Introduction

One of the ancient oil seed crops, sesame (Sesamum indicum L.), is a member of the Pedaliaceae family and is frequently referred to as the "queen of oil seeds" (Weiss 2000). It is one of the most important oil crops for export in Ethiopia. Sesame seed is a common spice and a substantial source of folic acid, oil, protein, unsaturated fatty acids, vitamins, and minerals. It is also used as an insecticide synergist and in pharmaceutical and skin care products (Anilakumar et al. 2010). Between 50 and 60% of the oil from the seed is stable due to the presence of antioxidants including sesamolin, sesamin, and sesamol, three natural antioxidants (Zerihun 2012). Sesame oil's fatty acid makeup varies greatly between its various cultivars around the world (Bahkali et al. 1998; Uzun et al. 2008). This crop is native to the tropics and certain temperate regions. It thrives in regions that are 500 to 800 m above sea level, but it may even grow as high as 1300 masl on well-drained soils with moderate fertility and with an optimum temperature for growth in the range of 27 °C to 35 °C (Baraki and Berhe 2019; Langham et al. 2008; Terefe et al. 2012).

The cultivated sesame have their origins and greatest morphological and genetic diversity in Ethiopia (Baraki and Berhe 2019; Teklu et al. 2021). One of the largest core collections of sesame genetic resources in Africa is kept by the Ethiopian Biodiversity Institute (EBI), which also preserves around 5000 genetically varied sesame germplasm resources (Woldesenbet et al. 2015). According to reports on the diversity of Ethiopian sesame germplasm, the sesame landraces gathered from the country's many areas exhibit significant morphological and genetic differences (Abate et al. 2015; Teklu et al. 2021). Although landraces naturally produce lower yields, they are the primary seed sources for sesame farming in Ethiopia, where farmers praise them for their distinctive flavor and aroma, ability to withstand hard conditions, and other qualities (Nyongesa et al. 2013).

Millions of farmers around the world are supported by the enormous sesame business (Berhe et al. 2022). It is estimated that 2,211,339 tons of sesame grains were traded globally in 2019 for a sum of 3.4 trillion USD, and the production is continuously rising (FAOSTAT 2020). Ethiopia is where, it is predominantly grown as a source of income and a staple food. Ethiopia's gross domestic product has greatly increased by the renowned sesame. In Ethiopia, the production of sesame was assigned 45.7% of the anticipated area needed to grow oil crops in 2020 (CSA 2020). Sub-Saharan African countries shipped 1,465,493 tons of unprocessed sesame worth 1.9 trillion USD in 2019, with Ethiopia accounting for 8.96% of all sesame exports and 307 million USD in total value (FAOSTAT 2020). Even though Ethiopia's sesame business is growing and is currently the country's leading agricultural export, biotic and abiotic factors have a significant impact on sesame seed production, productivity, and storage.

Due to numerous production constraints, sesame productivity in Ethiopia and other sub-Saharan African sesame growing regions are low and static (< 0.6 t/ha) (Teklu et al. 2021). One of the most crucial elements affecting sesame seed production and storage, both in terms of quality and quantity, is the presence of insect pests. In general, lack of high-yielding and well-adapted varieties, vulnerability to capsule shattering, the frequency of biotic and abiotic challenges, and lack of contemporary production technology, such as storage facilities, are the important factors contributing to sesame's low yield. (Berhe et al. 2022; Teklu et al. 2021; Usman et al. 2022).

Because sesame seeds are high in proteins and lipids, they are vulnerable to insect pest attacks (Adeleke and Babalola 2020; Rajendran and Chaya Devi 2004), which can results in weight loss, contamination, flavor loss, mold growth, and the creation of toxins. Beetles, including Tribolium spp. (Coleoptera: Tenebrionidae), and moths, including Ephestia elutella (Hübner) (Lepidoptera: Pyralidae), Plodia interpunctella (Hübner) (Lepidoptera: Pyralidae), and Corcyra cephalonica (Stainton) (Lepidoptera: Pyralidae), have been identified as significant pests of oil seed storage (Berhe et al. 2022; Rajendran and Chaya Devi 2004). Sesame seed bug (Elasmolomus sordidus (Fabricius) (Hemiptera: Lygaeidae)) is Ethiopia's most harmful post-harvest insect pest reported so far (Berhe et al. 2008, 2020). According to several research studies, the sesame seed bug damages sesame and groundnut seeds in the field as well as in stores (Berhe et al. 2022, 2020).

