Evaluation of Calpurnia aurea leaf extracts as natural insect repellents for stored product insect pests in Ethiopia

Abstract

The repellency of solvent extracts of Calpurnia aurea leaves was evaluated on the maize weevils and the red flour beetles. Nine-cm Whatman number 1 filter paper that is partitioned into treatment, neutral and non-treatment portions was used for the experiment. Each solvent extract treatment was applied at 2.5, 5 and 10% rates. Insects were released in the neutral portion, while the untreated part served as a control. A choice bioassay experimental method that is laid down in a completely randomized design within three replications was used. 5 and 10% rates of the polar solvent extracts of Calpurnia aurea leaves induced significantly (p < 0.05) higher percentage weevils and beetles repellency (≥50.00%) at 2 days after treatment than non-polar and partial polar solvent extract treatments and the untreated check. 10% dosage of Calpurnia aurea leaves’ polar solvent extract treatments caused 100% weevils and beetles repellency three days after treatment. Consequently, the polar solvent extracts of the tested plant were potent and could be used at 5 and 10% rates for the maize weevils and the red flour beetles’ management under farmers’ maize storage conditions. Nevertheless, their impact on cost-effectiveness, natural enemies and human beings needs additional study.

KEYWORDS:

• Calpurnia aurea
• cereals
• repellency
• Sitophilus zeamais
• stored maize
• Tribolium castaneum

Introduction

Maize is the third most vital cereal crop in the world next to rice and wheat in terms of both consumption and area-grown (Kornher 2018). It is also the most extensively cultivated staple crop in Sub-Saharan Africa (SSA), accounting for a larger area coverage of the farmland each year (Jacob 2018; Santpoort 2020). It could play a crucial role in the transformation of Sub-Saharan Africa’s agriculture, reducing poverty and improvement of food security (Erenstein and Kassie 2018).

Maize is also the most widely grown staple crop by poor farmers in Ethiopia. It has been cultivated for different purposes, including for human and animal consumption, for generating income, for construction and other uses. But, insect pests both in the field and storage have been affecting the yield, quality, quantity and market value of maize (Demissie et al. 2008; Hiruy 2018; Fufa et al. 2021). In addition, the storage insect pests are the most harmful of pests, as their loss is none recompensed (Chomchalow 2003; Hiruy and Getu 2018a). Not only in Ethiopia but also in all tropical worlds, storage insect pests are important problems that pose both quantitative and qualitative loss of food grains, including maize (Chebet et al. 2013). Of these storage pests, Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae) and Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae), respectively are revealed to be the most damaging primary and secondary pests that have been causing considerable loss of maize grains (Srivastava and Subramanian 2016; Hiruy 2018; Hiruy and Getu 2020). Management of these insect pests has heavily relied on the use of synthetic insecticides and fumigants (Jahromi et al. 2012; Goudoungou et al. 2018; Laizer et al. 2019; Mubayiwa et al. 2021) to which negative attributes such as environmental pollution, health risk to human and non-target organisms, among others have been associated for years (Cherry et al. 2005; Gu et al. 2008; Cosimi et al. 2009; Ofuya and Longe 2009; Sousa et al. 2009; Harish et al. 2013; Goudoungou et al. 2018; Mubayiwa et al. 2021). These impacts of synthetic pesticides have been commencing the exploration of bio-rational pest management options such as botanicals (Karunaratne and Karunaratne 2015; Hiruy 2018; Hiruy and Getu 2020).

Thus, protection of stored food grains such as maize from insect pests by designing and implementing affordable, effective, safe and environmentally sound management options such as repellent botanicals is critical and timely to ensure a constant supply of food and feed for resource-poor farmers of Ethiopia at stable prices year round. Isolation of homegrown effective botanicals is also indicated to serve as a cheap, safe, eco-sound and sustainable management alternative to synthetic insecticides for stored grains protection from insect pests (Hiruy 2018; Hiruy and Getu 2020).

