EFFECT OF SEASONAL TEMPERATURE AND HUMIDITY ON OVULATION, FECUNDITY AND RETENTION OF EGGS IN SILKMOTH, BOMBYX MORI (L.)

Studies OD reproductive biology in silkmoth, Bombyx morl (L.) have shown that temperature and relative humidity, prevailing at the time of rearing, play an important role in egg production and oviposition (Tikkoo et al., 1975; Tazima, 1978; Rahman et aI., 1980). In the tropical plains of West Bengal, there is a high degree of fluctuation in temperature and relative humidity throughout the year. During the period from March to June, temperature is comparatively higber with lower humidity and from June to September, both these parameters are on higher side. Therefore, the silkworm cocoon crops, obtained during the period from April to September, are comparatively poorer, when the season is unfavourable. The other half of the year from October to March, when both the average temperature and relative humidity are comparatively lower, these grow richer, and the season is called favourable (Krishnaswami et al., 1971).


INTRODUCTION
Studies OD reproductive biology in silkmoth, Bombyx morl (L.) have shown that temperature and relative humidity, prevailing at the time of rearing, play an important role in egg production and oviposition (Tikkoo et al., 1975;Tazima, 1978;Rahman et aI., 1980). In the tropical plains of West Bengal, there is a high degree of fluctuation in temperature and relative humidity throughout the year. During the period from March to June, temperature is comparatively higber with lower humidity and from June to September, both these parameters are on higher side. Therefore, the silkworm cocoon crops, obtained during the period from April to September, are comparatively poorer, when the season is unfavourable. The other half of the year from October to March, when both the average temperature and relative humidity are comparatively lower, these grow richer, and the season is called favourable (Krishnaswami et al., 1971).
It is observed in Bombyx morl that all the eggs produced in the ovary ate not oviposited, but some are retained. Therefore, studies have been launched to find out the effect of seasonal temperature and relative humidity on ovulation, fecundity and retention of eggs and their interdependance in multivoltine silkmoth, Bombyx morl (race nislari). The results of the study are discussed under two broad seasons, viz., unfavourable season (March-September) and favourable season (October-February).

MATERIALS AND METHODS
The indigenous multivoltine silkworm race nistari of Bombyx mort (L.), from West Bengal which is generally reared during the four commercial seasons, viz., Jaistha (May), Bhaduri (August-September), Agrahayani (November) and Chaitra (February), was selected as an experimental material. Larvae were re'ared in mass under the room conditions. On completion of spinning, cocoons were taken at random on the Sth day of pupation from the mass lot. After cutting the cocoons, 30 each of female and male pupae of approximate weight (0-9 g.:l:0·05 g.) were selected on the 5th day of pupation for the experiment. The 5th day pupae were selected, because their weight at this stage is less influenced by moisture content than that of the fresh pupae (Orooz, 1665). Pupae and moths were maintained under room conditions. On emerangence, mating of male and female moths were allowed to lay eggs for 24 hours on egg cards bearing serial number (1-30). Each female was then dissected to find out the number of retained eggs in the ovarioles ; the number of eggs laid were also counted individually. Prevailing data of room temperatu're and relative humidity were recorded four times a day (6 A.M. 10 A.M., 4 P.M. and 9 P.M·.) with the help of a dry and wet bulb zeal thermometer from rearing to oviposition. Average data of dry temperature and relative humidity were noted and. their mean seasonal readings are presented in Table 1  Analysis of the data presented in Table 1 an~ 3 has revealed that ovulation, fecundity and retention percent of eggs showed significant differences in the seasonal mean readings, as discussed below.
(i) Ovulation: The rate of egg production during the seasons of August-September and March-April differed significantly from that during May .. July. Number of eggs produced in the ovary was recorded maximum (292) during August·September, when the average temperature was high (27·6SoC ± 0-157°C) with high relative humidity (89 0 99% ± 0'434%).
(iii) Retention per cent of eggs: The phenomenon during the season of August-September differed significantly from that of the period during March-July.' Retention per cent of eggs was recorded (2'47%) during March.April, when the average temperature was high R8 (29049°C ± 0-429°C) with low relative humidity (55,95% ± 1-581 % ), while it was recorded maximum (6'51%) during August-September, wben the average temparature was high This indicated that high temperature with high humidity favoured high retention per cent of eggs in the ovary_ Retention per cent also showed positive significance (P < 0'01) correlation with ovulation and fecundity (Table.3).
(B) Favoorable seaSOD (October-February) : Analysis of data presented in Tables-2 and 3 showed significant difference amongst the sea,sonal mean-readings for the biological processes under study, as discussed below.
(i) Ovulation: The data of ovulation during the seasons of November-December and December-January differed significantly from each other and also individually from that of the seasons of October-November and January-February. However, there was no significant difference between October-November and January-February seasons_ ~otal number of eggs produced in the ovary was recorded maximum (497) during November-December, when the temperature was 2S'36°C ± 0'176°C with relative humidity 69-20% ± 1-037%. It decreased insignificantly (471) during December-January, when the temperature was comparatevely low (24 0 43°C :I: 0'94°C) with relative humidity 70 0 15% ± 0'782%, However, egg production in the ovary reduced to minimum of 408 (January-February) and 415 (October-November), when the temperature and relative humidity altered from 2S'36°C ± 0-176°C and 69'20% ± 1-037% (optimum) respectively. Ovulation C_ D. = Critical difference. N. S, = Not Significant, was found to have high positive significant (P < 0-0 1) correlation with fecundity and high negative significant (P<O'OI) correlation with retention percent (Table-3