The Ethiopian seed system is another problem that lowers the productivity and production of sesame. Ethiopian farmers cultivate sesame using landrace varieties, which are naturally low yielders, resulting in lower productivity and income. The primary factor driving Ethiopian sesame farmers to utilize landraces has been attributed to their high value for possessing traits favored by farmers, such as distinctive flavor and aroma, and their ability to thrive on marginal and low-input agricultural soils (Teklu et al. 2021). The lack of a formal sesame seed business has been cited as one of the biggest obstacles to sesame production in Ethiopia, along with limited access to new seed varieties, low yield improvements from cultivating the existing local cultivars, high seed costs, low yields, and poor quality seeds (Teklu et al. 2021). Furthermore, findings from research conducted in the Kafta Humera districts of Western Tigray, located in North Ethiopia, highlighted that primary hindrances to sesame production and its overall efficiency include the absence of extension services, limited technical expertise, and inadequate storage infrastructure. These factors collectively contribute to substantial losses during storage at different points along the value chain (Gebretsadik et al. 2019). Therefore, in order to reduce the impact of the storage insect pests, a thorough analysis of the insect species, infestation rates, insect population density, and farm-storage losses of sesame seed is required considering the growing importance of this cash crop to Ethiopia's agricultural economy. Since there is a lack of information on the insect population density and species composition on stored sesame, the post-harvest protection study in sesame in Ethiopia still has limits to manage the large loss the crops suffer from. The current study was initiated with the objective to assess the occurrence, distribution, insect species composition, and population densities of sesame storage pests and their associated quantitative and qualitative losses in stored sesame seeds across major sesame-growing districts of Ethiopia.

Materials and methods

Description of study areas

The survey was conducted in six significant sesame-growing districts, namely Kafta Humera, Tsegede, Metema, Tach Armachiho, Pawe and Gida Ayana, spread over four regional states of Ethiopia: Tigray, Amhara, Benishangul Gumuz, and Oromia. Detailed description of the study areas is presented in Table 1 and Fig. 1 below.

Table 1 Geo reference and temperature of the study areas
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Fig. 1
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Map showing selected sesame storage pest survey districts in Tigray, Amhara, Benishangul Gumuz, and Oromia regions, Ethiopia

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Sampling and data collection

Sampling approach and sample size

An assessment survey was conducted on 431 randomly selected farmers across the six study districts during the 2017—2018 growing season. Depending on the production area and size of households in the study districts, nine villages, locally known as "kebeles," were selected purposively from the study district, five from Tigray, two from Amhara, and one from each of the Benishangul Gumuz and Oromia regional states (Table 2). Each sesame-growing farmer or household was used as the sampling unit for this study. A sample was taken from each household after two houses. When asked for storage-related remarks, the next household was considered if any of the family's adult members were not present. From each sampling unit, one kilogram of seed sample was collected. Seed samples were taken from the top, middle, and bottom of the storage structures due to the uneven insect distribution in storage structures.

Table 2 Sampling districts, sub-districts (Keble) and sample size (N)
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Data collected

During sampling

At the time of sampling, information on the sesame seed variety, seed age, storage method, pesticide use, and bio-physical data (intra-granular temperature, seed moisture, relative humidity) were taken. Data on sesame seed variety, seed age, storage method, pest control method used were obtained through interview. Data on bio-physical parameters like inter-granular temperature, seed moisture, and relative humidity were measured using a moisture meter from the USAID Feed the Future Innovation Lab for the Reduction of Post-Harvest Loss (PHL-IL) (Armstrong et al. 2017). The moisture meter was placed in the bulk of the stored seed and allowed to stabilize for two to four minutes before recording the equilibrium inter-granular temperature (oC), equilibrium moisture content (% wet basis (wb)), and equilibrium relative humidity (%) from the meter's display. Three different readings were taken from each sample units, and the mean values for each of the measuring parameters were computed (Manu et al. 2019),

After sampling and incubation

Insect species identification and population density

The insect species identification, determination of population density, seed damage, and weight loss were conducted at the entomology laboratory of the Dry Land Crop and Horticultural Science Department at Mekelle University, Ethiopia. After collecting the samples, they were incubated, and one kilogram of the sample seeds was sieved using standard test sieves with mesh sizes of 2 mm and 0.425 mm from Supertek Scientific (Addison, Illinois, USA). The total number of insects, both dead and alive, as well as their species, were recorded for each location. Species identification was carried out under a stereomicroscope using basic morphological features and identification keys provided by Gorham (1991) and Kumar (2017). Following the removal of dead and live insects, the remaining sample seeds were placed in plastic jars with mesh lids to allow gas exchange while confining the insects inside. These jars were kept under laboratory conditions to observe the emergence of the first offspring (F1) over a six-week incubation period. The total number of live insects was determined by adding the initial parent insect counts to the number of F1 progenies that emerged. Insect pests were identified at the genus and species level using basic morphological features and identification keys provided by Gorham (1991) and Kumar (2017), with unidentified species being preserved in vials and sent to Department of Grain science and Inductry at Kansas State University for further identification.