Accordingly, an investigation of phytochemical analysis of Calpurnia aurea revealed that the leaves and roots are a chief source of terpenoids, saponins, tannins, flavonoids, steroids, glycosides and alkaloids (Nega et al. 2016; Kemal et al. 2020). This plant belongs to the subfamily Papilionoideae of the family Fabaceae. It is a small 3–4 m tall, multi-stemmed tree, which is widely grown in highland areas of Ethiopia (Birhanu and Asale 2015).

C. aurea is usually used in traditional medicine to treat different disorders and parasitic infections of animals and humans nationally. It is used to treat stomach disorders, amebic dysentery and eye diseases (Hiruy 2018). Its potency as a toxicant in the control of storage pests has also been reported (Blum and Bekele 2002; Hiruy 2018; Hruy and Getu 2018b). Nonetheless, information on the repellent potency of C. aurea in managing storage insect pests, in general, and weevils and beetles, in particular, is lacking nationally, and yet to be investigated. Moreover, the extract quantity and quality, the velocity of the extraction, the repellency, toxicity and anti-feedant activities as well as the biosafety of plant extracts were reported to be greatly influenced by the type and polarity of the solvent, and the plant parts used for extraction (Rafińska et al. 2019; Wakeel et al. 2019; Zhang et al. 2019). Extraction of toxicant and repellent constituents from botanicals was also shown to be affected by different factors, including solvent polarity and concentration, as well as temperature and time. Based on chemical composition, solvents of different polarities were used for extracting various plant phytochemicals, as no single solvent might be reliable to extract all secondary plant metabolites with repellent and toxicant effects (Nawaz et al. 2020). Consequently, the identification of an effective solvent to test the efficacy of botanicals against stored grain insect pests is considered to be very vital. Therefore, this study was initiated to evaluate the repellency of solvent extracts of Calpurnia aurea leaves on the maize weevils and the red flour beetles under laboratory conditions in Ethiopia.

Materials and methods

The study area and period

The study was conducted in the Insect Science Insectary of Addis Ababa University of Zoological Sciences Department of Life Sciences Faculty of Collage of Natural Sciences of Ethiopia between 1 October 2016 and 30 June 2017.

Collection, identification and preparation of the plant materials

The plant leaves (Figure 1) were collected from the Hadiya Zone of Southern Ethiopia (Figure 2) and identified as Calpurnia aurea species at the Life Sciences Faculty Herbarium of Addis Ababa University. In three weeks, known weight the fresh leaves of C. aurea were dried under shade in a well-ventilated room and powdered into fine texture via pestle and mortar. Then the powder was sieved to remove larger material and to get a more fine texture for solvent extraction (Figure 1) (Tekie 1999; Jembere 2002; Hiruy and Getu 2020).

Figure 1. Partial view of Calpurnia aurea: (a) tree in maize farm and (b) fresh leaves collected from the study area, (c) leaves dried under the shed and leaf powders for solvent extracts.

Figure 2. Map of the study site in Hadiya and Silte zones in Southern Ethiopia.

Mass culture of the test insects

Adults of the maize weevils and the red flour beetles obtained from farmers’ storages (Figure 2) were cultured at 27 ± 3°C and 55–70% RH in Insect Science Insectary of Addis Ababa University (Jembere et al. 1995; Zewde and Jembere 2010; Hiruy 2018). Shone variety of maize grains was used in culture, as it is the most commonly grown variety in the area and is considered to be susceptible to insect infestation. Before use in the raring experiment, the grains were kept in a deep freezer at −20 ± 2°C for two weeks to disinfect them from egg larvae, pupa and adults of any insect (Gemechu et al. 2013; Hiruy 2018). Besides, the grains were mechanically damaged in a subtitle manner for culturing the red flour beetles, as beetles cannot feed on sound grains. Fifteen pairs of the adult test insects then were placed in 12 one-liter glass jars containing 200 g of maize grains (Zewde and Jembere 2010; Hiruy 2018; Hiruy and Getu 2020). The jars were then covered with nylon mesh and held in a place with rubber bands to allow ventilation and prevent the escape of the experimental insects. After an oviposition period of 13 days, all the parental insects were removed from the grain through the sieve. The jars were then kept until the emergence of F1 progeny under the laboratory condition mentioned above. Up on emergence after 30 days, three to seven days old unsexed adults of the F1 progenies were sieved and used in the experiment.

