 6.7 ± 0.12 a |
Each value is the mean ± standard error of the three replicates. Values with same letter(s) do not differ significantly at P>0.05 using Tukey’s honest significance test.
DISCUSSION
The main tactic for controlling insect pests during storage has been centered on the use of synthetic insecticides and fumigants but they have several drawbacks. Alternative botanicals are a panacea for the problems caused by the use of synthetics insecticides and fumigants. The high insecticidal effect of M. charantia and A. wilkesiana recorded in this study may have been due to insecticidal properties. In addition, these powders may block the spiracle of the beetles, resulting in suffocation and death (Ileke and Olotuah, 2012; Oni, 2014; Oni et al., 2018a). The efficacy and effectiveness of these plant powders (M. charantia and A. wilkesiana) may also result from phytochemicals, such as glycosides, alkaloid, sapon, and tannis, among others, found in the plants. Since most of these compounds have been reported as causing insect mortality, the high C. maculatus mortality seen with the plant powders of M. charantia and A. wilkesiana could be associated with the presence of one or more of these compounds (Obembe and Ogungbite, 2017).
The study revealed that maximum oviposition suppression was observed with M. charantia and A. wilkesiana, while the minimum oviposition suppression was found in the control (untreated seeds). Increasing the dosage levels suppressed the ovipositions, as shown in Table 2, which agrees with Olaifa and Erhun (1998), Oni et al. (2018b), and Adesina et al. (2012), who found that a higher concentration of a powder made from Piper guineense and M. charantia significantly reduced the oviposition rate and that these plant powders might possess repellent and/or oviposition suppression properties. Oviposition suppression may be due to changes induced in the physiology and behavior in C. maculatus adults, as reflected in their eggs laying capacity.
The significant effects of the plant powders in inhibiting adult emergence could be attributed to a biocidal property that suppresses C. maculatus oviposition in stored cowpea seeds. A significant reduction in adult emergence was observed between the treatments. The efficacy of the plant powders (M. charantia and A. wilkesiana) was much stronger against F1 than against egg laying (oviposition), as found by Jayakumar et al. (2003), who reported that plant products have an obvious effect on the postembryonic survival of insects, resulting in a reduction in adult emergence with all concentrations of plant powders. Raja et al. (2001) stated that botanicals inhibit C. maculatus adult emergence in cowpea. He further stated that, when eggs are laid on treated seeds, the toxic substance in the plant products may enter through chorion, suppressing their embryonic development. In the present study, adult emergence was greatly reduced in the treated seeds, as compared with the control seeds (untreated seeds).
The significant reduction in weight loss in the treated cowpea grains may have resulted from the ability of the plant powders (M. charantia and A. wilkesiana) to suppress oviposition and adult emergence in the cowpea weevils in the stored cowpea seeds. These results support the findings of Adesina et al. (2012), who claimed that M. charantia leaf powder has great potential for use as an admixture in stored cowpea seeds for small scale farmers, reducing insect pest infestation damage, increasing mortality, and suppressing oviposition, adult emergent and weight loss.
CONCLUSION
The results of this study showed that the effectiveness of the plant powders (M. charantia and A. wilkesiana) was dependent on the dosage and exposure period, with significant effects on all accessed parameters that evidenced the entomocidal potential of protecting stored cowpea grains from C. maculatus infestations. Because of their relative availability, easy preparation, easy application, cheap cost, nontoxic properties, greater ability and higher influence on C. maculatus infestations in stored cowpea seeds, it is recommended that M. charantia and A. wilkesiana powders be used in Integrated Pest Management by small scale farmers or shops for protecting short term cowpea storage from C. maculatus infestations. Further studies are recommended to determine the mode of action responsible for the insecticidal properties and the phytochemical profile of the plant materials.
