Arthropod Density in a Weed Ecosystem Maintained Around a Rice Field of Tamil Nadu

Three field experiments were conducted during kharif and rabi 2000 and kharif 2001 to study the diversity of arthropods on weeds maintained around a rice ecosystem. The study revealed that phytophagous insects of weeds constituted about half of the population of all arthropods. Among the sucking hemipterans, brown planthopper (BPH) was the dominant species of insect, followed by white backed planthopper (WBPH) from the first week to fourth week (tillering stage) in all the three seasons. But, the occurrence of WBPH was absent through out rabi 2000. Green leafhoppers and ear head bugs were dominant, followed by red spotted bugs from the fifth week to the last week. Short horned grasshoppers occurred in all the three seasons and expressed the dominance to the extend of 0.79, 1.05 and 1.25 individuals in kharif and rabi 2000 and kharif 2001, respectively. All the lepidopteran insects showed less abundance compared to hemipteran insects during kharif 2000 and kharif 2001. However, Spodoptera, sphingid and pierid butterfly were absent through out rabi 2000. The coleopteran insects viz., blue beetle and pumpkin beetle were recorded. The blue beetle occurred in all the seasons and the pumpkin beetle showed very less abundance and occurred only in the last few weeks. The phytophage dipteran insects showed less abundance in the three seasons. The five groups of natural enemies viz.,Orthroptera, Odonata, Hemiptera, Coleoptera and Hymenoptera were recorded. In Odonata, dragonfly was dominant during kharif 2000, where as damselfly was dominant during rabi 2000 and kharif 2001. But, the occurrence of dragonfly was absent during rabi 2000. Predatory mirid bugs expressed more abundance in the three seasons. The coleopteran predators viz., ground beetle and coccinellid beetle occurred through out all the three seasons and showed less abundance. Spiders recorded > 0.60 individuals in the three seasons. In the parasitic hymenopterans, ichneumonfly, braconids, and bethylids recorded more abundance in the three seasons. A Total of 18 weed species acted as alternate hosts for polyphagous phytophage insects were recorded in partially weeded plot. Of them, Cyperus iria, C. diformis, C. rotundus, Echinochloa colonum, E. crus – galli, Ipomea aquatica and Marsilia quadrifolia were dominant.