Experimental design and repellency bioassay

The experiment was laid down in a completely randomized design (CRD) in three replications in a factorial arrangement. A choice bioassay method was used to conduct the repellency test (Talukder and Howse 1995; Jahromi et al. 2012; Hiruy and Getu 2020). 2.5%, 5.0% and 10.0% solutions of solvent extracts were prepared by diluting the dry fine powder of the test plant in different solvents with different polarity (N-hexane (partial polar), chloroform (non-polar) and ethanol, acetone, methanol, distilled water (polar). Nine-cm Whatman number 1 filter paper that is divided into treatment and non-treatment (control) portions (3.5 cm each), and the central neutral portion (2 cm) was used in the experiment (Figure 3) (Hiruy and Getu 2020). The partitioned filter paper discs were placed inside the Petri dishes. Then, in each non-treatment and treatment portion of the Petri dishes, three grams of the maize grains (slightly damaged for the red flour beetles and sound for the maize weevils) were placed, leaving the central neutral portion free. Using a pipette, the grains in one-half of the Petri dishes (treatment portion) were treated with 1 ml solutions of the respective plant solvent extracts, leaving the other side (untreated) as the control side (Figure 3). Three to seven days old ten unsexed experimental insects were released at the central portion of each Petri dish, after air-drying the treated halves for 30 min to allow complete evaporation of the solvents. Subsequently, the Petri dishes were covered with nylon mesh and held in a place with rubber bands and kept at 25–30°C, temperature and 65–70%, relative humidity (Jembere et al. 1995; Zewde and Jembere 2010). The number of insects found on the control (Nc) and treated (Nt) parts of the Petri dishes were counted at an interval of 12 h for three days. Percentage repellency (PR) was computed by the following formula (Thein et al. 2013; Loko et al. 2017; Estelle et al. 2018; Hiruy and Getu 2020; Gitahi et al. 2021). $\text{PR = }[(\text{NC}-\text{NT})\text{ / }(\text{NC + NT})]\times 100,$where Nc was the number of insects in the untreated area after the exposure interval and Nt was the number of insects in the treated area after the exposure interval. The mean percent repellency of each treatment was apportioned to the repellent classes ranging from 0 to V; Class 0 (PR ≤ 0.1%), Class I (PR  = 0.1–20%), Class II (PR = 20.1–40%), Class III (40.1–60%), Class IV (60.1–80%) and Class V (80.1–100%) (Loko et al. 2017; Estelle et al. 2018).

Figure 3. Partial view of the Petri dish chamber used for repellency test of solvent extracts of leaf powder of Calpurnia aurea on the maize weevils and the red flour beetles.

As adopted in prior studies (Nerio et al. 2009; Loko et al. 2017; Rajabpour et al. 2018; Estelle et al. 2018), the index of repellency (IR) was calculated by the following formula: $\text{IR }(\text{index of repellency})=\frac{2\text{T}}{\mathrm{T}+{\mathrm{C}}^{\mathrm{\prime }}}$where C and T represent the percentage of insects in the untreated and treated areas of preference, respectively. The IR values are between 0 and 2 (Gusmão et al. 2013) with, IR = 1 indicating a similar repellency (neutral treatment), IR > 1 indicating a lower repellency (being attractant) and IR < 1 representing a greater repulsion (Nerio et al. 2009; Loko et al. 2017; Estelle et al. 2018; Rajabpour et al. 2018; Gitahi et al. 2021).

Data analysis

Microsoft Excel version 2013 software (Microsoft Corporation, Albuquerque, New Mexico, U.S.) system was used for compiling the data on the percent repellency of the maize weevils and the red flour beetles due to the tested botanical leaf solvent extract treatments at different hours after treatment. Then the data were subjected to analysis of variance (ANOVA) of the Statistical Program for Social Sciences (SPSS) version 2016 for Windows (IBM, Chicago, USA). Two-way ANOVA or univariate analysis of the general linear model was employed for analysis, as there were two independent variables (solvent extract treatments and time). Significant differences between means of different treatments of different solvent extracts of Calpurnia aurea leaves applied at different rates at different times after treatment were separated using Turkey’s studentized (HSD) test at a 5% confidence interval. Correlation between the treatments of solvent extracts of C. aurea leaves applied at different rates at different hours after treatment application and the percent repellency was determined using Pearson’s correlation of the SPSS program of version 16.

