DIVERSITY OF PHYTOPHAGOUS AND PREDATORY MITES ON MANGROVE AND AGRI-HORTICUL TURAL CROPS IN SUNDARBAN BIOSPHERE RESERVE , WEST BENGAL

Sundarban Biosphere Reserve (SBR) is very rich with at least 36 species of mangrove vegetation of the 55 species of mangrove plants that are known to occur in the world. Besides, during the past 2-3 decades, agricultural horticultural crops have been started growing in different re-claimed areas of SBR which have further added to the vegetational richness of this area. Expectedly, the plant mite fauna w.s.r. to those occurring on mangrove vegetation and diverse types of agrihorticultural crops are also likely to be diverse. Unfortunately, the knowledge about the plant mite faunal wealth of this region is very inadequately explored as appeared from the fact that only 6 species are so far known occurring on mangrove vegetation of that region (Gupta, 1992) and practically nothing is known about the phytophagous and predatory mite fauna occurring on agrihorticultural crops. Hence, with a view to exploring the plant mite faunal diversity of SBR and to study their bio-ecological aspects, this project was taken up and the present paper gives an idea about the Species Richness Index, Species Diversity Index, Index of Dominance, Relative Abundance, based on the observations made at Bamankhali, Sagar Island, during November, 2000 to October, 2001. This is the first such attempt to collect information of this kind on plant mite fauna from SBR and surprisingly such information is rather unavailable from most parts of India.


INTRODUCTION
Sundarban Biosphere Reserve (SBR) is very rich with at least 36 species of mangrove vegetation of the 55 species of mangrove plants that are known to occur in the world.Besides, during the past 2-3 decades, agricultural horticultural crops have been started growing in different re-claimed areas of SBR which have further added to the vegetational richness of this area.Expectedly, the plant mite fauna w.s.r. to those occurring on mangrove vegetation and diverse types of agrihorticultural crops are also likely to be diverse.Unfortunately, the knowledge about the plant mite faunal wealth of this region is very inadequately explored as appeared from the fact that only 6 species are so far known occurring on mangrove vegetation of that region (Gupta, 1992) and practically nothing is known about the phytophagous and predatory mite fauna occurring on agrihorticultural crops.Hence, with a view to exploring the plant mite faunal diversity of SBR and to study their bio-ecological aspects, this project was taken up and the present paper gives an idea about the Species Richness Index, Species Diversity Index, Index of Dominance, Relative Abundance, based on the observations made at Bamankhali, Sagar Island, during November, 2000to October, 2001.This is the first such attempt to collect information of this kind on plant mite fauna from SBR and surprisingly such information is rather unavailable from most parts of India.

MATERIALS AND METHODS
The population data pertaining to phytophagous and predatory mites occurring on mangrove vegetation and agri-horticultural crops of SBR was collected from the experimentation station located at Bamankhali in Sagar Island about 100 kms.Southeast of Calcutta.Two species of Mangrove vegetation (Avicennia alba & Rhizophora mucronata.)and two types of horticultural crops (Psidium guajava & Citrus sp.) were selected.For A. alba and Rhizophora, 10 and 5 trees respectively were selected and tagged with labels.From each of the A alba tree, 10 leaves and from each of the Rhizophora tree, 8 leaves of same age and size were plucked and popUlation of phytophagous and predatory mites was counted from the under surface of the whole leaf under stereo binocular microscope and number of each of the species present was recorded.Hence, for Avicennia alba, a total of 100 leaves and for Rhizophora mucronata 40 leaves were examined.
Likewise, for Psidium guajava and Citrus sp., 8 and 5 trees respectively were selected and tagged with labels and from each guava tree 10 leaves (total 8 x 10 = 80 leaves) and from each citrus tree 10 leaves (total 5 x 10 = 50 leaves) were plucked and population was counted from under surface of each of the entire leaf.The observation was recorded at monthly interval.The phytophagous and predatory mite fauna collected from each type of tree was identified and respective number of each species was counted.
The data thus collected was used to calculate Relative Abundance (RA) which was assessed following Tamura (1967) season wise for determining the nature and proportion of mite assemblage, Species Diversity Index (H) was calculated following Shannon & Weaner (1949, 1949a), Index of Dominance( c) was calculated month wise to isolate the favourable and unfavourable months for diversity, Richness Index (d) was calculated following Menthnick (1964).The data towards abiotic factors like temperature, RH and rainfall were collected from Alipore Meteorological Department, Calcutta.

