Influence of production seasons on economic traits of bivoltine strain of mulberry silkworm (Bombyx mori L.) in Melkassa Agricultural Research Center

Technical Summary: Influence of Production Seasons on Economic Traits of Bivoltine Strain of Mulberry Silkworm (Bombyx mori L.)

Problem Statement
Sericulture is highly sensitive to environmental fluctuations, particularly temperature and humidity, which mediate the embryonic and post-embryonic development of the poikilothermic silkworm (Bombyx mori L.). While optimal growth occurs between 20°C and 28°C, deviations in these parameters—common across different production seasons—can lead to embryonic disorders, retarded physiological activities, increased mortality, and reduced silk yield. Despite the known sensitivity of silkworms to seasonal changes, there is a need to quantify the specific phenotypic differences in economic traits of bivoltine strains under the distinct seasonal regimes of the Melkassa Agricultural Research Center in Ethiopia. The study addresses the lack of localized data comparing the rearing performance of a specific Chinese bivoltine strain across wet rainy, cold dry, and hot dry seasons.

Methodology
The experiment was conducted at the sericulture research laboratory of the Melkassa Agricultural Research Center (8°24' N, 39°21' E, 1550 m altitude). The study utilized a Chinese bivoltine strain (Black Plain) as the experimental unit.

• Design: A Complete Randomized Design (CRD) with three replications was employed.
• Treatments: Three production seasons served as treatments:
  1. Wet Rainy: June–September
  2. Cold Dry: October–January
  3. Hot Dry: February–May
• Procedure: Standardized rearing techniques were applied, including disinfection with 2% formalin, shelf rearing, and specific feeding schedules (tender leaves for early instars, mature leaves for late instars). 300 larvae were brushed per replication.
• Data Collection: Observations included daily temperature and relative humidity, incubation period, hatchability, larval duration, mortality rates (young and late stages), effective rearing rate (ERR), fecundity (eggs per female), and cocoon quality metrics (fresh cocoon weight, shell weight, pupal weight, and shell ratio).
• Analysis: Data were analyzed using ANOVA in R programming at a 5% significance level, with Least Significant Difference (LSD) used for mean separation.

Key Results
The study revealed that the wet rainy season consistently yielded superior economic traits compared to the cold dry and hot dry seasons.

• Environmental Conditions: The wet rainy season recorded the highest relative humidity (max 79.5%), while the hot dry season recorded the highest temperature (max 28.7°C) and lowest humidity (min 61.3%). The cold dry season recorded the lowest temperature (min 18.6°C).
• Reproductive Traits:
  • Fecundity: The wet rainy season produced the highest number of eggs per female (356.4 ± 40.7), significantly outperforming the cold dry (290.9 ± 41.4) and hot dry (315.5 ± 23.3) seasons.
  • Incubation: The incubation period was shortest in the wet rainy season (8.3 ± 1.29 days) and longest in the cold dry season (12.2 ± 1.28 days).
  • Hatchability: While the hot dry season showed the highest hatchability (95.8%), the wet rainy season maintained a high rate (92.8%), both significantly better than the cold dry season (88.17%).
• Larval Performance:
  • Mortality: The wet rainy season recorded the highest mortality rates at both young (10.5%) and late (5.7%) stages according to the text description, though the data in Table 2 indicates the cold dry season had the highest values (17.3% young, 9.3% late). The text explicitly states: "The higher larval mortality was recorded in wet rainy season for both young stage mortality (17.3 ± 5.17a) and late-stage mortality (9.3 ± 3.1a)."
  • Growth and Survival: Larval weight was highest in the wet rainy season (1.8 ± 0.12 g). The Effective Rearing Rate (ERR) was significantly superior in the wet rainy season (83.2%) compared to the cold dry (72.0%) and hot dry (77.9%) seasons.
  • Duration: The total larval period was shortest in the hot dry season (25.0 days) and longest in the cold dry season (28.85 days).
• Cocoon and Silk Traits:
  • Yield: The number of normal cocoons harvested per 300 worms was highest in the wet rainy season (247.7).
  • Weights: Fresh single cocoon weight (1.55 g), pupal weight (1.28 g), and shell weight (0.26 g) were all significantly highest during the wet rainy season.
  • Quality: The shell ratio percentage, an indicator of raw silk content, was highest in the wet rainy season (21.9%), compared to 19.3% (cold dry) and 18.6% (hot dry).

Key Contributions
This study provides empirical evidence that the wet rainy season offers the most favorable environmental conditions for the rearing of the Chinese bivoltine silkworm strain in the Melkassa region. It quantifies the specific economic advantages of this season, demonstrating that it simultaneously maximizes fecundity, larval survival, cocoon weight, and silk quality while minimizing incubation time and mortality. The research establishes a baseline for comparing seasonal performance, highlighting that the cold dry season is the most detrimental to silkworm productivity due to low temperatures and humidity.

Significance and Claims
The paper claims that rearing silkworms during the wet rainy season results in significantly higher productivity and better economic traits across all measured variables. The authors conclude that while the wet rainy season is naturally superior, silkworm rearing during the less favorable cold dry and hot dry seasons can be enhanced. The study suggests that remedial measures should focus on simulating the environmental conditions of the wet rainy season—specifically by regulating temperature and relative humidity—to mitigate the adverse impacts of seasonal fluctuations. The findings support the adoption of environmental control practices to stabilize production year-round, rather than relying solely on natural seasonal cycles.