Result

The repellent effects of the tested botanical treatment on the tested insect pests differed significantly with the difference in polarity of solvent extracts, dosage and exposure time after treatment. Repellency of the maize weevils and the red flour beetles was increased with an increase in dosage, polarity and exposure time after the treatment of tested botanical solvent extracts (Table 1, and Figures 4–6).

Figure 4. Mean percent repellency of (a) the maize weevils and (b) the red flour beetles due to Calpurnia aurea leaves’ solvent extract treatment at a 2.5% rate. N.B. Means with different colors followed by error bars (±SE) at different days after treatment application are significantly different at p < 0.05, SE = standard error of the mean, h = hour.

Figure 5. Mean percent repellency of (a) the maize weevils and (b) the red flour beetles due to Calpurnia aurea leaves’ solvent extract treatment at a 5% rate. N.B. Means with different colors followed by error bars (±SE) on different days after treatment application are significantly different at p < 0.05.

Figure 6. Mean percent repellency of (a) the maize weevils and (b) the red flour beetles due to Calpurnia aurea leaves’ solvent extract treatment at a 10% rate. N.B. Means with different colors followed by error bars (±SE) on different days after treatment application are significantly different at p < 0.05.

Table 1. Cumulative mean percent repellency and the repellency index of the maize weevils and red flour beetles due to Calpurnia aurea leaves’ solvent extract treatment at different rates at different times after the treatment.

Pests tested for repellency	The solvent used for extraction	Rate	Cumulative mean % repellency within 12–72 h after treatment	Repellency class	IR (Index of Repellency)	Classification
The maize weevil	Ethanol	2.5%	24.17 ± 3.37^d	II	0.76 ± 0.23^b	Repellent
		5%	48.33 ± 5.62^c	III	0.52 ± 0.29^b	Repellent
		10%	83.33 ± 4.15^a	V	0.17 ± 0.04^b	Repellent
	Acetone	2.5%	20.83 ± 3.13^e	I	0.79 ± 0.25^b	Repellent
		5%	44.17 ± 5.57^c	III	0.56 ± 0.38^b	Repellent
		10%	80.83 ± 4.85^a	V	0.19 ± 0.06^b	Repellent
	Methanol	2.5%	16.67 ± 3.10^e	I	0.84 ± 0.24^b	Repellent
		5%	40.83 ± 5.84^c	III	0.59 ± 0.41^b	Repellent
		10%	78.33 ± 4.90^b	IV	0.22 ± 0.04^b	Repellent
	Distilled water	2.5%	12.50 ± 2.79^e	I	0.87 ± 0.07^b	Repellent
		5%	36.67 ± 5.95^c	III	0.65 ± 0.24^b	Repellent
		10%	75.83 ± 5.44^b	IV	0.24 ± 0.06^b	Repellent
	N-hexane	2.5%	0.00 ± 0.00	0	1.00 ± 0.58^a	Neutral
		5%	0.00 ± 0.00^f	0	1.00 ± 1.00^a	Neutral
		10%	1.67 ± 1.12^f	0	1.00 ± 0.58^a	Neutral
	Chloroform	2.5%	0.00 ± 0.00^f	0	1.00 ± 1.00^a	Neutral
		5%	0.00 ± 0.00^f	0	1.00 ± 0.58^a	Neutral
		10%	0.83 ± 0.83^f	0	1.00 ± 1.00^a	Neutral
The red flour beetle	Ethanol	2.5%	30.00 ± 3.92^d	II	0.70 ± 0.30^b	Repellent
		5%	58.33 ± 5.62^c	III	0.42 ± 0.29^b	Repellent
		10%	87.50 ± 3.92^a	V	0.12 ± 0.08^b	Repellent
	Acetone	2.5%	26.67 ± 3.34^c	III	0.73 ± 0.18^b	Repellent
		5%	54.17 ± 5.57^c	III	0.46 ± 0.03^b	Repellent
		10%	84.17 ± 4.17^a	V	0.16 ± 0.10^b	Repellent
	Methanol	2.5%	20.83 ± 2.60^d	II	0.79 ± 0.20^b	Repellent
		5%	50.84 ± 5.84^c	III	0.49 ± 0.29^b	Repellent
		10%	81.67 ± 4.24^a	V	0.18 ± 0.02^b	Repellent
	Distilled water	2.5%	15.83 ± 2.60^e	I	0.84 ± 0.15^b	Repellent
		5%	46.67 ± 5.95^c	III	0.53 ± 0.40^b	Repellent
		10%	78.33 ± 5.06^b	IV	0.22 ± 0.04^b	Repellent
	N-hexane	2.5%	0.00 ± 0.00^f	0	1.00 ± 1.00^a	Neutral
		5%	0.00 ± 0.00^f	0	1.00 ± 0.58^a	Neutral
		10%	1.67 ± 1.12^f	0	1.00 ± 1.00^a	Neutral
	Chloroform	2.5%	0.00 ± 0.00^f	0	1.00 ± 0.58^a	Neutral
		5%	0.00 ± 0.00^f	0	1.00 ± 1.00^a	Neutral
		10%	0.83 ± 0.83^f	0	1.00 ± 0.58^a	Neutral