For the determination of seed damage and weight loss one kilogram of sifted samples was divided into 25 g seed sub-samples using the quartering and conning procedures (Boxall 1998). The INDOSAW Seed Counter (Osaw Industrial Products Pvt. Ltd., Salarheri, and Haryana, India) and sensitive balance were used to separate damaged and undamaged seeds, count, and weigh each type of seed separately. Insect-damaged seeds might be seen based on crashed seeds or webbed left behind. Seeds with mechanical damage were considered for dockage. The following equation was used to determine the percentage of seed damage (Boxall 1998):

$$Seed\;damage\;(\%)=\frac{Number\;of\;damaged\;grain}{Total\;number\;of\;grain\;used}\times 100$$

Percent seed weight loss due to insect damage was calculated by the count and weigh method (Harris and Lindblad 1978) using the following formula:

\(Seed\;weight\;loss\;(\%)=\frac{(Wu\times Nd)-(Wd\times Nu)}{Wu\times (Nd+Nu)}\times 100\); Where: Wu = weight of undamaged seeds, Nd = number of damaged seeds, Wd = weight of damaged seeds and Nu = number of undamaged seeds.

Using the blotter method, 100 randomly selected seeds from each sampling unit were used to examine the germination of seeds (In 2014). The seeds were put in filter-paper-lined Petri plates made of polypropylene. The plates were set up in a germination chamber with fluorescent lights that cycled on and off for 12 h at a temperature of 25 °C. (Alemayehu et al. 2020). In order to prevent the filter paper from drying out, distilled water was added to the plate every day. The number of seeds that germinated were recorded after one week and percentage germination computed as follows:

$$Seed\;ger{\;\text{min}}\;ation\;(\%)=\frac{No.\;ger{\text{min}}ated}{Total\;No.\;of\;sample\;seed}\times 100$$

Dockage was computed using the seed sample unit of one kilogram. To ensure that no dirt had gotten into the seed, the sample was sieved. Manual inspection was also used to remove any coarse material, such as pods, stems, straw, stones, and others, from the sieve's top parts. The amount of mixed material that passed through the sieve's bottom and was still visible above the sieve was added, divided by the entire sample size, and multiplied by 100 to determine the dockage percent.

Statistical analysis

The qualitative and quantitative data that were collected through checklists and measurements on samples were statistically analyzed using R software version 4.2.2 (R core team 2022). Cross-tabulations were made for nominal factors, and descriptive statistics were calculated to give an overview of data on the variety of sesame seeds employed, storage techniques used, and insect prevalence. Fisher's exact test with two sides (Zar 1987) was used to determine the association between nominal parameters. To find variations between samples from the six study districts, nonparametric analyses was applied to measurement variables. Multiple mean comparisons were made using the Kruskal–Wallis test for significant difference. The correlation between measurement and count factors for insect, varieties, and insect infestation level (infested/non-infested) were investigated using the chi-square and Welch two-sample t-test respectively.

Results

Biophysical characteristics of seed samples

The results of the analysis revealed highly significant variations (P < 0.001) in the inter-granular temperature, moisture content, and relative humidity of the stored sesame sample seeds across the study districts (Table 3). The pairwise mean variation of each biophysical characteristic is presented in Fig. 2. The results showed that the samples from Tsegede, Kafta Humera, and Metama districts had the highest intra-granular temperatures, while those from Pawe and Gida Ayana districts had the lowest intra-granular temperatures which lead to the highest seed moisture content and relative humidity among the study districts (Table 3). Likewise, samples from the districts of Kafa Humera, Tach Armachiho, and Tsegede received highest intra-granular temperatures had the lowest seed moisture content and relative humidity (Table 3).

Table 3 Mean (± SE) inter-granular temperature, moisture content and relative humidity of stored sesame seed in different sesame growing districts of Ethiopia
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Fig. 2
figure 2

Mean difference in seed intra-granular temperature (.oC), seed moisture content (%) and relative humidity(RH) (%) of samples collected from sesame growing districts in Ethiopia. If an interval does no contain zero, the corresponding means are significantly different(ADD Q)

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Insect species composition and population density

In this study, thirteen species of stored sesame insect pests were identified from the samples collected from six major chickpea growing districts across Ethiopia (Table 4). The most common and prevalent insect species in this study were Flour beetles, Tribolium spp. (T. castaneum/ & T. confusum). Both the T. castaneum (red flour beetle), and T. confusum (confused flour beetle) were predominated in stored sesame, however T. castaneum was the most abundant (77%) compared to T. confusum (23%), which together accounted for 44.26% of the overall insect population density (Table 5). Rice moth (Corcyra cephalonica), Lesser grain borer (Rhyzopertha dominica), and Rice weevil (Sitophilus oryzae) were the second abundantly found insect pests in varying degrees after Tribolium spp with a percentage population density of 28.2%, 15.72%, and 4.88%, respectively (Table 5). The cumulative abundance of the remaining eight species represents 6.94% of the total insect population count detected in the total 431 samples across the study districts (Table 5).

