A number of factors including hormonal, chemical, environmental, physical, behavioural aspects etc. have been attributed to be significant in successful egg deposition by female silk moth (Raabe, 1986). Among them, photoperiod, temperature, humidity, duration, frequency ofmating etc. (Punitham et.al., 1987; Chattopadhyay, 1995; Leopold, 1976; Davey, 1985; Raabe, 1986) have been significant in oviposition behaviour. Also the reproductive physiology, as mating, vitellogenesis, ovulation, oviposition (Davey, 1985), weight of pupae and pharate adult, surface texture and inclination also are important factors for viable egg deposition by adult female moth. Photoperiod in insects serves as a clock indicating the seasonal changes and influences their life cycle, distribution and abundance. Photoperiod had been reported (Yamaoka and Hirao, 1975, 1981; Wang, 1988) to play an important role on oviposition. Studies of Engelmann (1970) indicated that egg laying or total egg production in insects do not necessarily give a true picture of the actual reproduction potential unless factors like atmospheric condition during mating period and oviposition and nature of laying substratum are considered either individually or on a cumulative effect on the egg production. Egg laying capacity of various breeds of silkworm, Bombyx mori L. have been reported to vary even under identical nutritional, atmospheric, mating and laying conditions (Hassanien and EL-Sharaawy, 1960, 1962; Yokoyama, 1963 and 1973; Narayanan et al., 1964; Jolly et.al., 1966; Sidhu et al., 1967; Ueda et al., 1971; Nakasone, 1970; Sengupta et.al., 1973; Pillai and Krishnaswami, 1980; Rahman et.al., 1980; Ito, 1967). Hatching of silkworm is predominantly diurnal. It seems to be under the circadian control (Sivarami Reddy, 1993; Sivarami Reddy and Sasira Babu, 1990).

Keeping this point in view the present study was undertaken to study the reproductive performance of multivoltine and bivoltine breeds and a bivoltine hybrid of silkworm, Bombyx mori L. to assess seasonal variation in egg laying potential, hatching performance and egg laying rhythmicity, that can be utilized by the silkworm seed grainier formaximum egg recovery.

Four silkworms breeds/hybrid i.e, Nistari (Multivoltine), NB18, P5 (Bivoltine) and P5 x NB18 (bivoltine Hybrid) were selected for the behavioural study on the egg laying rhythm. The rearing were conducted in four commercial seasons i.e. February？March (S1: favourable seasons, 20 - 30℃ and 45-75%), May？ June (S2: unfavourable seasons, 32 - 38℃ and 75- 95%), September？October (S3: unfavourable seasons, 30 - 37℃ and 75-98%) and November？December (S4: favourable seasons, 20 - 30℃ and 45-80%) for three consecutive years. Five disease free layings (dfls) of each breed/hybrid were brushed following the rearing procedures as suggested by Krishnaswamy (1978, 1979) with minor modifications. A standard well-practised mulberry variety S1635 was used for the experiment purpose. Soon after adult emergence, 300 female and 300 male moths of each breed/hybrid were allowed to copulate for 210 min. After decoupling, female moths were allowed to lay eggs on egg sheets. One batch of female moths were allowed to copulate at 6 am and decoupled at 9.30 am, whereas a second batch were allowed to copulate at 12 noon and decoupled at 3.30 pm and subsequently allowed to lay eggs in BOD condition (25 ± 1℃ and 85 ± 5% RH) and at ambient conditions (23 - 28℃ and 74.7 ± 5%) separately. Ten replications were kept per treatment to record the egg laying rhythm; moths were shifted to fresh sheets every two hours and allowed to lay eggs under both conditions. Thus the moths were shifted for a total of 16 times, every two hours. A total of 36 h duration was given continuously to lay eggs.

The reproductive output of the female moths of different silkworm breeds & hybrid was studied for recording the total number of eggs laid and also the egg retention in the reproductive tract. Finally the data were subjected to statistical analysis. The hatching percentage i.e., number of eggs hatched with respect to total number of eggs laid by the female moth plays an importance role in cocoon productivity for which it was considered during the study.

The ideal condition of silkworm rearing is 25 -28℃ and 70-80% RH. Amongst the four commercial

crop seasons of West Bengal i.e. February？ March with 20 - 30℃ and 45-75%) and November ？ December with 20-30˚C and 45-80 % are considered as favourable seasons, where as , May？June with 32 - 38℃ and 75-95% and September ？October with 30 - 37℃ and 75-98% are considered as unfavourable seasons. Therefore, two favourable and two unfavourable seasons were selected for the experiments.

