Introduction

One of the greatest constrictions towards ensuring food security is the mitigation of post-harvest loss of stored food grains quality and quantity against stored product insect infestation. The cowpea beetle, Callosobruchus maculatus (Coleoptera: Chrysomelidae) is a major post-harvest pest of stored highly nutritious pulse grains that causes sizeable loss to cowpea in terms of germination impairment, nutritional and weight loss when not effectively managed (Langyintuo et al. 2003; Oluwafemi 2012; Ndong et al. 2012; Mailafiya et al. 2014; Tiroesele et al. 2014; Kpoviessi et al. 2018). Contamination of stored cowpea grain with the powdery waste and damaged seeds through adult exit holes reduces its market price.

For several years now, the chemical control method remains the foremost and active routine means of protecting and controlling stored food grains against insect infestation and grain/seed damage (Adesina et al. 2012). The inadequacy in the use of synthetic insecticide to human health, environment vulnerability, and residue in food grains have raised apprehension and curiosity in probing for an unconventional material that is least injurious or mild to the environment and such is the use of medicinal and aromatic plants as natural source insecticide.

The use of botanical insecticides can be dated back as early as the beginning of agriculture, but it was neglected after the modern agricultural system introduced synthetic chemicals. It is noteworthy that several researchers have resorted again to the use of plants as natural available pesticide alternatives. Many plants have been investigated and screened to contain hundreds of secondary metabolites that are biologically active against various insect pests (Akinkurolere 2007; Adesina et al. 2012; Awosolu et al. 2018). These bioactivities included stomach poison, feeding deterrent, repellent, oviposition deterrent, or inhibitory and growth regulatory effects. So far, only a few of botanical insecticides were commercialized, in spite of rich plant resources that occur, particularly in tropical and subtropical countries (El-Massad et al. 2012).

Datura metel L. belongs to the Solanaceae family, and commonly found in Asia and Africa. It is known as thorn or devil’s apple, Jimson weed, angel's trumpet, Myaramuo (Igbo), Zakami (Hausa) and Apikan (Yoruba) (Abdullahi et al. 2003). Datura metel is an herbaceous, leafy plant which can grow up to 200 cm in height. The leaves are alternate and can be up to or more than 26 cm long and 21 cm wide. Datura metel is believed to be a medicinal plant widely used in phytomedicine to cure diseases and various parts of the plant (leaves, seeds, roots, and fruits) are used for different purposes in herbal medicine (Imo et al. 2019). In view of this, the objective of this study is to evaluate the susceptibility and reproductive capabilities of Callosobruchus maculatus (Coleoptera: Bruchidae) to Datura metel extracts.

Materials and methods

Collection and preparation of Datura metel extracts

Fresh leaves of D. metel were collected from a refuse dump site in Owo, Ondo State. A Sample of the plant was identified at the herbarium, Department of Forestry and Wood Technology, Rufus Giwa Polytechnic, Owo, Ondo State, Nigeria where specimens of the plant were deposited for further reference. The plant samples were air-dried for a period of two weeks in a shade. The crisp dried plant leaves were then pulverized into powder using an electric milling machine. Thereafter, the powder was further sieved with 1mm2 perforations and were packed in black ploythene bags and stored in a cool dry place prior to use.

Pulverized plant leaves (200 g) were used for the extraction in a Soxhlet apparatus for 7—8 h sequentially with a series of solvents of increasing polarity namely: hexane, ethyl acetate, chloroform, acetone, and methanol followed by evaporation until the solvent was dried. The extracts were collected and stored in an amber coloured glass vial and then kept in the refrigerator at 4 °C with proper labeling prior to use.

Rearing and maintenance of Callosobruchus maculatus

Stock culture of Callosobruchus maculatus used was obtained from naturally infested grains sourced from food grain dealers. The insects were reared and maintained on cowpea grains in a kliner jar under ambient laboratory conditions of temperature 30 + 2 °C, relative humidity 65 + 5% and 12 h light and darkness regime (Adesina et al. 2016). Adult insects in the jar were maintained for seven days to mate and oviposit and were subsequently removed from the jar. From this stock culture, subcultures of the insect were reared on a local susceptible variety of cowpea in 1L plastic jars covered with muslin cloth, secured firmly by a rubber band in the laboratory to allow for aeration and prevent the escape of insect in order to obtain insect of the same age (1–2 days old). Insects used for the bioassay were taken from this stock.