Table 4 List of insect pest’s species composition identified in stored sesame seeds in six major sesame growing districts of Ethiopia
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Table 5 Population density of the major insect species and their infestation stratus across the six major sesame growing districts of Ethiopia (N = 431)
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In most of the districts investigated the five commonly existing stored sesame insect species namely, T. castaneum, T. confusum, R. dominica, S. oryzae and C. cephalonica and other spp were discovered during this study (Fig. 3). These dominating species was present in all the study districts in varying percentages of frequency distribution within each district (Fig. 3) as well as Across all the study districts (Fig. 4). The infestation distribution of each species within each districts indicated that, Tribolium spp in Kafta Humera district showed the highest infestation distribution (68.8%), followed by Tach Armachih0 (66.7%), Metema (64.1%), Tsegede (57.1%), Gida Ayana (45.2%), and Pawe (40.0%) districts (Fig. 3). Likewise, C. cephalonica also showed the highest infetation distribution in the districts of Kafta Humera (65.45%), followed by Tsegede (53.97%), Gida Ayana (41.94%), Tach Armachiho (35.19%), and Pawe (33.33%) (Fig. 3). Rhyzopertha dominica was observed predominating in Kafta Humera with a infetation distribution of 66%, followed by 56%, 51%, 50%, 30%, and 23% in the districts of Tach Armachiho, Tsegede, Metema, Pawe, and Gida Ayana, respectively (Fig. 3). Sitophilus oryzae was present in all the research districts, but its distribution was less widespread than that of the other major species. Sitophilus oryzea was found in all districts only with a maximum detection rate of 34.92% in Kafta Humera and a minimum detection rate of 17.19% in samples taken from Metam (Fig. 3). The insect pest species found in stored sesame seeds were observed to infest 93% and 77% of the sampled seeds in Kafta Humera and Tsegede districts, both located in the Tigray region, respectively. This was followed by infestations in 72.22% and 71.88% of the sampled seeds in Tach Armachiho and Metama districts, both situated in the Amhara region, respectively. Additionally, 56.67% and 54.848% of the sampled seeds from Pawe district in Benshangul Gumuz and Gida Ayana district in the Oromia regional state were also affected by these insect pests (refer to Fig. 3).

Fig. 3
figure 3

Frequency distribution of infested samples within each district of Ethiopia

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Fig. 4
figure 4

Frequency distribution of infested samples across the six assessed districts in Ethiopia

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When examining the distribution of infestations for each species across all the study districts, it was observed that Kafta Humera district, followed by Tsegede, Metema, and Tach Armachiho, exhibited the highest levels of infestation. In contrast, Pawe and Gida Ayana districts displayed the lowest infestation rates for each species (see Fig. 4). These observations were made in the context of the total infested samples for each species, which included Tribolium spp (N = 269), R. dominica (N = 234), S. oryzae (N = 112), C. cephalonica (N = 216), Other spp (N = 116), and the total insect population (N = 335).

Out of the total seed samples collected (N = 431) across the six study districts, 344 samples (79.81%) were found to be infested with stored sesame insect pests (Table 5). Those infested samples (79.81%) had a total population density count of 5795 insects, with a mean count of 13.44 (min/max = 0/143) insects per kilogram of the sample seed. Among the 13 different stored sesame insect species identified in this study, the highest population density count was recoded from Tribolium spp., with a total of 2565 insect population count and a mean count of 5.95 (min/max = 0/70) insects per kg of the sample seed (Table 5). Corcyra cephalonica ranked second in total population density count (1634) with a mean count of 3.79 ((min/max = 0/67) insects per kg of the sample seed. Rhyzopertha dominica was the third populated stored sesame insect species across the study districts with total population density of 911(0.55%) insects and a mean count of 2.11(min/max = 0/25) insects per kg sample seed. Sitophilus oryzae showed 283 total insect count with a mean count of 0.66 (min/max = 0/12) insects per kg of the sample seed. The remaining other 8 insect species score a total of 402 insect counts with a mean count of 0.99 (min/max = 0/15) insects per kg sample seed.

