Index

1. INTRODUCTION

Cowpea, Vigna unguiculuta (L. Walp) is an important food crop in tropical countries particularly in West Africa (Adedire and Ajayi, 2003). It has been reported that the crop is also a source of livestock feed and revenue in the tropics (Onekutu et al., 2015). Nigeria contributes a substantial amount of world cowpea production with about four million hectares cultivated annually (Ofuya and Longe, 2006). Cowpea seed is a main source of plant proteins for the families that consume it in many countries of Western and Central Africa (Ileke et al., 2012).

Callosobruchus maculatus is a principal pest of stored legume seeds, particularly cowpea, Vigna unguiculata (Ofuya, 2001). C. maculatus infestations have been reported to cause substantial reduction in quality and quantity of cowpea seeds -within three to five months of storage (Ileke et al., 2012). Furthermore, Ashamo (2007) opined that the biggest challenge facing Nigerian farmers is that of post-harvest losses associated with insect pest infestation. This is because preservation of the cowpea seeds for the next cropping season is a major target of the farmer.

Currently, synthetic insecticides application is the major means of controlling beetles infestations in stored cowpea seeds (Onekutu et al., 2015). This could be in form of fumigation of stored product with phosphine or carbon disulphide and or dusting with carbaryl, pirimiphos methyl or permethrin (Ileke et al., 2012). However, consequent upon reported ozone depletion by methyl bromide and carcinogenic concerns with phosphine, conventional fumigation technology is under scrutiny in the developed countries (Adedire et al., 2011). Ileke et al. (2012) further highlighted problems associated with the  use of conventional synthetic insecticides  to include  high mammalian toxicity, high level of persistence in the environment, poor application knowledge, exorbitant cost prices, pest resurgence, genetic resistance by the insect pest and deleterious effects on non-target organisms. One possible way to overcome the short comings of synthetic insecticides is to substitute synthetic insecticides with naturally-occurring plant insecticidal materials (Ileke et al., 2012; Khater, 2012). According to Lale (1995) plant products are neither scarce nor expensive; they are safe for humans and environmentally safer than the conventional synthetic insecticides. Since the control of this coleopteran insect pest with synthetic insecticides has been reported to be unsafe to humans (Ileke et al., 2012; Onekutu et al., 2015) the present study sought to assess powders of five medicinal plant materials which are peppers and spices against C. maculatus infestation on stored cowpea seeds in three different agro-ecological zones of Nigeria.

2. MATERIALS AND METHODS

2.1. Standardization of Seeds

Cowpea variety TVX3236 used in this study was obtained from the Seed Technology Centre, Federal University of Agriculture Makurdi, Benue State, Nigeria. The seeds were cleaned and frozen at -20 ± 2oC for one week to disinfest them and then, stored at 4oC to prevent re-infestation (Sulehrie et al., 2003). Prior to the bioassay, cowpea seeds used for the experiment were kept in muslin-covered plastic containers to acclimatize for four weeks. As measured by the standard oven method, this stabilized the moisture contents of the seeds at about 14 % under laboratory mean temperature and relative humidity of 31.2 oC and 69.8 % respectively.

2.2. Mass Rearing of Callosobruchus maculatus

Callosobruchus maculatus adults were obtained from the stock culture of the Department of Crop and Environmental Protection, Federal University of Agriculture Makurdi, Benue State, Nigeria. The insects were reared on a susceptible cowpea variety, Ife brown in 1.5-litre glass bottles at laboratory ambient temperature and relative humidity of 31.2 oC and 69.8 % respectively. Freshly emerged bruchid adults of 1 - 2 days old were then used for the experiment.

2.3. Plant Materials

The five medicinal plant materials and their identities used in the investigation are presented in Table 1. Air dried samples of the plant materials were bought from spice sellers at Modern Market, Makurdi, Benue State, Nigeria. The plant materials were ground separately into powder using clean mortar and pestle. The powders were sieved (mesh size: 1 mm2) to produce fine powders (Ileke and Oni, 2011) which were used immediately for the experiment. The choice of “test concentration” of plant powder was based on Lale (2002) which stressed that the concentration of plant powder used against stored product pests should not be greater than 2.0 % w/w to be economically-justified. Therefore, the plant materials were assessed at 1.5 % w/w concentration.

Table-1. Plants evaluated for insecticidal properties against C. maculatus Fab. in stored cowpea seeds

2.4. Bioassay

A quantity of 0.3 g powder of each of the plant materials: M. fragrans, C. annuum, Z. officinale, P. nigrum and A. melegueta was added respectively to five plastic jars (each of 100 ml capacity) containing 20 g of cowpea seeds (TVX3236) to give 1.5 % w/w. Comparable rate of permethrin (Rambo: 0.6 % D) was added into a sixth jar containing 20 g of cowpea seeds. The plant powders and seeds were mixed thoroughly to enhance coating of the seeds. An untreated control was also set up with no plant material powder and permethrin added. Five pairs (5 females + 5 males) of freshly-emerged adult C. maculatus were introduced into each jar which was covered with a muslin cloth and held firmly with a rubber band. Female and male C. maculatus adults were sexed using the method of Raina (1970). Adults with shorter antennae and bright-spotted elytra were identified as female while those having longer antennae and dull-coloured elytra were characterized as male. The treatments were arranged in a completely randomized design on a workbench in the laboratory. The experiment was observed daily for 4 days and data on percentage adult bruchid mortality were collected every 24 hours and recorded. A bruchid was considered dead if it did not respond to a probe with a pin. Percent mortality was calculated using the standard formula:

All dead and live C. maculatus adults were sieved out immediately after mortality count to ensure that the emerging adults were direct consequence of the number of eggs oviposited in 4 days (Ileke et al., 2012). The eggs laid on the cowpea seeds after 96 hours in a no-choice protocol were counted and the average number was calculated per treatment. The examined seeds were returned to their respective containers and left under the same experimental conditions for another 26 days, giving a total of 30 days under storage and emerged adults from seeds of different treatments were counted and recorded. The experiment was replicated four times.