Means followed by the same letter within the column are not significantly different at p ≤ 0.01.

Accordingly, the distilled water, methanol, ethanol and acetone (the polar solvents’ extract treatments) of C. aurea applied at all rates induced significant (p < 0.05) mean percent (≥6.67%) repellency of the maize weevils and the red flour beetles in comparison to non-polar and partial polar solvent extract treatments and the untreated check at all days (1, 2 and 3 days) after treatment. Of which ethanol extract treatments applied at higher rates (5% and 10%) caused higher (≥ 43.33%) percent repellency (class III repellency, ranging from 40.1 to 60%) of the maize weevils and the red flour beetles than the treatment applied at a lower rate (2.5%) at one day after treatment. Following ethanol extract treatments, high percent repellency of the maize weevils and the red flour beetles was caused by acetone (≥ 36.67%) (Class II) and methanol (≥ 33.33%) (Class II) extract treatments of C. aurea leaves applied at 5% and 10% rates at one day after treatment than the treatment applied at a rate of 2.5%. Water extract treatments of C. aurea leaves applied at rates of 5% and 10% also caused substantial (≥ 26.67%) (Class II) percent repellency of the maize weevils and the red flour beetles at one day after treatment than the treatment at the rate of 2.5%. These class III and II repellency suggests the induction of substantial repellency by polar solvent extract treatments of C. aurea against the test insects at 5 and 10% dosage at one date after treatment (Table 1 and Figures 4–6).

Nevertheless, all polar solvent extract treatments of C. aurea leaves applied at dosages 5% and 10% induced significantly (p < 0.05) higher (≥50.00%) percent repellency of the maize weevils and the red flour beetles at two days after treatment (≥ class III repellency), implying higher repellency of the tested botanical treatments on the maize weevils and the red flour beetles at 5 and 10% rates. Of which ethanol extracts caused higher (≥60.00) (Class IV) repellency, followed by acetone (≥56.67%) (Class III) and methanol (≥53.33%) (Class III), while water extract induced considerable repellency (≥50.00%) (Class III) (Table 1 and Figures 4–6). The index of repellency ranged from 0.12 to –0.87 for polar solvent extract treatments of C. aurea leaves, indicating possession of a great repellant activity of all polar solvent extract treatments against the test insects (Table 1). Furthermore, all polar extract treatments of C aurea leaves applied at rates of 10% caused 100% (Class V) repellency of the maize weevils and the flour beetles three days after treatment (Figures 4–6).

The correlation between the treatments of C. aurea leaves applied at different dosages, and the percent repellency of the maize weevils was highly (p < 0.01) significant and strongly negative (r is −0.604**) (Table 2). The correlation between the treatments of C. aurea leaves applied at different rates, and the percent repellency of the red flour beetles was also highly (p < 0.01) significant and strongly negative (r is −0.650**) (Table 3).