Sesame varieties and storage materials used by farmers

A total of 10 varieties, 9 local cultivars and 1 improved variety, were recorded from the total number of sesame seed samples (N = 431) gathered from the six sesame growing districts (Fig. 5). Hirhir was the number one most popular variety cultivated in all the study districts, mostly farmers in the Kafta Humera (90.5%) and Tsegedae (71.4%) districts, were grew this variety. Gojame-azene, which was mostly grown by farmers in Tach Armachoh (59.3%), Metame (31.2%), Tsegede (11.1%), and Kafta Humera (1.6%), was the second most popular sesame variety grown in the four districts. Abasena was grown in three districts, with 53.3%, 27.8%, and 26.6% in Tach Armachiho and Metema and Pawe, respectively. Metema (14.1%), Pawe (3.3%), and Tsegede (3.2%) were the three districts where Beshbesh variety was grown, whereas bawunji, Nech_zer, Setit-1, and Tikur_zer were only found in two districts. Most of the sesame growing farmers (93.5%) in Gida Ayana districts grew Wollega-type sesame exclusively. However, in Tsegede, Kafta Humera, Gida Ayana, and Metema, respectively, 7.9%, 4.2%, 3.2%, and 3.1% of the farmers were cultivating mixed sesame types.

Fig. 5
figure 5

Farmers’ variety preference across the major sesame growing districts in Ethiopia

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All farmers in the study district use polypropylene (PP) to keep their sesame seeds or grains until they are used for personal use or sold to markets, except for 10% and 7.8% of farmers in Pawe and Metama, respectively who uses plastic-silo locally called “jerican” and “bermil.

Seed damage, seed weight loss and insect counts

There were significant variations among the sesame samples in percentages of insect-damaged seed (P < 0.001), seed weight loss (p = 0.02), and total Corcyra cephalonica insect count (P < 0.001) per kg of sample seed (Table 6). The percentage of insect-damaged seed ranged from 5.96% in Tach Armachiho to 9.98% in the Kafta Humera district, with a mean of 8.75% (min/max = 0/39.66%) (N = 431). While the mean percentage of the seed weight loss in this study was 1.80% (min/max = 0/12%) which was ranged from 1.29% in Tach Armachiho to 2.54% in Gida Ayana. The average number of Corcyra cephalonica count per kilogram of sesame seeds that were collected from all sample units (N = 431) throughout the research regions was 3.79 insets (range: 0 to 67 count/kg). The maximum Corcyra cephalonica count per kg of sample seeds were detected in Tsegede, and Kfta Humera and Gida Ayana while the least count was recorded in Metama and Tach Armachiho.

Table 6 Means (± SE) insect-damaged seed (%), seed weight loss (%) and count of insect population (count/kg) of sesame samples collected from different six sesame growing districts tes in Ethiopia
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Percentage of Seed grain damage, seed weight loss and insect count across different sesame varieties

There was a significant difference among the sesame varieties in terms of the proportion of insect damaged seed (x2 = 26.40, DF = 9), seed weight loss (x2 = 29.40, DF = 9), and mean total insect count (x2 = 29.64, DF = 9, P < 0.001) (Table 7). The Bawunji cultivar had the highest percentage of insect-damaged seed, seed weight loss, and insect population density in this study followed by the mixed cultivar. Tikur_zer followed by Nech_zer and beshbesh showed the least insect population count, whereas Tikur_zer and Abasena had the lowest seed damage (1.15%) and seed weight loss (1.15%) respectively (Table 7).

Table 7 Means (± SE) insect-damaged seed, seed weight loss and count of insect population (count/kg) across the sesame seed varieties
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The mean insect counts of Rhyzopertha dominica and Corcyra cephalonica (x2 = 37.36, DF = 9, P < 0.001 and x2 = 34.50, DF = 9, P < 0.001, respectively) showed significant different among the sesame varieties while Tribolium spp. and Sitophilus oryzae did not (Fig. 6). Rhyzopertha dominica counts that ranged from 2.74 per kilogram of sesame seed in Hirhir to 2.29 in Gojam_Azene varieties were the highest while the Nech_zer cultivar showed the lowest count. Mixed cultivars had the highest insect population of C. cephalonica, followed by Hirhir and Wellega varieties, while beshbesh and Tukur_zer had the lowest count (Fig. 6).

Fig. 6
figure 6

Means (± SE) insect population of Rhyzopertha dominica (count/kg), Corcyra cephalonica (count/kg), and corresponding seed damaged and weight losses for the difference sesame cultivars and improved seed variety

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Seed germination and seed dockage

This study discovered significant variability in the seed germination throughout the sesame growing regions (P < 0.001) (Table 8). While the mean seed germination rate for all the 431 sample units was 77% (min/max = 12/100 percent), it ranged from 72.48% in Kafta Humera to 85% in Metema districts. The proportion of seeds that germinated showed substantial difference between insect infested and non-infested samples (t = 22.08, P < 0.001, DF = 422) (Fig. 7A). The mean seed germination for infested sesame samples (N = 344) was lower (72.5 ± 1.00%) than the mean for non-infested samples (N = 87) (97.48 ± 0.44%) across the study districts that demonstrated a significant difference in the infestation level among them (Fig. 7B). This study discovered a significant difference in seed dockage with mean of 1.79% that ranged from 1.11% to 2.14% (Table 8).