3. STATISTICAL ANALYSIS

The statistical software was SPSS for Windows® (version 21.0). Data were subjected to one - way analysis of variance (ANOVA). Following the outcome of Levene’s test for equality of variances, data were transformed using arcsine and log N transformations, respectively (Somta et al., 2008). Tukey’s Honest Significant Difference (HSD) test was used to separate significant means from non - significant ones at α = 0.05. 

4. RESULTS

4.1. Effect of the Plant Materials on Mortality of Adult C. maculatus

Effect of the plant materials on adult mortality of C. maculatus is presented in Table 2. Percent mortality varied significantly (P < 0.01) among the treatments. All the test materials caused mortality in various degrees to the beetle after 24 hours of exposure, reaching 100 % in 96 hours. Piper nigrum caused highest bruchid mortality at all exposure periods and this differed significantly from mortalities caused by the rest of the plant materials. The conventional synthetic insecticide, permethrin, caused higher bruchid mortality than P. nigrum at 24 - hour post - treatment period. However, at 2nd to 4th day exposure periods, the efficacy of P. nigrum matched permethrin in the mortality of the adult caused. Significant differences did not occur among M. fragrans, C. annuum and untreated control in terms of beetle mortality after 24 hours of exposure. The A. melegueta seed powder did not cause considerable C. maculatus mortality within 48 hours of exposure. However, powder of C. annuum which was the least active killed all the beetles after 96 hours as recorded for other treatments.

4.2. Effect of the Plant Materials on Oviposition and Adult Emergence of C. maculatus

Table 3 presents results on oviposition deterrence by the plant materials caused to C. maculatus females as well as effects of the plant materials on adult emergence. The number of eggs laid by female C. maculatus adults was also significantly different (P < 0.01) among treatments. Piper nigrum deterred oviposition most. This was followed by Z. officinale. Capsicum annum gave the highest support to oviposition. However, no plant material matched the efficacy of permethrin in deterring oviposition in the bruchids. Adult emergence was significantly lower in cowpea seeds protected with powder of P. nigrum and highest in seeds protected with powder of C. annum. Piper nigrum matched permethrin in reducing adult emergence. Aframomum melegueta deterred oviposition and reduced adult emergence significantly when compared to the unprotected cowpea seeds; but not when compared to permethrin. Cowpea seeds treated with M. fragrans, Z. officinale and A. melegueta did not differ significantly in the number of emerged adults.

Table-2. Effect of plant materials [1.5 % (w/w)] and permethrin powder on mortality of adult C. maculatus Fab.

*Treated control (conventional synthetic insecticide)

Data are means ± SEM of four replications

Means in a column followed by the same letter are not significantly different (α = 0.05)

Mortality was significant at P < 0.01 by Fisher’s F-test

Table-3. Effect of plant materials [1.5 % (w/w)] and permethrin powder on oviposition and adult emergence by C. maculatus Fab.

*Treated control (conventional synthetic insecticide)

**After 4 days

***After 30 days

Data are means ± SEM of four replications

Means in a column followed by the same letter is not significantly different (α = 0.05)

All indices were significant at P < 0.01 by Fisher’s F-test

5. DISCUSSION

This investigation revealed that powder of P. nigrum had the highest insecticidal activity against C. maculatus. Its toxic effect on the insect did not significantly differ from permethrin in 24 hours. Adult emergence deterrence by P. nigrum also showed that the black pepper has good potentials for controlling bruchid infestations in stored cowpea seeds. This agrees with the findings that members of the family Piperaceae show some form of insecticidal activity (Adedire and Ajayi, 1996; Okonkwo and Okoye, 1996; Adedire and Lajide, 1999). Okonkwo and Okoye (1996) reported that the bioactive agents in some members of Piperaceae are piperine and chavicine and these are highly insecticidal to various crop pests. Lale (1995) reported piperine and alkaloids as the major active ingredients in P. guineense seeds. This can be attributed to the reason why P. nigrum was significantly better at evoking insecticidal activity. However, the results presented here indicate that the conventional synthetic insecticide permethrin was superior to P. nigrum on account of the latter lacking strong evidence of oviposition deterrence.

The failure of A. melegueta seed powder to cause considerable C. maculatus mortality within 48 hours of exposure corroborates the findings of Adedire and Lajide (1999) and Onekutu et al. (2015) it has been reported that both seed powder and extract of A. melegueta did not affect oviposition and egg hatchability of C. maculatus significantly (Ofuya, 1990). Differences in mortality data of untreated control and mortality caused by M. fragrans and C. annum after 24 hours of exposure suggest that the plant materials would not give good protection to cowpea seeds. This opinion is further supported by the findings of Onekutu et al. (2015) that any natural material which cannot control C. maculatus at an economically justified concentration within 24 hours is unlikely good control agent against the insect pest. The modes of action of the test materials are contact and fumigation (Adedire and Lajide, 1999; Asawalam and Emosairue, 2006; Asawalam et al., 2007; Franccedil et al., 2009; Ukeh et al., 2010). P. nigrum that most effectively protected against bruchid infestation is edible and thus, poses no risk to human health and the environment, unlike permethrin. Results obtained from this study demonstrate insecticidal potentials of these plant powders against C. maculatus in stored cowpea seeds.

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