Performance of Different Mulberry/ Morus Sp/ Genotypes and their Effect on Mulberry Silkworm, Bombyx Mori (Lepidoptera: Bombycidae)

Introduction

Silkworm (Bombyx mori L.) is essentially monophagous insect feeds solely on mulberry leaves (Morus spp.). Leaf quality is an important parameter used for evaluation of varieties aimed at selection of superior varieties for rearing performance. Growth and development of silkworm Bombyx mori L. is known to vary depending on the quality and quantity of mulberry leaf used as food source, which in turn indicated by commercial characteristics of cocoon crop [1]. Mulberry varieties regarded as one of important factors that effect on number of laid eggs, fecundity, hatchability, larval period and weight in silkworms. This variation will lead to various physiological state and cocoon production. Maximum of larval growth and uniform cocoon production determined by mulberry leaves varieties and caused that long silk fiber in silkworm fed [2].

It also been observed that the growth and development of silkworms and quality of silk cocoon produced are directly influenced by the variety and quality of leaves fed to the worms [3]. Morphological characters of leaves contribute to acceptability by silkworms [3]. Gogoi and Goswami, studied genotypes and observed variation in leaf yield in different genotypes [4].

Therefore, selection of mulberry genotypes is an important criterion for better growth and development of silkworm for proper nourishment to obtain better fecundity and higher cocoon productivity [5]. However, very little information is available on the different mulberry genotypes and its performance on silkworms in Ethiopia. Therefore, the objective of this study was to evaluate the agronomic and their rearing performance of different genotypes of mulberry on silkworms for improving silk production.

Materials and Methods

Experiment conducted on field and laboratory, for field it was tested across different Agricultural Research Center (Melkassa, Jimma, Wondogenet, Hawassa) and Alage ATVET College whereas, for the laboratory it was conducted under Melkassa Agricultural Research Center.

About 6 genotypes of mulberry namely, Nekemte, Jimma, M-4, K-2, S-13 and Local were used as a treatment and evaluated under field and laboratory conditions. The study was carried out under rain fed condition with supplemental irrigation during dry periods. Mulberry cuttings were planted with a spacing of 60 cm within plants and 60 cm between rows on a plot size of 3.6 cm * 3.6 cm. The treatments were arranged with RCBD in three replications in the field.

For laboratory mulberry silkworm was reared on the 6 mulberry genotypes. The silk worm rearing room and equipment’s were cleaned, washed and disinfected with 2 % formalin solution at the rate of 800 ml per 10m2 before the commencement of the experiment [6]. This silk worm was reared following cellular techniques starting from brushing till silkworms at larval stage was fed four times a day with tender leaves until III instars and mature leaves until V instars. The grown up worms were picked and left on the mountages for spinning. On the sixth- eighth day of spinning, the cocoons were harvested, counted and weighed [7]. The experiment was arranged in Completely Randomized Design (CRD) in three replications. In each replication, 200 worms/tray were used and allowed to complete the larval period to cocoon spinning on the six genotypes.

Results and Discussion

Evaluation of mulberry genotypes in field and its rearing performance in laboratory were carried out. Mulberry silkworms fed with the leaves of different genotypes of mulberry and their response was evaluated. Results showed that in all locations displayed significant (P<0.05) differences for a number of agronomic and yield characters for different mulberry accessions as compared to local check.

The qualitative and quantitative traits of mulberry genotypes can be expressed in terms of their morphological differences such as, plant height, stem thickness and number of branches, internodes and leaf area. Therefore, such differences in mulberry genotypes will lead to different directions of their utilization. Among all treatments, S-13 and K-2 gave significantly (P<0.05) higher yield as compared to local check and other treatments. They gave better and similar results in most of the measured parameters (Table 1).

Plant height, number of primary branches, stem thickness as well as internode length showed statistically significant differences among mulberry genotypes. Highest plant height was recorded from (277.04 cm) from Wondogenet site and the shortest was 194 cm from local mulberry genotype. However, the highest primary branch was from K-2 (21.5) and minimum was recorded from Nekemte (8.2). In addition, long internode length was registered by M-4 (8.3 cm) but the shorter was by S-13 (5.7 cm). Moreover, maximum leaf area was recorded 333 cm3 but the short one was 144cm3. In another way, high stem thicker was obtained from local (14.2 cm) but the thinner one was from K-2 (9 cm) (Table 4). During the experimental period, disease particularly leaf spot was becoming serious problem. As a result, disease incidence and severity was recorded. Therefore, significant difference was observed among the genotypes. Thus, highest disease incidence was recorded from Nekemte (64.24 %) at Alage site, but the lowest was recorded from S-13 (8.3 %) at Melkassa site. Similarly, the highest disease severity was recorded from local check (53.8 %) but lowest was recorded from S-13 (10.5 %) (Table 5).