Table 2. Correlation among percent repellency efficacy determining parameters of Calpurnia aurea leaves’ solvent extracts on the maize weevils.

Efficacy parameters	Solvent extracts	Rate	Time after treatment	Percent repellency
Solvent extracts	1	0.000	0.000	−0.604**
Rate	0.000	1	0.000	0.511**
Time after treatment	0.000	0.000	1	0.260**
Percent repellency	−0.604**	0.511**	0.260**	1

Correlation coefficients with two asterisks (**) represent the highly significant association at P values < 0.01 (two-tailed) and those without asterisks are non-significant.

Table 3. Correlation among percent repellency efficacy determining parameters of Calpurnia aurea leaves’ solvent extracts on the red flour beetles.

Efficacy parameters	Solvent extracts	Rate	Time after treatment	Percent repellency
Solvent extracts	1	0.000	0.000	−0.650**
Rate	0.000	1	0.000	0.474**
Time after treatment	0.000	0.000	1	0.240**
Percent repellency	−0.650**	0.474**	0.240**	1

Correlation coefficients with two asterisks (**) represent the highly significant association at P values < 0.01 (two-tailed) and those without asterisks are non-significant.

Discussion

The repellency of the tested botanical treatments on weevils and beetles varied significantly with a difference in dosage and exposure time, and it was increased with an increase in dosage and exposure time after treatment. This might be due to an increase in the concentration of active metabolites and the associated volatiles of the tested botanical as the dosages of plant extract treatments are increased. Consistently, Gitahi et al. (2021) revealed dose-dependent repellency activities of T. diversifolia and V. lasiopus extracts on weevils. Dose-dependent repellency of methanol extracts of the leaves of Cinnamomum tamala, Rosmarinus officinalis, and Pelaragonium graveolens on red flour beetles was also confirmed (Manonmani et al. 2017). Concentration and exposure time-dependent increment on the repellent effect of A. indica, T. vogelii, and L. camara powders against adult P. truncates was also reported (Chebet et al. 2013). Karakas (2016) showed the dosage and solvent polarity-dependent mortality of weevils by leaf extracts of dill, Anethum graveolens and purple basil, Ocimum basilicum. The mortality of ticks was shown to increase with the increased exposure time after treatment and dose of C. aurea (Kemal et al. 2020). Concentration, solvent polarity and exposure time-dependent weevils mortally due to C. aurea leaves’ solvent extract treatments were also confirmed (Hruy and Getu 2018b).

Percent repellency of the maize weevils and the red flour beetles due to the tested botanical leaf extract treatments also varied with the polarity (type) of solvent used in extraction; the highest being in ethanol extracts, followed by extracts in acetone and methanol, while water extract induced considerable repellency. Broad organic compounds’ solubility properties in ethanol might be a contributor to the relatively higher repellant efficacy of the tested botanical leaf ethanol extracts. Consistently, Mulatu (2020) revealed that the percent yield of the plant extracts varies from solvent to solvent due to their different polarities and extracting potential. According to him the great efficacy of ethanol extract of C. aurea leaves in antibacterial activity is attributed due to ethanol’s ability in dissolving the polar and nonpolar phytochemical constituents (i.e. broad solubility property of ethanol). Of various solvents such as water, ethanol, acetone and petroleum ether used for plant extract preparation, ethanol is revealed to be an accepted solvent for its broad solubility properties (Amoh-Amoah 2010). Hruy and Getu (2018b) also indicated concentration, solvent polarity and exposure time-dependent weevils mortally due to C. aurea leaves’ solvent extract treatments. Jembere (2002) also revealed the effectiveness of the aqueous extracts of M. ferruginea, following acetone and ethanol extracts against the maize weevils after three days post-treatment exposure.

