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1. Natural or biological weed control agents are those of biological origin, which suppress or kill the weeds without significantly affecting the desirable plants. They include insects, animals, fish (like Chinese carp), snails, birds (like duck), microbes (fungi, bacteria, viruses, nematodes, etc.), their toxic products, and plants (parasite plants, competing plants) or their products.
2. Experiments on rizi-pisciculture, in India and other countries have shown that herbivorous (macrophytophagous) and algivorous (microphytophagous) fish can be successfully used in the control of weeds and algae in the rice fields. The fish found useful in this purpose are Puntius javanicus in Asia, Tilapia rendalli and T. zilli in Africa, which are herbivorous and T. mossambica in Asia, which are algivorous.
3. Animals, like pigs feed on the tubers of purple nutsedge (C. rotundus) in the off-season, in India. In control of Ludwigia parviflora in rice fields, steel blue beetle (Haltica cyaamea) and larvae of Bactra verutana were found to bore into shoots of Cyperus rotundus.
4. Plant pathogen, Colletotrichum gloeosporioides sp. Aeschynomene was found effective for control of Aeschynomene virginica and Colletotrichum gloeosporioides sp. Jussiaceae for the control of Ludwigia decurrens .Other Pathogens that have been shown potential as biological control agents for controlling weeds in rice, include Drechslera monoceras for the control of Echinochloa crusgalli , Epicoccosorus nematosporus for the control of Echinochloa kuroguwa. Biocontrol of E. kuroguwai has become possible by means of a plant pathogen, Epicoccosorus nematosporus, and of Scirpus planculmis by a pathogen Alternaria sp.
Weed Management in Rice, DRR Training Manual