As it can be realized from the results of the present investigation, mulberry genotypes showed wide variation in their quantitative and qualitative traits. Consequently, these differences resulted in significant variation in rearing performance and feeding efficiency of silkworms when leaves of these genotypes were used as a feed material.

There were significant differences (P<0.05) in number of leaf production, fresh and dry leaf weight among treatments in different locations. Maximum fresh leaf weight (26503 kg/ha) and dry leaf weight (8268 kg/ha) were recorded. However, the least fresh (9435 kg/ha) and dry leaf weight (2453 kg/ha) was recorded in local accession from Alage site, respectively (Table 3).

Evaluation of any crop is a continuous process to evolve new varieties suitable for specific zones for commercial utilization. The present scenario of sericulture industry demands new varieties suitable for various agro climatic conditions. Suitable parent material needs to be identified from large number of germplasm accessions for the purpose. Moreover, estimates of genetic diversity and relationship between various collections from diverse origin help in efficient management and utilization of germplasm [9]. Several studies have already highlighted the variability of mulberry germplasm and association of different agronomical traits was also studied in detail [10].

Studies on nutritional ecology of an insect are very important for its commercial exploitation [11]. The suitability of host is determined through estimation of rate of ingestion, digestibility, conversion efficiency of food and growth rate of the animal [12]. Nutritive value of mulberry (Morus spp.) leaf is a key factor besides environment and technology adoption for better growth and development of the silkworms and cocoon production. It is a confirmed fact that, leaf quality differs among mulberry varieties which in turn responsible for the difference in silkworm rearing performances [13]. Leaves of superior quality enhance the chances of good cocoon crop.

When genotypes perform consistently across locations, breeders should able to effectively evaluate germplasm with a minimum cost in a few locations for ultimate use of the resulting varieties across wider geographic areas [14]. However, with high genotype by location interaction effects, genotypes selected for superior performance under defined environmental conditions [15]. Therefore, it could be implicated that selection of better performing genotypes at one location may not enable the identification of genotypes that can repeat nearly the same performances at another location.

The results indicated that mulberry genotypes of Nekemte, Jimma, M-4, K-2, S-13 and and local check resulted significant variation in rearing performances of the worms. Insects do vary in efficiency of conversion of digested food due to the varied level of nutrients intake, quality of the food and total biochemical components of the leaf supplied to the insects [3]. Among different genotypes of mulberry, silkworms fed on leaf of S13 and K2 gave better results. In accordance to yield, maximum cocoon weight was recorded in those larvae which were fed on the leaves of S-13 (1.11g) and followed by that of K-2 (1.03g). The minimum cocoon weight (0.866 g) was recorded in local check. Similarly, the highest pupal weight was recorded from S-13 and K-2 (0.924g and 0.865g) respectively and the lowest was from local check (0.73 g) (Table 6).

Shell weight of cocoon revealed significant variation when fed with the different mulberry accession. Mulberry genotypes of S13 and K2 revealed significantly higher shell weight (0.187g and 0.168g) respectively as compared to local check and other treatments however, the least shell weight (0.136g) was recorded from local check. In parallel with shell weight, silk ratio was found significantly highest in S-13 and K-2 (16.82 % and 16.35 %) respectively but the lowest in local check (14.07 %) (Table 6).

Hatching of silkworm egg showed wide significant variation fed on different mulberry genotypes from 62 % to 75.33 %. The maximum hatching was recorded in M4 (75.33 %) closely followed by K2 (70.7 %), but lower hatching percent was obtained from Jimma (62%). Larval duration in days was recorded when the beginning is day and hour of larval brushing and the end is day and hour when the feeding is stopped and larvae mounted. Significant differences were observed in hatching percentage among treatments. Longer larval durations (34 days) were recorded in the worms fed on Nekemte, Jimma and local genotype whilst, S-13 and K-2 genotype showed shorter larval duration (31.5 and 32 days) respectively, as compared to other treatments. Silkworm fed on S13 recorded significantly higher fecundity (305.5) followed M4 (289.9). The lowest fecundity was recorded from local check (259.5). Effective rate of rearing (ERR) has also revealed significant difference when mulberry silkworm fed on different mulberry genotypes. Mulberry silkworm fed on S13 (81.5 %) recorded higher ERR closely followed by M4 (77%) and K2 (75.6 %). The least ERR was obtained from local check (68.83 %).

