A PRELIMINARY STUDY ON THE SUCCESSION OF INSECT VISITORS AND THEIR SYMBIOTIC INTERACTION FOR EFFECTIVE POLLINATION IN BRASSICA JUNCEA (L.) OF SOUTHERN WEST BENGAL

1. Post Graduate Department of Zoology, Vidyasagar College, Sector – 2, CL Block, Salt Lake City, Kolkata – 700091, India. 2. Zoological Survey of India, North Eastern Regional Centre, Risa Colony, Shillong-793003, India. 3. Assistant manager WWF, India, West Bengal State Office, Tata centre 1st floor, Jawaharlal Nehru road, Kolkata700071, India. 4. Scientist – C, Zoological Survey of India, Kolkata – 700053, India. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History

A preliminary study was conducted for nine days in a commercial crop field at Haripal in Hooghly district of West Bengal during the flowering phase of Brassica juncea L. to observe the periodical occurrence of various insect visitors and their role in pollination. 29 insect species belonging to 16 families under 7 orders were recorded as visitors of Brassica. As a whole, Lepidoptera had maximum number of species occurrence followed by Coleoptera and Hymenoptera. It has also been observed that species were found to visit in different parts of plant but most of them had a common location with flowers. It is indicated that most of the insect species has specific intension of nectar collection which led the pollination efficiency. Symbiotic interaction of the visited insect species was also studied.

…………………………………………………………………………………………………….... Introduction:-
Survey of insect pollinators in any cultivation field is a common field work practice to get an idea of abundance of pollinator species or visitors (Mitra, 2010;Mitra et al., 2005). The blooming phase of any plant is the most crucial period as the diversity of insects, both occasional and regular is higher than any other phase of that plant species (Bhalchandra et al., 2014;Rasheed et al. 2015;Ali et al., 2015;Nicholls and Altieri, 2012;Belavadi and Ganeshaiah, 2013). However, there are many species which perform their activities only as visitor of flowers or sometimes remain as local inhabitants only. So, overall survey and inventory of insect species at the blooming phase of nectar plants keep records of taxa, their availability and abundance.
However, the major aims of the present study was to record the insect diversity and their visitation pattern, visited plant parts, presence and absence during the flowering period, abundance, and moreover their biological role on Brassica juncea L. According to Atmowidi et al. (2007) and Kunjwal et al. (2014), the location of the collected specimens on plant individuals, like flower, leaf, stem etc. and also their pollination status like effective pollinators, visitors etc. can give an idea of plant-insect interaction of Brassica sp. There are earlier studies also which revealed The mutual occurrence of species-pairs is a co-species or con-generic interaction where more than two species assemblage for the same resources (Kulloli et al., 2011;Hu et al. 2005). The presence of a species or any particular taxon may affect the occurrence of another species or some taxon at the same time period (Alagumurugan et al. 2011;Duara and Kalita, 2013). The symbiotic interactions between species-pairs among the insect species were also noticed in the present study. But the conclusion in details this type of symbiotic information could not be possible from the present study as the records of mutual occurrences were mostly based on limited observation.
The present study reports 29 insect species belonging to 16 families under 7 orders of insect visitors of Brassica juncea L. Of them, Lepidoptera shares maximum number of species (8 species), followed by Coleoptera (7 species) and Hymenoptera (5 species).

Methods:-
The study was conducted at Krishnapur, Haripal in Hooghly district of West Bengal (22 0 49 ' N, 88 0 6' E). The insect species on Brassica juncea L. during its blooming phase was recorded from 22 nd January to 19 th February of 2016. The study site (almost 937.56m 2 ) was a cultivation land of Brassica. The samplings were planted keeping an average distance of 20cm x 2 cm. The flower visitors were observed for an interval of each 2 minutes within quadrate plots of 1 m 2 each. The study was carried out for 9 days of flowering session. The overall observation was recorded by four session, viz., 08:00-10:00 hours (Session 1), 10:00-12:00 hours (Session 2), 12-14 hours (Session 3) and 14-16 hours (Session 4) of each day (GMT+5:30).
Collection of insects was made by insect sweeping net and hand picking methods. The insect species were collected from the samplings of the study sites and from four different locations of the plant individual. The specimens were identified with the help of the experts of Zoological Survey of India, New Alipore, Kolkata.

Result:-
Of the collected insect species, Delias eucharis (Drury, 1773), Pseudozizeeria maha (Kollar 1848) and Eurema hecabe (Linnaeus, 1758) were found exclusively on flowers. It was also noticed that Lipaphis erysimi Kaltenbach, 1843 and Modicogryllus sp. were found to visit different parts of Brassica other than flowers. One species, Modicogryllus sp. was found to visit only close to root of the plant species. No species were found to visit only stems or leaves of any individual plant. Among the 29 species reported as visitors of Brassica juncea L., the maximum number of species were found from flower (27), followed by Leaf (22) and Stem (10) ( Table 1). From the present investigation ( Fig.1) it was also revealed that, the insect found to visit maximum in both the plant parts i.e., Flower and Leaf (15)   The reported species of the present investigation was categorized as per the previous records and references (  Modicogryllus sp. Y Considering to the availability and abundance of the insect groups visited Brassica, the number of species of order Coleoptera was found to be higher from session 1 to session 2, and lesser in session 3 and session 4 accordingly, lepidopteran species were found in higher abundance from session 1 to session 4. The number of the recorded species of order Diptera was found to be increased gradually throughout the study period ( Figure 2).   Revathi and Remadevi, 2011). In the present study, the session-wise abundance of order Diptera has negatively related with the order Coleoptera and positively related with the order Hemiptera.
The overall study showed the species abundance of different orders in the blooming stage of Brassica juncea L. Of them, Lepidoptera had maximum number of species occurrence followed by Coleoptera and Hymenoptera. The blooming period of Brassica juncea L. varies from 20 days to 22 days depending on many environmental factors (Roy et al., 2014). This study also revealed that, most of the insect species start to visit at its full grown time and reduces their occurrence gradually. Similarly, many species occur at the post blooming phase (after 8 to 9 days of blooming) and stay still they are dried up. There are also some species which were found irregularly throughout the blooming session and there was no specific interaction with other co-taxa.
In the present study, there was no species which was collected exclusively from stem or leave. The species which was collected from stem and leaves was mostly local inhabitants and frequently available. Many species were found in different plant parts but most of them had a common location with flowers. It clearly indicates that most of the insect species has specific intension of nectar collection which led the pollination efficiency. Present study also focused that presence of other non-specific insect groups may also act as indicator of specific pollinator groups and their symbiotic interaction may cause the effective pollination of that particular crops.