Table 8 Means (± SE) seed germination and dockage in sesame samples collected from major growing districts of Ethiopia
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Fig. 7
figure 7

Seed germination variation (A) in infested and non-infested sesame sample seeds and infestation level (B) difference across the study districts in Ethiopia

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Seed germination across the sesame varieties

The mean seed germination percentage showed a significant difference (x2 = 59.75, P < 0.001, DF = 9) among the sesame varieties grown throughout all study districts (Fig. 8). The overall mean across the 10 sesame varieties was 77.57 ± 0.08% ranged from 12 to 100%. In this study seeds of beshbesh and Gojam_Azene cultivars showed the highest seed germination while seeds from Bawunji followed by Hirhir resulted in lower seed germination (Fig. 8). The incidence of insect infestation and varietal differences among the sesame growing districts in this case may be resulted in the variation in seed germination percentage seen among the study districts.

Fig. 8
figure 8

Seed germination (%) comparison among sesame seed varieties collected from major sesame growing districts in Ethiopia

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Interrelationships between biophysical characteristics, insect population density, seed physical characteristics and insect damage variables

The findings indicated a significant correlation (P < 0.001) among several variables, including biophysical characteristics (intra-granular temperature, seed moisture content, and relative humidity), insect population density, and seed physical characteristics (germination and dockage), as well as insect damage variables (seed damage and seed weight loss) (Fig. 9). Unexpectedly, the intra-granular temperature displayed an unexpected negative correlation (P = 0.05) with seed moisture content and relative humidity (r = -0.3 and -0.4, respectively). However, a highly significant positive correlation (r = 0.80, P < 0.001) was observed between seed moisture content and relative humidity. Additionally, the study identified a significant positive association (P < 0.001) between total insect population density and insect damage variables, including seed weight loss (r = 0.65) and seed damage (r = 0.65) and the biophysical character including temperature (r = 0.52). Furthermore, two other seed physical parameters, percentage of seed germination (r = -0.67) and dockage (r = 72), were found to be strongly linked to insect population density.

Fig. 9
figure 9

Pearson correlation matrix among biophysical characteristics, insect damage variables and insect population density. Note. ‘***’P < 0.001, ‘**’P = 0.01, ‘*’P = 0.05, ‘’not significant at 0.05 level of confidence’

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Discussion

The findings of current investigation showed that there were significant differences among the study districts in the inter-granular temperature, seed moisture content, and relative humidity of the stored sesame sample seeds. The study districts with the highest intra-granular temperatures were Tsegede, Kafta Humera, and Metama, whereas the lowest intra-granular temperatures, highest seed moisture content, and highest relative humidity were found in Pawe and Gida Ayana. The variation in environmental conditions among districts and the variation in insect populations inside the storage bags may have resulted in variation in the biophysical features. The results are in agreement with earlier findings that stated the inter-granular temperature within a stored seed can be affected by the ambient air temperature, heat produced by the grain's respiration, and any insects present in the storage (Assefa and Srinivasan 2016; Befikadu 2014; Demissie et al. 2014; Mengistu 2022).

How effectively sesame seeds are preserved depends on the moisture content of the seeds. The maximum safe moisture level for storing sesame seeds is said to be between 6 and 8% (Neme et al. 2021; Usman et al. 2022). According to the results of the current study, the mean moisture content of sesame seeds stored for about eight months across the study districts during the sampling time was 8.40%, ranging from 7 to 10%, which is above the recommended moisture content for storage. The highest seed moisture content and relative humidity were recorded in both Gida Ayana and Pawe districts. Therefore, the higher relative humidity in those two districts may be the cause of the increased seed moisture content there. The significant strong positive correlation shown between seed moisture content and relative humidity in our study can confirm that the environmental relative humidity had an effect on the sesame seeds moistures content stored in the traditional storage materials.

The variation in biophysical factors inside the storage materials in this study may be attributed to the variation in environmental factors among the study districts and storage methods used. As the storage containers used by the farmers were open, they allow the biophysical factors to interact with the outside environment. This finding is in agreement with the previous research outputs that indicated, variations in biological activities of living organism inside the stored seeds could be a result of variations in bio-physical components inside the storage materials (Danso et al. 2019).