The polar solvent extracts of the tested botanical leaves applied at all rates caused significant (p < 0.05) percent repellency of the maize weevils and the red flour beetles in comparison to non-polar and partial polar solvent extract treatments and the untreated control. This suggests the possession of more polar solvent-soluble volatile active biochemical constituents by the C. aurea leaves, which are responsible for considering the maize weevil’s and the red flour beetles’ repellency. Accordingly, Gebre-slassie and Eyasu (2019) indicated that the ethanol extract of C. aurea leaves yields the highest active secondary bio-chemicals, followed by ethyl acetate than petroleum ether and chloroform extracts, and they suggested that the leaves extract of C. aurea possessed more polar compounds than non-polar. The strongest repellent effect of methanol and acetone solvent extract treatments of M. viridis and O. gratissimum on C. maculatuswas was reported, and the solvents’ methanol and acetone were suggested to be more effective in extracting bio-active compound/s of the tested plants. Previous phytochemical screening investigation from the leaf, bark, stem, and root of C. aurea through aqueous and ethanol extraction methods indicated the existence of alkaloids, terpenoids, steroids, saponins, tannins, flavonoids and phlorotannin phytochemicals (Adedapo et al. 2008; Nega et al. 2016). Recent secondary metabolites screening study on C. aurea also confirmed the existence of different major active phytochemicals; alkaloids, cardiac glycosides, flavonoids, phenols, phytosteroids, saponins, terpenoids, and tannins in its different parts, including its leaves (Mulatu 2020). Kemal et al. (2020) also showed that C. aurea leaf methanol extracts possessed phenolic, alkaloids, steroids, flavonoids, saponins, phlobatannin, glycosides and tannins. Adedapo et al. (2008) also revealed much higher antibacterial activity of the methanol extracts of the leaves of the C. aurea than the stem. The active ingredients in the leaf extract of the castor bean plant tested against ectoparasites of animals were confirmed to reside in the polar fractions and suggested as the active principals are polar in nature (Amante 2016). Jembere et al. (2005) reported that M. ferruginea seed water extract induced high Z. subfasciatus mortality and justified it as it is due to the presence of high water-soluble chemicals in the seeds of the botanical. Methanolic, aqueous and ethanolic extracts of Euphorbia balsa mifera, Lawsonia inermis, Mitracarpus hirtus and Senna obtusifolia were also shown to have repellent potential against Sitophilus zeamais of stored sorghum (Suleiman et al. 2018).

As mentioned previously, various phytochemicals screening investigations revealed that the leaves, roots, seeds and stems of C. aurea are a rich source of terpenoids, saponins, tannins, flavonoids, steroids, glycosides and alkaloids by (Adedapo et al. 2008; Nega et al. 2016; Kemal et al. 2020; Mulatu 2020). The higher repellency of C. aurea leaves’ polar solvent extracts applied at rates of 5 and 10% on the test insects after two days of treatment exposure and 100% repellency at 10% dosage application at three days after treatment might probably be due to the synergism of some or all of these active volatile plant phytochemicals’ olfaction to repel away the tested pests from the treatments. This higher repellent efficacy of the polar solvent extracts of C. aurea leaves might probably be more significant than our former study on mortality, as repellency pushes away the insects from the grain and reduce contamination with remains of dead body parts, which could be retained through toxicity. Nevertheless, supplementary investigation on screening the major contributors for such repellency of the test pests due to the tested botanicals is needed. Consistently, it was revealed that plant essential oils and/or solvent extracts are composed of several secondary active metabolites, and repellency to insects is typically enhanced by the additive effects of the individual compounds (Zhang et al. 2013). Plants are indicated to be a chief source of bioactive metabolites, which display repellent, toxic and anti-feedant effects in a wide range of insects (Jahromi et al. 2012) and consequently, their extracts and products have been considered to be an alternative means of controlling insect pests (Tripathi et al. 2009). Tsegay et al. (2018) reported the highest repellency (80%) of termites by C. aurea leaf powder treatment at 1 h after treatment application. Berhanu et al. (2006) also reported C. aurea as one of the major mosquito repellent botanicals used in Ethiopia, among many ethno botanicals surveyed by him.