The silkworm is an of economic insect used for silk production and Sericulture or silkworm rearing depends on mulberry leaves as the sole natural food of the silkworm *Bombyx mori* L., the quality of the mulberry leaves has a direct bearing on the normal growth of the larvae and the quality of the cocoon [2]. The composition of mulberry leaves plays an important role in the growth and development of silkworms and other traits important to the economic production of these animals [16]. Significant seasonal variations occur in the nutritional value and composition of mulberry leaves depending on factors such as the weather, pests and diseases as well as agricultural practices such as fertilization, irrigation and other current practices [17]. This variation impacts both qualitatively and quantitatively upon the silkworm cocoon production. Weakness of nutritive value of mulberry leaves will lead to significant decrease of silk production [16].

The study by Rajesh et al on the increase of larval weight, cocoon and pupal weight and silk ratio exhibited by the silkworm fed on leaf was explained due to the higher rate of food ingestion, food assimilation and respiratory activity [18]. The involvement of these factors in increasing the larval body substance has been reported by Stockner [19]. In general, S-13 and K-2 showed better results in agronomic performances in the field and also gave better results in rearing performance of silkworms in the laboratory. Mulberry varieties regarded as one of important factors that affects on number of laid eggs, fecundity, hatchability, larval period and weight in local strains and obtained in silkworms. This variation will lead to various physiological state and cocoon production [20].

| Treatment | Melkassa | Jimma | Wondogenet | Hawassa | Alage |
| :--- | :--- | :--- | :--- | :--- | :--- |
| PH | NL | PH | NL | PH | NL |
| Nekemte | 202.6a | 224.9cd | 242.4a | 206.56 b | 242.41a | 142.22ab | 238.89a | 126.72 b | 220.5a | 138.93 bc |
| Jimma | 225.6a | 278.4bc | 276.48a | 165.29b | 276.48 a | 139.45b | 243.11a | 171.46 ab | 222.2 a | 123.91c |
| M-4 | 202.7a | 167.7d | 277.04 a | 208.85b | 277.04 a | 140.85 ab | 254.72 a | 182.94ab | 229.12 a | 166.24abc |
| K-2 | 211.3a | 303.6b | 260.89a | 373.11 a | 260.89a | 188.44ab | 256.94 a | 209.61a | 203b | 232.77ab |
| S-13 | 227a | 371.3a | 219.08a | 340.78 a | 219.08a | 177.82ab | 226.50 a | 227.89 a | 218ab | 272.07a |
| local | 194a | 217cd | 269.93a | 203.11 b | 269.93 a | 144.85 ab | 242.22 a | 158.45ab | 216ab | 136.14 bc |
| CV | 10.7 | 13.8 | 19.9 | 17.6 | 19.9036 | 17.15869 | 14.4 | 21.7 | 4.2 | 32.91 |
| LSD | 41 | 65.3 | 93.29 | 80.1 | 93.29 | 48.574 | 63.8 | 70.9 | 16.64 | 106.8 |

Means within the same column with a common letter are not significantly different (P<0.05), PH= plant height (cm), NL= Number of leaves per plant.

Means within the same column with a common letter are not significantly different (P<0.05), PH= plant height (cm), NL= Number of leaves per plant, 1ry=primary branches, 2ry= secondary branches. Table 2: Means for primary and secondary branches production seasons of 2013/2014 to 2016/2017 at harvesting stage of mulberry accessions grown across locations.

Treatment Fresh and Dry leaf weight (kg/ha) at harvesting stage, respectively Melkassa Jimma Wondogenet Hawassa Alage FLW DLW FLW DLW FLW DLW FLW DLW FLW DLW Nekemte 14645b 4592b 13199d 4272.6b 12478b 4822b 11535 c 4055.6ab 11116b 2890.1 b Jimma 17846b 5168ab 13793 cd 4375.5 b 13513b 4736b 12162c 4216.1 ab 12017b 3124.4b M4 16586b 4800b 19808bc 5083.3 b 19688ab 7044ab 14640bc 4726.9ab 11889b 3091.1 b K-2 24419a 7162ab 26333a 7698.6 a 26330a 8268a 21063a 6236.6 a 20597a 5355.3 a S-13 26503a 8027a 24053ab 6728.4 a 20865ab 6232ab 18830ab 5831.8b 18975a 4933.6 a Local 13046b 6245ab 13551d 4152.3b 14321b 4651b 10548c 3520.1 b 9435b 2453.1 b CV 169 28 17.9857 14.61082 26.444 23.79 22.40623 25.5 15.9 15.9 LSD 5820 3089.5 6039 1431.4 8595.2 2578.6 6031.5 2207.8 4051.2 1053.3 Means within the same column with a common letter are not significantly different (P<0.05), FLW= Fresh leaf weight, DLW= dry leaf weight, TFLWHA= total fresh leave per hectar, TDLWPP= total fresh leave weight per plant. Table 3: Means for fresh and dry leaf weight (kg/ha/year) at harvesting stage of mulberry accessions grown across locations.