Regarding the insect species composition and population density, 13 species of stored sesame insect pests were found in the current investigation, with the flour beetle, (Tribolium spp. (T. castaneum/T. confusum)), Lesser grain borer (R. dominica), rice moth (C. cephalonica), and rice weevil (S. oryzae) being the most prevalent insect species. Those dominant insect species were found in all the studied districts in variable amounts, both within each district and across the whole study districts. The variation in insect species composition and population density among study districts might be due to difference in altitude, temperature, and relative humidity of each district and also storage methods, control practice and variety used by different farm households.

Although there were no reports to date indicating either of the insect species identified in stored sesame seeds in Ethiopia, our current findings were consistent with earlier research that suggested that C. cephalonica was noted as a significant sesame storage pest in India (Babu et al. 2020; Dilipsundar et al. 2019). Similar to this, C. cephalonica was also identified as a significant insect pest of many stored products in the tropics, affecting a variety of commodities, including oil seeds like groundnuts, cotton seeds, flax seeds, and oil-cakes (Bhandari et al. 2014; Debbarma et al. 2021; Vincent et al. 2021). Typically, the destructive larval stage of C. cephalonica prefers to infest cracked or damaged grains, where it feeds beneath the formed silken threads, which over time gather density and bulk up the contaminated items (Chaudhuri and Senapati 2017). Following the infestation of sesame stored products by this pests, the entire stock of seeds or grains turns into a black, webbed mass that emits a unique stench, and the seeds deteriorate and become unsafe for consumption or marketing (Babu et al. 2020). In general, by consuming seeds and weaving silken webs, this pest significantly damages and contaminate sesame seed. Despite the fact that this pest is affecting sesame seeds in Ethiopia, no prior research to our knowledge has shown that it poses a serious threat to sesame storage there, except reviewed as pest of sesame and groundnut storage in 1997 (Getnet et al. 1997). Therefore, this study is the first to recognize C. cephalonica as a significant and damaging pest of the Ethiopian’s stored sesame.

The discovery of R. dominica in stored sesame seeds, specifically from Ethiopian stored sesame., is also reported for the first time in this study. However, it has been noted that R. dominica, commonly known as the lesser grain borer, is primarily found in warmer climes, where both the adult and larvae significantly damage grains and seeds (Pires and Nogueira 2018). Rhyzopertha dominica had been described repeatedly as a destructive internal feeder that mostly destroys stored grain seed (Giunti et al. 2021; Mahroof et al. 2010), including Ethiopian wheat, maize, rice, oats, barley, and millet (Kalsa et al. 2019; Tadesse et al. 2006).

With their larvae and adults, Tribolium spp. are known to target damaged grains or grain products. In agreement with our findings, Tribolium spp was reported as pest of sesame storage (Babu et al. 2020). Similary, before two decades and half, T. confusum was reviewed as storage pest of sesame and groundnuts in Ethiopia (Getnet et al. 1997). Recent findings from Ethiopia, however, claim that Tribolium spp. are the primary pests of stored sorghum, maize, and wheat (Dibaba 2020; Kalsa et al. 2019; Tadesse and Subramanyam 2019). Tribolium spp. does not immediately harm grain, but once it is impacted, it still shows signs of infestation; in this case, it had formed an infestation, leaving the products with a lingering bad smell (Walker et al. 2018).

According to reports, S. oryzae, commonly known as the rice weevil, is a significant pest of stored rice, maize, cotton, almonds, and other cereals (Ahmad et al. 2021). Similar to our findings, Sitophilus spp., was reviewed from stored sesame (Usman et al. 2022) and was also reported occasionally affecting pulses. Sitophilus oryzae targets healthy and undamaged grain and after being harmed, the grain is more vulnerable to other secondary pests (Doherty et al. 2023). As far as we are aware, no stored sesame grain samples were reported infested with S. oryzea in Ethiopian storage facilities. This makes the work a groundbreaker in identifying this major pest in sesame seeds kept in farm-storage facilities in the Ethiopian case.

Landraces are the main seed sources for sesame farming in Ethiopia, where farmers appreciate them for their flavor and scent, capacity to endure harsh conditions, and other features. Despite their naturally lower yields, landraces are valued for these qualities (Nyongesa et al. 2013). Based on the knowledge data gathered during sampling period, the 431 sesame seed sample units in this study were divided into nine farmer cultivars and one improved variety of sesame known as setit-1. The Humera Agricultural Research Center (HuARC) released Setit-1 in 2011, concentrating on the Kafta humera district in North Western Ethiopia (Baraki and Berhe 2019; Gebremichael 2017). Farmers in all of the study's districts grew Hirhir, one of the nine cultivars; however, it was mostly farmed in the districts of Kafta Humera and Tsegede. In the Tach Armachiho and Metame districts farmers primarily grew the Gojame-azene and Abaseana cultivars. Farmers in the Pawe district most frequently cultivated Abasena. While Setit-1, the improved sesame variety, was only planted in the Kafta Humer and Tegede districts, while the Welega-type cultivar was the most extensively grown sesame cultivar in the Geda Ayana district. As many reports have been indicating, Ethiopia is the center of origin and diversity for cultivated sesame (Baraki and Berhe 2019; Teklu et al. 2021). According to studies on variabilities of Ethiopian sesame germplasm, the sesame landraces collected from the nation's diverse sesame growing areas have notable morphological and genetic distinctions (Abate et al. 2015; Baraki and Berhe 2019; Teklu et al. 2021).