RESULTS AND DISCUSSION
The identification of mite species indicates the presence of 26 species (Table-I) collected during the study period, of those, 15 belonged to predatory group and 11 to phytophagous group.Tydeus sp.
The Relative Abundance (RAJ: The Table 3, indicates that the most abundant species was Amblyseius largo ens is (Muma) and Amblyseius pruni Gupta among predatory mites while among phytophagous mites, Tetranychus sp.Schizotetranychus hindustanicus (Hirst) and Eotetranychus hicoriae (McGregor), on all the plants in post monsoon period.During premonsoon period, the most dominant species were Amblyseius largo ens is, Amblyseius multidentatus (Swirski & Shechter), Amblyseius pruni, Amblyseius coccineae Gupta, among predatory group and Schizotetranychus hindustanicus and Eotetranychus hicoriae among the phytophagous group.The dominant predatory and phytophagous species during monsoon months were Amblyseius largo ens is, Amblyseius alstoniae Gupta, Amblyseius pruni, Amblyseius coccineae in the former group and Schizotetranychus hindustanicus and Eotetranychus hicoriae in the latter group.The remaining species, as per their frequency of occurrence, belonged to subdominant or rare category.
While studying the occurrence of different mite species in different groups of plants (Table -2), it was found that predatory mites like Amblyseius largo ens is, Amblyseius pruni, Pronematus f/eschneri Baker and phytophagous mites like Tetranychus urticae Koch, Brevipalpus phoenicis (Geijskes) were dominant on Avicennia alba during postmonsoon period, Amblyseius alstoniae, Amblyseius fallacis (Garman), Amblyseius coccineae and Tetranychus neocaledonicus Andre were sub dominant on the same host while the occurrence of other species was rare.
In case of Citrus sp., as usually, the dominant species were Amblyseius largo ens is among predatory species and Schizotetranychus hindustanicus, and Eutetranychus orientalis (Klein) among phytophagous mites were dominant in post monsoon period.During premonsoon period, the dominant species were Amblyseius largo ens is, Amblyseius pruni, Amblyseius coccineae, among the predatory mites and Schizoteranychus hindustanicus and Eutetranychus orientalis among the phytophagous group were found dominant.Finally, during monsoon period, Amblyseius largoensis, Amblyseius pruni, Amblyseius coccineae, Pronematus fleschneri, and Tydeus sp.among the predatory    3).
In case of Guava (Psidium guajava), the most abundant species were Amblyseius largo ens is and Tetranychus sp., during post monsoon period.In premonsoon period, Amblyseius pruni, Amblyseius coccineae, Amblyseius multidentatus among the predatory mites and Tetranychus sp. and Eotetranychus hicoriae among phytophagous mites were dominant.Almost the similar trend in population prevailed during monsoon period excepting the addition of Amblyseius largoensis as one of the dominant predatory species.

DISCUSSION
From the data presented in Tables 2-6, it is evident that the species diversity is not uniform in all the four plants on which study was conducted.It also appeared that the dominant pest species on guava were Tetranychus sp. and Eotetranychus hicoriae and on Citrus the dominant pests were Eutetranychus orientalis and Schizotetranychus hindustanicus.Surprisingly, both the mangrove plants were relatively free from phytophagous species though harboured a reasonably good number of predatory species.The overall predatory species which were found dominant during the entire study period on all the four types of plants were Amblyseius largoensis, Amblyseius pruni, Amblyseius coccineae and Amblyseius multidentatus.This pioneer study attempted to find out the incidence of Species name 1 to 26 are listed in the Table-I.R. A. Value> 5 = Dominant species.o. A. Value 2 to 5 = Sub dominant.R. A. Value> 2 = Rare.

Table - I
. List of the Phytophagous and predatory mites.

Table - 2
. Relative Abundance of mite fauna in different plants (RA).

Table - 3
. Showing Relative Abundance against total mite population in three different periods.Schizotetranychus hindustanicus and Eutetranychus orientalis among the phytophagous group were found dominant.The other species were sub dominant or rare (Table Species name I to 26 are listed in the Table-I.R. A. Vaue > 5 = Dominant species.R. A. Value 2 to 5 = Sub dominant.R. A. Value> 2 = Rare species. Species Diversity Index (H) :indicates that in Avicennia alba, the maximum species diversity index (1.826) was found inMarch, 2001 and minimum (1.127)was in October, 2001.In case of Citrus sp and guava, the highest values 1.491 on Citrus and 1.637 on guava were found in April, 2001 and their respective lowest values were 0.470 and 0.582 were during January, 2001.In Rhizophora, the highest diversity value (1.972) and lowest (1.072) were seen during March, 2001 and January, 2001 respectively.Index of dominance (c) : Table-5 indicates that the index of dominance values were different in four different plants chosen for this study.In Avicennia alba, the maximum (c) value (0.388) was found in January, 2001 and minimum (0.173) was in March, 2001.In Rhizophora mucronata, the highest (c) value (0.407) was seen in October, 2001 and lowest (0.145) seen in February, 2001.In Citrus, the highest was in January, 2001 (0.764) and lowest (0.120) was in February, 2001.Finally, on guava, the highest (0.663) and lowest (0.211) were found in January and April, 2001 respectively.Richness Index (d) : Table-6 gives the values of species richness index in four species of plants studied.In Avicennia alba, the highest value (1.152) was in November, 2000 and lowest (0.363) was in August and September, 2001.In Rhizophora, the highest value (1.077) was in December, 2000 and lowest (0.383) was in October, 2001.In Citrus, highest (0.639) and lowest (0.384) were seen in April and in September, 2001, respectively while on guava, the highest (0.532) and lowest (0.011) were found in April, 2001 and December, 2000 respectively.