).
Keeping all the seasons in view in the tropical plains of the state, the phenomena of reproductive biology, viz., ovulation, fecundity and retention per cent of eggs of Bombyx mori were greatly influenced by both temperature and relative humidity. Wigglesworth (1972) reported that the rate of egg product~on varies with temperature. It is accelerated up to point and then falls rapidly. The range of temperature, at which reproductioncan occur, is of~en much narrower than that of other normal activities of the same species. Further, if the temperature be of adaptive importance for a particular species, different aspects of life-cycle sh~uld be under the strict control of temperature-bound factor (Schnebel and Joseph, 1986). Congdon and Logan (1983) found that at high temperature (up to 31°e), fecundity decreased in Bank Grass Mites (BGM), Ollgonlchul paratensis. Nickel (1960) found that in high humid condition (85-90%) BOM, rr'etrcmychus desertorum Banks laid fewer eggs. Boyne and Hain (1983) also noted a similar relationship with Olig""ychus ""unguis.. Boudreaux (1958) reported that four Tetranychus 8pS. laid more eggs at a faster rate in dry condition ( < 35% R. H.) Data presented in Tables 1-3 reveal only one factor alone, i. e., temperature or.: relative humidity was not responsible for increasing/decreasing ovulation, fecundity and retention of eggs, but indicated that it was a combined effect of temperature and relative humidity.
Considering all the seasons of unfavourable and favourable period in view, it indicated that temperature at 25-36 c C :l: 0·176°C with relative humidity 69°20% :I: _·037% was found optimum (November-December) for ~maximum production of eggs in the ovary (495), and minimum retention of eggs (0°37%). Any increase r:or decrease in temperature and relative humidity from the optimum level increased retention of eggs or decreased egg production and fecundity respectively. Fecundity was found to have high positive significant (P < 0·01) correlation with ovulation irrespective of seasons.
However, with retention per cent of eggs, it exhibited positive significant (P <0·01) correlation during unfavourable season and highly negative significant (P <001) correlation during favourable season (Tabl~3). Stamenkovic (1985) reported that the effect 0 temperature plays a significant role on the fecundity of summer fruit tortix moth, Andoxophyes orana. Cunnington (1985) suggested that the most favourable condition for oviposition by gain mite, Acarus siro were 20-25°C and 80-90% relative humidity_ Ouhelli and Pandey (1984) also found that 2SoC temperature and 84% relative humidity were suitable for laying viable eggs in Hyalomma lusitanicum. q'he above studies indicate that an inter-relationship might exist between temperature and relative humidity affecting oviposition. Keeping seasonal conditions in view, above findings corroborate our results that about 25°C temperature and 70% relative humidity is most favourable for maximum egg production and oviposition with minimum retention in multivoItine silkmoth, Bombyx morl (race-nistari).
The results reported are useful for predicting silkworm seed production as a function of seasonal temperature and relative humidity in the tropical plains. This is an important aspect of efforts to determine their impec,t on silkworm rearing and silk yield, because it allows one to determine the de J rees of seasonal synchrony with potential egg productioD.

SUMMARY
In the tropical plains of Wes't Bengal, there is a high degree of fluctuation in temperature and humidity throughout the year. With this, ovulatioD, fecundity and retention of eggs in mulivoltine silkmotb, Bombyx mari (L.) [race nistari] vary in different seasons. Maximum ovulation and fecundity with minimum retention of eggs was recorded in November-December, 1984 when the temperature was 25'36°C ± 0'17°C and relative humidity 69'20/~ ± 1'03 (optimum). Any fluctuation of temperature and humidity from the optimum level decreased ovulation and fecundity and increased retention of eggs. All these phenomena together with their possible interactions during favourable (October-February) and unfavourable (March-September) season have been discussed.