EFFECTS OF MONOCULTURE ON COLLEMBOLA AT A CROP FIELD IN NADIA (WEST BENGAL)

The distribution, abundance and seasonal changes in the population density of Collembola were investigated both in India and abroad by numerous workers. Notable amongst them are Bellinger (1954), Sheals (1957), Harrlov (1960), Christiansen (1964), Choudhuri and Roy (1972), Mitra et. ale (1977), Addison (1980), Hazra (1984), Hazra and Choudhuri (1983, '90), Alfred et. ale (1991) and Hazra and Sanyal (1996). Though Mitra et. al studied the distribution of Collembola in· accordance with different vegetation, the influence of sugarcane crop on the distribution of Collembola in India has not been reported so far. The present report forms part of a study of a longterm project on the qualitative and quantitative composition of coHembolan fauna of a controlled monoculture of sugarcane crop persisting in the field through out the period of observation.


INTRODUCTION
The distribution, abundance and seasonal changes in the population density of Collembola were investigated both in India and abroad by numerous workers. Notable amongst them are Bellinger (1954), Sheals (1957), Harrlov (1960), Christiansen (1964), Choudhuri and Roy (1972), Mitra et. ale (1977), Addison (1980), Hazra (1984), Hazra and Choudhuri (1983, '90), Alfred et. ale (1991) and Hazra and Sanyal (1996). Though Mitra et. al studied the distribution of Collembola in· accordance with different vegetation, the influence of sugarcane crop on the distribution of Collembola in India has not been reported so far. The present report forms part of a study of a longterm project on the qualitative and quantitative composition of coHembolan fauna of a controlled monoculture of sugarcane crop persisting in the field through out the period of observation.

MA TERIAL AND METHOD
A total of 108. soil samples were drawn, 9 from each of the 3 plots (5 sq. m) at monthly intervals over a period of one year. Each plot was sampled at random by using stainless ste~l samplers having 8.55 cm 2 in cross sectional area. The soil samples were extracted through the Tullgren funnel, modified by Macfadyen (1953).

THE EXPERIMENTAL SITE
The site was located within the experimental crop fields of Sugarcane Research Station at Bethuadahari in the district of Nadia, West Bengal, about 120 km north of Calcutta. The field underwent monoculture of sugarcane throughout the year. No chemical or organic manure was applied except a cover of sugarcane leaves undergoing decomposition with grasses like Cynodon dacty,loll, Dichanthium annulatum, Physelis sp., Arundinella sp. growing naturally in the field. The soil was brownish in colour and sandy silt in texture.

OBSERV ATION
The analysis of extracts revealed the presence of 19 collembolan species belonging to 18 genera. The most predominant was Salina indica (35.0%) followed by Lepidocyrtus sp. (a)

MONTHLY DYNAMICS IN POPULATION
The collembolan population showed its highest peak (24.26%) in the month of February , 97 and minimum (0.86%) in the month of December '96. A second peak (22.01 %) was found in August followed by smaller peaks (12.820/0 and 10.740/0) during January and March '97 (Fig. 2). Salina indica occurred through out the year being specially abundant during the rainy season (July-September) with highe~.t peak in August (16.63%), and with moderate build up (0 .. 86%-6.41 0/0) during rest of the year barring May and December when it was only 0.17% of the total collembolan ·population. The second dominating species Lepidocyrtus sp. (a) was also found through out the sampling period and it was maximum during February (7.970/0). The lsotomurus sp. was dominant during February (12.13%) but it was infrequent in occurrence. While, Cyphoderus javanus had minimum population build up (0.17%-1.03%) (being absent in samples of October to December) but showed its peak of population during August (2.25%).
Seira cf. indica appeared in samples of May (0.17%) and June (3.11 %) i.e., at the very beginning of the cultivation. Sminthurides sp. and Isotomiella minor comprising 2.59% and 2.0% respectively of the total populatien did not show any regular occurrence throughout the sampling period (Table. 1).

DISCUSSION
The result presented here is based on the random sampling from a monoculture crop field (sugarcane) which is retained in the field for over a year. The collembolan population from this sugarcane field was represented by 18 genera involving 19 species. In earlier reports, the peak of population of Collembola was observed in the month of July-August and minimum in May by Choudhuri and Roy (1972), Mitra (1976) and Hazra (1976) in vegetational sites other than sugarcane, dealt with in this study. In the present study, Collembola exhibited the highest peak in February and the minimum in December. This difference might be due to the effect of no cultivation in the February (Mitra, 1993). The plantation at the site was done in late March when there was no, undergrowth in the field. Further, no pesticides or insecticides were used throughout the period of cultivation. Eventhough, the field underwent handweeding during monsoon, the diversity of species was significant in comparison to the individual population buildup of each species. SUMMARY the present investigation is based on the monthly soil sampling made during April' 1996 to March '97 with a view to assesing the diversity of collembolan fauna, both qualitatively and quantitatively, in a sugarcane crop-field at Nadia, West Bengal.
Extraction of ] 08 soil samples yielded ] 154 examples of Colle'mbola belonging to 18 genera and 19 species. The species belonged to Salina sp., Lepidocyrtus sp., Isotomurus sp., Cyphoderus sp., Seira sp., Sminthurides sp., Isotomiella sp. Frequency of occurrence of most of the species during this period has been infrequent barring Salina indica (Imms), being most predominant in the sugarcane field. Two peaks of population were observed, one duri ng February, followed by the another in August.

ACKNOWLEDGEMENT
Authors are grateful to Dr. J. R. B. Alfred, Director of Zoological Survey of India, for providing laboratory facilities. Authors are also deeply indebted to Dr. A. K. Ghosh, former director for his constant inspiration and support for conducting this work.