Treatment Stem thickness, leaf area and Internode length at harvesting stage, respectively Melkassa Jimma Wondogenet Hawassa Alage ST LA IL ST LA IL ST LA IL ST LA IL ST LA IL Nekemte 9.8 258 6.3ab 13 276ab 6.5 13.3 276ab 6.5 11 144c 6.4 11.6 147.55b 6.7 Jimma 9.4 258 5.9ab 13.9 333 a 6.5 13.8 333 a 6.5 11 220ab 6.5 11.8 167.69b 6.5 M-4 9.2 267 5.5b 13.6 326a 5.9 13.6 326 a 5.99 13 229 a 7.9 12.98 242.67 a 8.3 K2 9 241 6.4a 12.9 248ab 6.4 12.9 248 ab 6.4 13 168 abc 6.4 13.3 169.53b 6.7 S13 10.6 194 5.7ab 11.1 205 b 6.7 11.1 205 b 6.7 11 139c 6.3 11.49 138.89b 6.7 Local 9.4 225 7.28 14.2 313ab 6.9 14.2 313 ab 6.86 12 150 bc 7.7 12.53 150.22b 7.9 CV 11.4 17 0.8 21 21 13.6 21 21 13.64 23 22 16.5 21.7 22.97 18.3 LSD 1.94 74 0.73 5 111 1.6 5 110.5 1.6 5 70.5 2.1 4.86 70.82 2.36 Means within the same column with a common letter are not significantly different (P<0.05), ST= stem thickness, LA= leaf area, IL= internode length. Table 4: Means for stem thickness, leaf area and Internode length during the production seasons of 2013/2014 to 2016/2017 at harvesting stage of mulberry accessions grown across locations.

Treatment Incidence (%) and Severity (%) at harvesting stage, respectively Melkassa Jimma Wondogenet Hawassa Alage Incidence Severity Incidence Severity Incidence Severity Incidence Severity Incidence Severity Nekemte 23.24a 37.4a 28.607 ab 29.073 bc 28.607ab 29.073bc 21.737 a 22.667ab 64.243 a 51.447 a Jimma 22.23ab 33.7a 26.770 abc 37.150ab 26.770abc 37.15ab 18.253 a 18.167 bc 56.6ab 49.163 a M4 16.9ab 18b 21.037 bc 16.813 d 21.037 bc 16.813d 20.370a 14.017 c 25.5d 30.280b K2 10.6ab 17b 18.620 c 23.923cd 18.620 c 23.923cd 15.133 a 15.777 c 38.887 c 26.833b S13 8.3b 10.5b 20.230 bc 17.967 d 20.230 bc 17.967 d 17.983 a 19.643 abc 27.23d 30.663b Local 21.4ab 36.7a 30.533 a 40.627 a 30.533 a 40.627 a 19.640 a 25.473a 47.853 bc 53.833a CV 45.3 20.4 20.33731 21.94813 20.33731 21.94813 42.90386 17.93634 14.38443 25.11686 LSD 14 9.5 8.9906 11.017 8.9906 11.017 14.715 6.2947 11.356 18.447

Means within the same column with a common letter are not significantly different (P<0.05), B: Hap= hatching percentage, LaD= larval duration (HR), Larw =larval weight, CoW= cocoon weight, Shw =shell weight, SiR= silk ratio, fecund= fecundity, ERR =effective rate of rearing; Table 6: Performance of bivoltine mulberry silkworm strain fed on different mulberry varieties.

Conclusion and Recommendation

The present study revealed that S-13 and K-2 performed best results in most important agronomic parameters in the field and laboratory conditions as compared to other accessions. Since mulberry varieties have strong influence on mulberry silkworm rearing performance thus, selection of mulberry varieties for rearing mulberry silkworms is very important in order to get better yield. Therefore, based on agronomic and laboratory results S-13 and K-2 were found to be the best promising mulberry variety for rearing of mulberry bivoltine silkworm and will be recommended for mulberry silkworm research and development efforts in future.

Acknowledgments

We would like to acknowledge Ethiopian Institute of Agricultural Research (EIAR) especially Melkassa Agricultural Research Center (MARC) for covering all the costs of the study and providing facilities. We would also like to acknowledge all MARC laboratory staff and the collaborative centers for their cooperative during the period of the study.