The average percentage of insect-damaged seed and seed weight loss varied significantly among the sesame samples in each district. The insect-damaged seeds ranged from 5.96% in Tach Armachiho to 9.98% in the Kafta Humera district while the seed weight loss was varied from 1.29% in Tach Armachiho to 2.54% in Gida Ayana. Our finding is in line with earlier studies that showed storage pests in Ethiopia under various circumstances caused storage losses of 2% – 94.7% in sesame seeds (Berhe et al. 2022). Similar to this, it was observed that several species of Indian meal moth insects caused a storage loss of 5 to 26% (Usman et al. 2022). Insect pests in sesame storage facilities, according to this author, included Sitophilus spp., Tribolium spp., and P. interpunctella (Hübner) (Lepidoptera: Pyralidae) (Usman et al. 2022). Between 10 and 30% of grain is thought to be lost on average in Ethiopia due to storage insect infestations; in extreme cases, this number could potentially reach 50%(Tadesse and Subramanyam 2019). According to recent reports by Debebe (2022), in Ethiopia a total post-harvest loss of 23.36% in cereal grains and 23.25% in pulses and oil seeds were recorded..

Like the insect damage variables, the maximum C. cephalonica count per kg of sample seeds were recorded in Tsegede, Kfta Humera and Gida Ayana while the least count was recorded in Metama and Tach Armachiho. Hence, the identified pests were mainly responsible for the significant of seed damage and weight loss in the study districts.

Beside the variation across the study districts, this study looked at a significant difference between the sesame variety in terms of the proportion of insect-damaged seed, seed weight loss, and mean total insect count. While Tribolium spp. and S. oryzae did not exhibit a significant variation across the sesame varieties, the mean insect counts of R. dominica and C. cephalonic showed significant differences among the varieties employed.

This study found significant variation concerning seed germination among the sesame varieties grown in all the study districts, and between samples with and without insect infestations. The mean seed germination rate for all the 431 sample units was 77%. In all the study districts, the mean seed germination for infested sesame samples was lower than the mean for non-infested samples, indicating a substantial difference in infestation levels between the districts. The variance in seed germination observed among the study districts may be attributed to insect pest infestation and varietal differences. Previous studies revealed that seed germination rapidly decreased as the severity of the damaged seeds grew, which accelerated the loss of nutrients during the early stages of seed germination and prevented the seed from developing normally into seedlings (Koptur 1998). The study's highly significant correlations between the percentage of seed germination and the insect-damaged seeds confirm that sesame seeds with infestations have a lesser chance of germinating.

Conclusions

Significant amounts of sesame grain/seed produced in Ethiopia are lost after harvest. The amount of sesame seeds or grains for local and high-value international markets may rise if post-harvest storage losses for sesame are reduced at the farm level in this situation. Among 13 species of stored sesame insect pests discovered during the course of this study, flour beetle, Tribolium spp. (T. castaneum/T. confusum), Lesser grain borer (R. dominica), rice moth (C. cephaloni), and rice weevil (S. oryzae) were found the most common and abundant ones throughout the six main sesame-growing districts of Ethiopia. Insect infestations severely decreased both the quantity and quality of sesame seeds. In every district surveyed, mostly PP bags and infrequently local plastic silos were the two most popular storage options used to store sesame grains/seeds. The percentage of insect-damaged seed, seed weight losses, seed germination, and insect population density differed significantly among the sesame seed varieties used and the assessed study districts. Thus, effective management options should be developed for the reduction of losses in stored sesame seeds under farm storage conditions.

However, due to only one-time single sampling and concentrated on sesame seeds kept at on-farm storage condition, the current findings may not accurately reflect the real dynamics of insect pests and losses across all regions and the nation's entire sesame storage facilities. Hence, as sesame is one of Ethiopia's main exports commodity, more research is needed to look at the economic losses caused by insects in addition to the quantitative and qualitative loss they have caused. Therefore, further detailed investigations across all sesame growing areas under both on-farm and warehouse stores conditions using repeated sampling at different time will be needed to get accurate and reliable information across entire sesame storage facilities in all regions of the country to support one of the country’s leading export commodity.