Multivoltine, Nistari, was selected because it is robust, indigenous and local breed (Kangayam et al., 2002). It can be reared 4-5 times per year. Its adaptability in all the seasons both favourable and adverse is more than any other breed. The eggs are nondiapause in nature. So it can be treated as control. Improved pure bivoltine, NB18 and P5 as well as robust bivoltine hybrid (NB18 x P5) were selected for comparison with the control.

Fecundity and hatching percentage: Reproductive performances of the selected breeds/hybrid studied during different seasons have been presented in Table 1 and the fecundity represented graphically in Fig. 1. The hatching potential of the breeds/ hybrid during the favourable and adverse seasons have been depicted in Fig. 2A and 2B.

Nistari: In multivoltine breed, Nistari the maximum fecundity of 438 ± 21.43 was recorded in season S1 followed by S4 at 392 ± 4.53. The lowest fecundity

was noted in adverse climatic season of S2 at 307 ± 19.28, whereas maximum egg retention (27 nos.) was recorded in season S3 followed by S2 (19 nos.) and minimum egg retention (5 nos.) was observed in S₁.

This clearly demonstrates that climatic conditions during adverse seasons have a greater impact on the race fecundity with maximum egg retention. Total egg production (laid + unlaid) was found to follow the same trend like fecundity. Results on hatching percentage revealed that maximum hatching at 92.33% ± 0.83 was obtained in season S3 followed by S4 at 92.21 ± 2.54 whereas minimum hatching was recorded in season S2 at 83.51% ± 4.33.

NB18: In bivoltine breed, NB18 the maximum egg production was recorded in season S4 at 568 ± 28.08, followed by S1 season at 525 ± 38.81 whereas minimum fecundity was recorded during S2 season at 417 ± 41.70. This was expected to be true in case of bivoltines that generally are difficult to rear during adverse climatic situations as observed in S2 and S3. Eggs unlaid were more in season S3 at 22 nos. and less in S1 season at 7 nos. Total egg production followed the same trend. The maximum hatching percentage of 91.11% ± 4.57 was observed in season S4 followed by almost similar results in season S₁ and S₃, followed by season S2 at 79.80%± 3.19.

P5: The fecundity and total egg production of another potential bivoltine breed P5 followed the same trend as observed with NB18. Eggs unlaid were found to be more in season S3 with about 32 nos. followed by 17 nos. in season S2. This was followed with season S1 with 10 nos. and season S4 with 4 nos. Although, the hatching percentage was found to be maximum at 91.72% ± 4.72 in season S2 followed by seasons S3, S1 and S4.

P5 x NB18: The hybrid represented a cross between pure lines, P5 and NB18 and therefore had the best fecundity in comparison with the breeds in almost all the seasons reported in the study. There was variations observed in the fecundity and total egg production (laid + unlaid) among the seasons with maximum fecundity of 562 ± 18.29 in S₄ season followed by 518 ± 27.82 in S1 season. The lowest fecundity recorded for the hybrid was in season S2 at 461 ± 19.25. The egg retention was found to be the highest at 24 nos. in season S3 followed by S1. However, hatching percentage was recorded the maximum in S3 at 96.75% ± 0.5, followed by season S4 at 90.18% ± 1.83, with the lowest hatching percentage observed in S1 at 86.79% ± 3.45.

Seasonal variation was observed among all the 4 experimental breeds & hybrid, with Nistari showed lesser fecundity in all the seasons compared with the others breeds & hybrid. The highest fecundity was recorded in breed NB18 (568 ± 28.08) compared to other breeds & hybrids in all seasons. While highest hatching percentage was recorded in hybrid NB18 x P5 compared to all other breeds.

The total number of eggs lay therefore associated with the seasons and races, with S1 and S4 being the most favourable season. This confirmed the observations of Tazima (1958), who observed the variation in the number of eggs according to the season and climate. Khan et al., (2003) also reported the suitability of spring and autumn seasons to be most favourable in West Bengal for all the races. Jha (1997) reported that, variations in temperature and RH can cause incomplete fecundity and prolongation in oviposition and extreme conditions result in poor development of embryo and increase in sterility. Bhasker et al., (1992) recorded the gradual decline in the fecundity of the moths at higher temperatures. This can be correlated with the blocking of juvenile hormone release from corpora allata. The influence of juvenile hormone seems to one of the contributing factors in determining the growth rate of the larvae and fecundity. Hence, temperature could be considered as an influential factor in determining the growth and fecundity of silkworm through the mediation of juvenile hormone. Low RH (less than 70%) can also lead to weak egg laying with high proportions of unfertilized eggs, resulting in low hatchability, while high RH (above 80%) causes larval mortality by favoring disease infestation (Ifantidis, 1982; Patil and Vishweshwara, 1986). At low humidity, larvae lose considerable proportion of water from their body which results in inefficient metabolism and less conservation of energy for pupal and adult stages, eventually affecting egg laying ability of the female moths. This may be the probable reasons for lower fecundity and hatching % during the unfavourable seasons.