Source of cowpea seeds

Cowpea seeds were purchased from a local market in Owo, Ondo State. Uninfected cowpea seeds with no feeding holes were selected and bagged in a polyethylene bags prior to experiments, the cleaned cowpea seeds were disinfected by storing in a deep freezer at -5 °C for one week to kill any immature stages or egg laid (if any). Twenty-four hour prior to research, the stored cowpea was removed from the freezer and air-dried on the laboratory bench to attain the normal room temperature so as to prevent mouldiness (Olotuah et al. 2007).

Toxicity of Datura metel extracts on survival of Adult Callosobruchus maculatus

Each plant extract was admixed at 50, 100 and 150 µl/20 g (2.5, 5 and 7.5 µl/g) of clean and uninfected cowpea seeds in a Petri dish respectively. The cowpea seeds and extracts were thoroughly mixed with a glass rod for about 2 min to ensure uniform coating of the seeds. Petri dishes were left open for 2 min to ensure dryness of the extracts (Adesina and Mobolade-Adesina 2016). Thereafter, 10 unsexed newly emerged adult C. maculatus of 1–2 days old were introduced. A negative control made of untreated seeds was also set up. The Petri dishes were covered with Petri plates, sealed with paraffin wax and were arranged in the laboratory at room temperature. The whole experiment was replicated three times. The survival of the C. maculatus adult was recorded after every 24 h for four (4) days. At each observation the dead insects were removed, counted and recorded to calculate percentage adult survival. Insects were considered dead on failure to respond to pricking using a safety pin. The experiment was maintained under ambient laboratory conditions of temperature 30 + 2 °C, relative humidity 65 + 5% and 12 h light and darkness regime (Adesina et al. 2016).

Toxicity of Datura metel extracts to Callosobruchus maculatus reproductive potential

After 96 h of C. maculatus infestation from contact toxicity as described above under same conditions, all living insects were removed, and the dishes with treated seeds and control were kept undisturbed in the laboratory for hatching adult emergence to take place. Inhibition of adult C. maculatus reproductive potential by D. metel extracts was determined based on the number of eggs laid and percentage egg hatchability at 4 and 6 days after infestation respectively (Howe and Currie 1964) and emergence of F1 progeny. The number of eggs laid on treated and control seeds were counted using a hand lens, and percentage reduction in the number of eggs laid was calculated as follows:

$$\mathrm{\%\ Reduction}= \frac{\mathrm{A}-\mathrm{B}}{\mathrm{A}} \mathrm{x }100$$

A = Number of eggs laid in control dish, B = Number of eggs laid in treated dish. Total number of eggs hatched were counted 6 days after removal of living insects, to determine the percentage egg hatchability. Hatched eggs were determined based on the presence of larval frass (Arun et al. 2001) and changed in transparent egg or fresh egg colour into white opaque and presence of excreta within the egg chorion (Sushmita et al. 2019).

$$\mathrm{\%\ egg\ hatchability}= \frac{\mathrm{Mean\ number\ of\ eggs\ hatched}}{\mathrm{mean\ number\ of\ eggs\ laid}} \times {100}$$

On the same experimental units, at 28 days after treatment (DAT) the number of adults emerged from the treated cowpea was counted and recorded. Percentage adult emergence reduction was calculated thus;

$$\mathrm{\%\ Adult\ reduction\ emergence}= \frac{\mathrm{C}-\mathrm{D}}{\mathrm{C}} \times 100$$

C = Number of emerged adults from control dish, D = Number of emerged adults from treated dish (Adesina et al. 2015).

Effect of Datura metel extracts on percentage infestation and weight loss

The grain damage (percentage infestation and tolerance) was determined by counting damaged seeds and adult emergence holes on the surface of the stored grains, 28 days after infestation (Adesina and Mobolade-Adesina 2020). Data for percentage infestation and weight loss were collected after sieving the treated and control dishes of insects and frass through a 3 mm sieve (Idoko and Adesina 2013).

$$\mathrm{\%\ infestation}= \frac{\mathrm{number\ of\ seeds\ with\ adult\ exit\ hole}}{\mathrm{total\ number\ of\ seeds }} \times \frac{100}{1}$$

(Deshpande et al. 2011)

$$\mathrm{\%\ insect\ tolerance}= \frac{\mathrm{Number\ of\ seeds\ without\ adult\ exit\ hole}-\mathrm{Number\ of\ seeds\ with\ adult\ exit\ hole}}{\mathrm{Number\ of\ seeds\ with\ adult\ exit\ hole }} \times \frac{100}{1}$$

(Adesina et al. 2020).

