Odonata is the largest insect order, which is entirely predaceous in rice ecosystem. Both naiads and adults are voracious predators on other insects. Gunathilagaraj et al. (1999) recorded 16 species of Odonata in rice fields of Coimbatore, Tamil Nadu, India. Barrion and Litsinger (1994) listed 14 species of Odonata in rice fields of Asia and Africa. As for their predatory role in rice fields, the damselflies Ishnuera senegalensis, Agriocnemis pygmaea, Ishnuera delicata and Ceriagrion coromandelianum (Krishnasamay et al., 1983) were reported to be predating upon leaf-and planthoppers. They also reported the dragonflies Orthetrum Sabina, Crocothemis servilia, Pantala flavescens and Diplocodes nubulosa as effective predators of rice Lepidoptera. The present paper deals with the diversity of Odonata in rice ecosystem of Madurai, Tamil Nadu.
MATERIALS AND METHODSThe diversity and relative abundance of Odonata was studied at the Wetlands of Agricultural College and Research Institute, Madurai, Tamil Nadu. The study area receives water from the Vaigai dam. The minimum and maximum temperatures of the experimental area were 28 0 C and 40 0 C, respectively, during Kharif (2000). The average rainfall of study area recorded was 928mm. Four popular rice varieties, viz., MDU 5, ADT 36, ADT 39 and ADT 43 were used during this season. Each variety was replicated into two treatments namely weeded plots (all the weeds removed) and partially weeded plots (10 weeds allowed/ m 2 ). The size of the experimental plot was 8 x 6m. The collection of adult dragonflies and damselflies in rice canopy ecosystem was done with sweep net. The collected adults were sorted out into respective taxa based on taxonomic characters and number of individuals in each taxon recorded. Sampling was taken at weekly intervals and a total of nine samplings taken during Kharif (2000). In the present investigation, Jaccard coefficient index of similarity (Jaccard, 1908) was used to study the diversity of Odonata between weeded and partially weeded rice ecosystems.
RESULTSThe present study revealed that a total of 12 taxa of Odonata comprising of nine Anisoptera and three Zygoptera were recorded (Table 1). Among the Anisoptera, Pantala flavescens (Fabricius.), Diplocodes trivialis (Rambur), Crocothemis servilia (Drury) and Tramea limbata (Desjardins) were the dominant species. Other species, Orthetrum Sabina (Drury), Rhyothemis variegata (Linnaeus), Neurothemis tullia (Drury), Anax guttatus, and Trithemis sp. were recorded only at the tillering stage of the crop. Among the three species of Zygoptera, Agriocnemis femina femina Brauer was the dominant species than Agriocnemis pygmaea Rambur and Ishnura sp. The similarity statistics of dragonfly exhibited the similarity values between 0.77-0.88 in the first week and showed greater diversity (0.50) during fourth, fifth, sixth, seventh and eighth weeks (Table 2). Damselfly registered higher diversity (0.50) from the second week to seventh week. Both the dragonfly and damselfly exhibited 0.50 similarity in...