The higher repellency C. aurea leaves’ polar solvent extracts applied at rates of 5 and 10% on the test insects two days after treatment, and 100% repellency at 10% dosage application at three days after treatment (especially, the water extracts which are cheap and safe) in the present study might have the potential utility of this plant leaves extracts in the management of insect pests of stored maize as benign pest management alternative to synthetic insecticides under resource-poor farmers traditional storage condition in Sub-Saharan Africa, in general, and Ethiopia, in particular. Accordingly, sustainable use of botanicals with pesticidal properties is shown to be worth in the enhancement of food security, especially in areas where the use of synthetic pesticides for pest control is risky full and uneconomic (Qwarse 2015). Tsegay et al. (2018) also reported the highest repellent (80%) potency by C. aurea leaf powder treatment at 1 h after treatment application on a termite. Berhanu et al. (2006) also reported C. aurea as one of the major potent mosquito repellent botanicals used in Ethiopia, among many ethno botanicals surveyed by him. Meragiaw and Asfaw (2014) also reported C. aurea as one of the potent repellent, pesticidal and antimalarial plants in Ethiopian traditional herbal medicine. Pavela and Benelli (2016) also indicated C. aurea as one of the repellent plants used against mosquito vectors in Africa.

The non-existence of the co-evolutionary interaction between the tested botanical and the tested insect pests is suggested as one of the contributors to the higher repellent effect of the tested botanicals applied at rates of 5 and 10% at two days after treatment, as the test insets have been mainly restricted to the grain stores (Hiruy and Getu 2020). Nevertheless, the great efficacy of C. aurea leaves’ polar solvent extracts applied at rates of 5 and 10% on the maize weevils and the red flour beetles repellency two days after treatment application in the current study was against the finding of Zorloni et al. (2010), in which C. aurea crude extracts didn’t induce any repellent effect on ticks. This difference in repellency efficacy might probably be due to inherent differences in plant chemistries that may be genotypic or Spatio-temporal variations caused by abiotic factors such as altitude, rainfall and soil type or different chemistries expressed in different plant parts (Sarasan et al. 2011; Belmain et al. 2012).

Conclusion

The current study provides some insight regarding the existence of the possibility of using the repellency of C. aurea solvent extract on the maize weevils and the red flour beetles’ management. Accordingly, the distilled water, methanol, ethanol and acetone (the polar solvents) extract treatments of C. aurea induced significantly (p < 0.05) higher repellency of the maize weevils and the red flour beetles in comparison to non-polar and partial polar solvent extract treatments and the untreated check at all days after treatment at 5% and 10% rates. Thus, the polar solvents’ extract treatments of C. aurea were potent for the management of the maize weevils and the red flour beetles at 5% and 10% dosages. There is also the possibility of using these polar solvents in the preparation of plant extracts for testing repellency and toxicity, screening phytochemicals and other possible bioassays against the maize weevils and the red flour beetles. Furthermore, the water extracts of the tested plants’ leaves could be used for the management of the maize weevils and the red flour beetles under traditional storage conditions in Ethiopia at 5% and 10% dosages, as their preparation is cheap and easy, safe and environmentally sound. Thus, the water extracts at the aforementioned rates can be recommended for managing the maize weevils and the red flour beetles in stored maize grain under farmers’ storage conditions in Ethiopia and elsewhere with similar environmental conditions. Nonetheless, the crude extracts’ effect on human beings, natural enemies and cost-effectiveness under traditional storage conditions need additional study before wide implementation of the outcomes of this study.

Authors’ statement

This manuscript describes original work that has not been published elsewhere and is not under consideration by any other journal.

Authors’ contributions

BH was responsible for planning the research proposal as well as searching literature reviews and writing up the paper. EG contributed to the endorsement of the research strategy, groundwork, and standardization of the manuscript. Both authors had equal responsibility for the preparation and approval of the final manuscript for publication.

Ethical statement

This study does not involve any human or animal testing.

Acknowledgements

The authors sincerely thank Arba Minch University and the Zoology Department of Addis Ababa University for providing them with financial support to conduct the study.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The data that support the findings of this study are available in OSF repository [https://osf.io/] at https://osf.io/5xtcs.

Additional information

Funding

The authors received funding from Arba Minch University for this research.