The weight loss (%) of cowpea seeds in the treated and control sets was calculated after reweighing of the seeds at the termination of the experiment:

$$\mathrm{\%\ weight\ loss}=\frac{\mathrm{initial\ weight}-\mathrm{final\ weight}}{\mathrm{initial\ weight }} \times \frac{100}{1}$$

(Ileke and Oni 2011)

Statistical analysis

The experiment was laid out in a Completely Randomized Design with three replications. The effectiveness of D. metel extracts on insect infestation and damage was determined by calculating mortality (%), reduction in egg laid (%), egg hatchability (%), adult emergence (%), tolerance (%) infestation (%) and weight loss (%) of treated and control. All the data were reported as mean value ± SE. The statistical analysis was performed by one-way analysis of variance (ANOVA) and significant treatment means were compared by using Duncan New Multiple Range Test (DNMRT) at the 5% level of significance (P < 0.05). Prior to ANOVA, arcsine transformation (x /100) was applied to all calculated percentages to meet the conventions of normality and homogeneity of variance (Asiry and Zaitoun 2020).

Results

In Table 1, results shown the contact toxicity of D. metel extracts on adult C. maculatus survival. At 24 h exposure, insects treated with D. metel ethyl acetate extract had the highest survival recorded in 2.5, 5 and 7.5 µl/g (33.17, 46.67 and 71.57% respectively). There was no significant difference (p > 0.05) in C. maculatus survival when reared in contact with extract of acetone (50.23%), methanol (50.51%) and chloroform (50.75%) D. metel extracts using 5 µl/g at 48 h exposure time and also, 63.43, 63.29 and 62.29% respectively for 72 h exposure. At 96 h exposure, D. metel ethyl acetate extract applied at 50 µl achieved highest adult C. maculatus survival rate (93.33%). The result shows non-significant difference (p > 0.05) between the survival rate attained by D. metel ethyl acetate (93.33%) and chloroform extracts (93.33%) at 5 µl /g (96.66%) and 7.5 µl/g (100%) respectively.

Table 1 Effect of contact toxicity of Datura metel extracts on adult Callosobruchus maculatus survival
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Lethal dose of D. metel extract that kill 50% of C. maculatus was presented in Fig. 1. The least LD50 (0.202 µl) recorded was in the D. metel ethyl acetate extract at 96 h after exposure, followed by methanol extract (9.45 µl) at 72 h.

Fig. 1
figure 1

LC50 of D. metel extract on C. maculatus

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For the dosage 2.5 and 5 µl/g (Table 2), methanol extract of D. metel was the highest in reducing egg laying (59.92 and 59.31%). While, D. metel ethyl acetate extract at 7.5 µl/g was the highest and significantly different (p <0.05) from other extracts. Ethyl acetate extract of D. metel achieved the lowest percentage egg hatchability recorded for all the measurement used (15.0% (50 µl), 22.63% (5 µl/g) and 23.89% (150 µl)), with significant difference (p < 0.05) from other extract. The highest percentage reduction of C. maculatus adult emergence was recorded in cowpea treated with 2.5 µl/g ethyl acetate (50.51%) extract of D. metel but not significantly different (p > 0.05) from chloroform (46.06%), methanol (45.33%), and acetone (48.55%) extracts of D. metel respectively Percent Reduction in adult emergence was highest with the D. metel methanol extract in 5 µl/g (89.33%) and 7.5 µl/g (91.67%).

Table 2 Effect of contact toxicity of Datura metel extracts on Callosobruchus maculatus reproductive capabilities
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Contact toxicity of D. metel extracts on grains’ susceptibility to infestation is presented in Table 3. In 2.5 µl/g, D Datura metel extracts of hexane (13.39%), ethyl acetate (16.31%), chloroform (15.47%) and control (16.49%) exhibited non significantly difference (p > 0.05) but exerted significantly difference (p < 0.05) from D. metel acetone (9.03%) and methanol extracts (8.52%). Likewise, acetone and methanol extracts of D. metel was not significantly different (p > 0.05) from each other but grains treated with D. metel methanol extract had the lowest percentage infestation in 5 and 7.50 µl/g (8.22 and 6.01%). Percent tolerance was lowest with ethyl acetate extract of D. metel (72.80% and 58.67%) in 2.5 and 7.5 µl/g respectively. Meanwhile, D. metel methanol extract was lowest (74.61%) in 5 µl/g, but not significantly different (p > 0.05) from ethyl acetate (76.69%) and chloroform (76.06%) extracts of D. metel.

