Major predators in rice ecosystems and their potential in rice pest management

Abstract

Survey conducted in three heterogeneous rice ecosystems of Kerala viz., Kuttanadu rice fields of Alappuzha district with high load of pesticide, double cropped rice fields of Thiruvananthapuram district with moderate level of pesticide and Pokkali rice fields of Ernakulam district with no pesticide during two consecutive seasons at the vegetative, tillering and reproductive stages of the crop revealed a distinct change in the composition and status of natural enemies and pests in rice ecosystem. While pests dominated in Kuttanadu and double cropped rice fields of Thiruvananthapuram, natural enerm es dominated In Pokkali ecosystem. Among the natural enemies, population of predators was signi ficantly higher 111 all the three ecosystems. Population of pnr as lt o i d s was very low. Among the predators, insect predators dominated the predator fauna followed by the spiders in the three ecosystems. The important insect predators observed were the mirid bug Cyrtorhinus livi dipennis Reuter and the coccinellid beetle Micraspis di scol or (Fabricius) and the major spider predators recorded were Tetragnatho maxillosa Thorell and Lycosa pseudoannulata (Boesenberg et Strand). Among the pests seen in th~ three ecosystems, the sucking pests were dominant followed by the defoliators. Stage specific sucking pests like the ricebug Lept ocorisa acut a (Thunb) and earhead thrips Hapl ot hrips ganglb aueri Schmutz too were seen infesting the crop during the respective crop stages. Hoppers were the predominant sucking pests in all the three rice ecosystems. Population of other sucking pests was very low. Among the hoppers Nephot ettix sp. and Sogatella furcifera (Horvath) were predominant. Comparatively, population of the brown planthopper, Nilaparvata lugens (Stal) was

low. Population of other pests considered as major ones in the State like yellow stem borer Scirpophaga incertulas (Walker), gall fly Orseolia oryzae (Wood-Mason), case worm Parapoynx stagnalis Zeller etc. was negligible. The population of neutrals was low in all the three rice ecosystems. On the other hand, high population of these 'other insects' was seen in the adjoining vegetation. Greater diversity of arthropod spec ies was observed in the insecticide-free Pokkali rice ecosystem and insecticide-rationally-used double cropped rice ecosystem of Thiruvananthapuram. Less diversity of species was seen in Kuttanadu rice ecosystem where insecticides were applied indiscriminately. Studies on the suitability of different prey for mass culturing C. l ivi dip enni s, M. discolor, T. maxillosa and L. pseudoannulata under controlled conditions indicated that the best prey for C. lividipennis was the first instar nymphs of N. I ugens ,for M. discol or its eggs, for T. maxillosa, Nephotettix sp. and for L. pseudoannul at a, N. lugens. Efficiency of the predators 111 suppressing hopper pests when assessed in terms of searching capacity, feeding potential, prey preference and hyperpredation revealed that the insect predators C. lividipennis and M. discolor showed better ability to search for plant hoppers, especially N. lugens and S. furcifera. Comparatively, the searching capacity of the spiders for hoppers was low. The feeding potential and prey preference of C. livi dipenni s was significantly high for first instar nymphs of N. l ugens . Similarly, M. discolor preferred more of its own eggs than the other prey tested. The spider, L. ps eu do annul at a was observed to be a significantly better predator of

hoppers than T. maxill os a. None of the predators tested showed cannibalism. Hyperpredatory activity was observed in Crocothemis sp., Agri ocne mis sp., Polytoxus fuscovitt at us (Stal), T. maxillosa, Oxyopes sp. and L. pseudoannulata. The mirid bug C. lividipennis was a highly preferred prey for hyperpredation. The commonly used insecticides for nee pest control viz., carbaryl, phosphamidon, monocrotophos, quinalphos and methyl parathion caused 80 to 100 per cent mortality of predators at the dose normally recommended for pest control. The results of the study emphasized the need for periodical assessment of the arthropod community Jl1 nee ecosystems as agro- ecological conditions tend to shift the status of phytophages and entomophages from time to time. Based on the results, growing of varieties tolerant to major pests and regulation of incident pests by conserving naturally occurring predators through optimal habitat modification and use of predator-friendly insecticides when absolutely needed will go a long way in sustainable production of rice in Kerala.