Egg production depended on the phase relationship between photo- and thermo period and on the frequency of the temperature oscillations (Ratte, 1985). Photoperiod played a phase-setting role in the ovipositional rhythm. In this study, hatching percentage was found to be independent either on changes in environmental condition or silkworm breed and pattern of hatching more or less followed the same pattern in case of all the selected breeds and hybrid. Hatching in mulberry silkworm, Bombyx mori L was predominantly diurnal under natural day conditions. It seemed to be under circadian control (Sivarami Reddy, 1993). Rhythmic patterns

in hatching of Bombyx mori as influenced by photo- period had also been reported (Anantha Narayana, 1980; Sivarami Reddy, 1993).

Experiment on egg laying rhythm (the case where the decoupling of the female moth happened at 9.30 am) was conducted both at ambient temperature and BOD conditions and are depicted in Fig. 3A and 3B. Maximum eggs laid were recorded from 5.30 to 7.30 pm in case of multivoltine race, Nistari, whereas minimum egg laid was observed during morning hours. In bivoltine silkworm breeds as NB18 and P5, the egg laying was found to be the highest during afternoon (1.30 pm to 5.30 pm) and minimum during morning hours. The bivoltine hybrid (P5 x NB18) followed the

similar trend of pure bivoltines with maximum egg laying reported from 1.30 pm to 5.30 pm and minimum during morning hours.

Experiment on the rhythmicity of egg laying (the case where the decoupling of the female moth happened at 3.30 pm) was also conducted both at ambient temperature and BOD conditions and are presented in Fig. 4A and 4B. In multivoltine breed Nistari, maximum egg laid was observed just after decoupling at about 3.30 to 7.30 pm and minimum during morning hours as reported while decoupling the female moth at 9.30 am. In NB18,maximum egg laid was recorded after or at the time of decoupling. In P5 and bivoltine hybrid, P5 x NB18, similar observations were noticed for the egg layings. It was therefore from both the possibilities that egg laying rhythmicity patterns remain more or less committed on similar lines excepting for the differences in oviposition time.

Results of the present investigations also revealed that 28.01% and 66.63% eggs were laid by Nistari during late photoperiod in ambient (room) and BOD conditions respectively when the female moths were allowed to lay eggs at 9.30 am and 20.47% and 33.93% eggs were laid during Scotoperiod in both the conditions respectively, but little or few number of eggs were laid during early photoperiod. The remaining % of eggs was laid in the next late photoperiod in both the conditions. The observations are in conformity with the findings of Fugo (1984). Bivoltines and their hybrid showed that about 90.21% - 95.42% of eggs were laid during late photoperiod in both conditions whereas during Scotoperiod few numbers of eggs were laid.

In another experiment the female moths were allowed to lay eggs at 3.30 pm. 61.57 and 32.26% eggs were laid by during late photoperiod and Scotoperiod. The bivoltines showed a similar pattern of egg laying behaviour like reported previously. Maximum ovulation and oviposition with minimum retention of egg was reported at 25.36 ± 0.17℃ (optimum) temperature and 80 ± 5% relative humidity. Any fluctuation of temperature from optimum level leads to decreased ovulation, oviposition rate, fecundity and increased retention of eggs (Mathur and Lall, 1994).

Logen and Harwood (1965) have reported that in Culex tarsalis, oviposition showed a bimodal pattern, in which egg laying occurred in the first few hours following the onset of the photo-phase and again after the beginning of the Scoto-phase (Flitters, 1964). Moths of genus Crambus have been shown to oviposit mainly during the early Scoto-phase (Crawford, 1966; Banerjee and Decker, 1966).

The study therefore established the potentiality of the hybrid over the pure breeds in laying the eggs in adverse and favourable seasons. It also was proved that congenial climate is a boon as it leads to healthy fecundity with less egg retention and satisfactory hatching percentage, though the variations in some cases may be marginal. The February-March and November - December seasons have been found to provide with good reproductive performance in comparison to adverse seasons. It was also revealed that late photoperiod and early scotoperiod were favourable for egg laid in case of multivoltine breed and bivoltine prefers late photoperiod for egg laying.