Table 3 Effect of contact toxicity on Callosobruchus maculatus susceptibility to infestation on stored grains
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In Table 4, the lowest percentage weight loss of treated grain in 2.5 µl/g was methanol extract of D. metel (4.41%) but not significantly different (p > 0.05) from hexane (5.43%) and ethyl acetate (5.83%) extracts of D. metel. In 5 µl/g, percentage weight loss ranges from 3.33% (chloroform) to 21.75% (control). The lowest percentage weight loss (2.50%) was recorded in 7.5 µl/g chloroform D. metel extract followed by methanol D. metel extract (2.57%) and ethyl acetate D. metel extract (2.87%) with no significant difference (p > 0.05).

Table 4 Effect of D. metel extracts on percentage weight loss of treated grains
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Discussion

Plant materials are regarded as a storehouse of bioactive chemical compounds that can serve as an alternate source of insect control measures to synthetic insecticides or fumigants in order to guarantee food security in third-world countries where the bulk of food crop production lies in the hands of resource-poor subsistence farmers, that lack modern storage know-how.

D. metel shows contact toxicity against C. maculatus in this study, as every parameter considered was significantly different from the untreated grains. This congruent with other reports on Datura spp as a potential bioinsecticide against stored grain pests (Habib et al. 2011; Nilesh et al. 2016; Obadofin et al. 2018). This particular ability is suggested to be due to the phytochemical content of this plant, has its solvent extract contains alkaloids, tannins, cardiac glycosides, flavonoids, carbohydrates, amino acids, and phenolic compounds (Al-Snaf 2017; Krishnan et al. 2017). Some of these metabolites might be toxic to the insect body system by blocking the ion channels, inhibit enzymes, or interfere with neurotransmission, loss of coordination, and death (Aniszewski 2007).

In this study, different solvents were employed to extract metabolites from D. metel against C. maculatus survival. Within 24 and 96 h of exposure, ethyl acetate extract of D. metel achieved the highest mortality recorded, followed by methanol. The polarity of plant material in a particular solvent also determines its effectiveness against an insect pest. This determine the level in which bioactive metabolites in the plant will be fully extracted. The mortality rate of different D. metel extracts on C. maculatus after 96 h of exposures is ethyl acetate > methanol > acetone > hexane > chloroform. Highest mortality of ethyl acetate extract is in accordance with Adesina and Mobolade-Adesina (2020) where an ethyl acetate extract of Anchomanes difformis achieved highest mortality when compare with acetone, hexane and methanol extract. Also, Khalequzzaman and Islam (1992) reported that the methanolic extract of D. metel leaf was more toxic than other extracts on T. castaneum.

The methanolic extract of D. metel effectively reduced egg laid at lower measurement while ethyl acetate was most effective at the highest measurement level. Botanical has been reported to affect the oviposition activity of insect pests of the stored products (Adesina and Ofuya 2015). This may be due to the changes induced by toxic metabolites in the physiology and behaviour of the adult which now reflect on their egg-laying capacity. This deterrent activity range was 30.56—83.38% and this was within the range of 12.31—84.66% reported by Jayakumar (2010) when considering different plants extract against C. maculatus.

It was also observed that egg hatchability was obviously affected by ethyl acetate extract of D. metel followed by chloroform. Reduction in egg hatchability was also observed by Khani et al. (2013) and it was attributed to phytochemical action of the compound present in the plant extract. Contact toxicity of ethyl acetate extract of D. metel also induced much reduction on adult emergences at the lowest measurement used while methanol extract attained the highest adult emergence reduction at higher measurement. The plant oil could as well block the air passage (micropyle region) of the egg chorion, leading to embryo death as a result of oxygen depletion and therefore reduces adult emergence (Pandey et al. 2011). Akinneye et al. (2018) examined the contact toxicity of Cleistopholis patens and Eugenia aromatica powders on Sitophilus zeamais eggs, the powder significantly reduced the number of adults that emerged. It was further revealed from this study, that the percentage infestation of C. maculatus was mostly reduced by methanol extract of D. metel in all measurements. Also, ethyl acetate also achieved the lowest infestation at 150 µl and had the least tolerance C. maculatus, followed by methanol.

There was a significant difference in the percentage weight loss on cowpea treated (8.33–2.50%) and untreated (21.75%). This can be related to the adult emergence and percentage infestation observed in this study, which is as a result of antifeedant, repellent and suppressant properties possess by D. metel. These three aforementioned properties debar the insect from making contact or having a bite on the grain after the plant ingestion (Haghighian et al. 2008).

Conclusion

The contact toxicity of D. metel against C. maculatus and its potential to protect cowpea grain have been studied. Ethyl acetate has proven to be most effective among the solvent used for extraction. This study suggest the possibility of using these extracts as cowpea natural protectant against C. maculatus. Further studies are recommended to identify the bioactive compounds responsible for the insecticidal activity present in the plant and also to know the site of action in the insect body system.