System of Rice Intensification – An overview on pest diversity=

System of Rice Intensificat= ion – An overview on pest diversity

G.Ravi

Tam= il Nadu Rice Research Institute (TNAU), Aduthurai – 612101, TN

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Rice is the predominant sta= ple diet for more than half of the human population, and is the most consumed cereal in Asia and Latin America. Recognizing the benefits of System= of Rice in Intensification (SRI) for the total factor productivity (Uphoff and Fernandes, 2002), the technology was introduced in Tamil Nadu during 2002 (Thiyagarajan, 2002). Now more and more farmers, scienti= sts, extension personnel and NGOs have gained confidence on SRI principles.=

&nb= sp; At Tamil Nadu Rice Research Institute, a detailed = study involving SRI concepts was taken up for the past four years (2002-2006) in = both dry and wet seasons. The TRRI represents irrigated rice ecosystem in Cauvery = Delta Zone, the granary of&n= bsp; Tamil Nadu and one of the oldest = rice growing regions in East Asia.=

The overall results of the experiments brought substantial amount of evidences that in SRI ecosystem some of the major ins= ect pests viz., stem borer, gall midge, Orseolia oryzae, whorl maggot <= i>Hydrellia sasakii and brown plan= thopper Nilaparavata lug= ens are less active. However, the rice leaffolder a key pest in rice crop reaches higher proportion in SRI = fields in all the seasons studied and cause significant damage in 8 crop seasons o= ut of 10 seasons observed. In the on-farm trials there were 11 sites wherein SRI practicing farmers have repo= rted leaffolder out break in the reproductive phase = (boot leaf damage) warranting pesticide application more than one round. Studies = on role of individual SRI components indicated that the intermittent irrigation practice showed negative impact on whorl maggot. Planting young seedling (14 days o= ld) avoids thrips damage both in nursery and main f= ield. Significant reduction in colonization and population build up in BPH was observed in wider spacing, intermittent irrigation and rotary weeder operation.&nb= sp; On the other hand, wider spacing as well as frequent intercultural operation using rotary weeder has resulted in h= igher leaffolder incidence due to darker foliage, broader l= amina and widened chlorophyll A:B ratio. = &nb= sp;

&nb= sp; The SRI ecosystem was also simultaneously monitored for the community patterns = of arthropod and their diversity, species richness and abundance (Michael, 198= 6; Magurran, 1988).&nbs= p; Based on the sum of all the arthropod taxa in each category, the relative abundance of pests, predators, parasitoids, neutrals and other categories, it is visualized that the arthropod recruitm= ent and species diversity were less in SRI than conventional rice cultivation a= nd the effect was more in wet season than in dry season. The SRI discourages significantly = the activities of aquatic arthropods. Members of the groups Miridae, Odonata, Mesoveliidae, Veliidae, <= span class=3DSpellE>Gerridae are less in SRI.<= span style=3D'mso-spacerun:yes'> Intermittent irrigation practice a= nd intercultural operation using rotary weeder contributed maximum for the declined arthropod diversity in SRI.=

From our research experience (both on-farm and on - station) , it is learned that the crop growing environment in SRI permits <= span class=3DSpellE>leaffolder upsurge in many occasions and green leafho= pper (GLH) Nephotettix spp. to some extend. However reduced incidence of insect pests such as brown planthopper (BPH), stem borer (SB), whorl maggot, thrips and gall midge were observed. With SRI, rice plants are well gro= wn and healthy due to the improved soil quality and root system. The effects of mineral nutrient su= pply on crop growth/yield are well known in terms of their physiological, struct= ural and biochemical functions in plants. The expression of resistance/tolerance mechanism in cultivars is considerably influenced by nutrient status and nutritional quality of the p= lant and these phenomena might be one of the reasons for changing pest scenario = in SRI.

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There are comprehensive information available on effect of mineral nutrition status of crop rice on pathogen and insect pest incidence (Reddy, 2005). Thicker stem, compact leaf sheath,= high degree of silicon in the stem due to the powerful and flourishing root syst= em in rice crop are known for stem borer resistance in rice. Occurrence of stem borer in less severity was also reported elsewhere in Lao= s (Vongsakid, 2002) and Indonesia (Gani et al., 2002) in SRI experiments

Why SRI is intervening with pest scenario and biodiversity?

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Differential response to pest attack between SRI and conventional rice cultivation practices warrants understanding on dynamics = and principles in SRI to take best advantage of the insight SRI can provide for pest and disease management. The principles behind SRI are explained hereun= der for their possible role in regulating pest incidence in the rice crop.=

1. T= ransplanting young seedlings: SRI prin= ciple encourages use of seedlings not more than 15 days i.e. before they start their fourth phyllo= chron of growth so that the genetic potential for tillering<= /span> and root growth is retained. This practice has advantage in pest management= as the early shifting of seedling from nursery will make the seedlings to esca= pe from thrips, green leafhopper and stem borer. In tungro endemic regions, the smaller nursery area helps protecting seedling from the invading GLH vector by use of wire nets as physical barrier.

2.   &= nbsp;     Planting the seedling wider apart: This SRI practice, besides leading greater ro= ot growth and accompanying tillering, encourages g= reat sun light penetration and thereby changes the microclimate of the pest nitch. = Pest species affected largely by this practice incl= udes BPH and gall midge. Transplanted in a square pattern pe= rmits sun light penetration upto base of the plant ma= inly in the early phase of the crop. Since the environment remains bright= er and warmer, the micro-climate of the crop is changed significantly. Such environment might inhibit colonization by BPH, stem borer, gall midge and w= horl maggot. However, may favour multiplication of GLH a sun loving insect whic= h acts as vector for the deadly disease tungro. Increa= sed distance between plant reduced mite population i= n SRI by increased exposure to sunlight and wind flow. This was reported on rice panicle mite Steneotars= onemus spinki, which= is involved in mite/fungi disease complex which causes up= to 30 % yield loss in rice in Cuba (Perez, 2002). Since panicle mite is an emerging problem in rice in states = like Andhra Pradesh and Karnataka this needs to be investigated. In our experime= nts rice leaf mite Oligonychus oryzae was the regular problem in dry season crop= . However, there was no difference observed in mite incidence between SRI and convention rice cultivation practice. In a observation = Gasparillo (2002) reported that the rats shy away from wider open spacing in the SRI fields.

3. K= eeping the soil well drained rather than continuously flooding through adoption of alternate wet and dry irrigation (AWDI): This practice is an important variable in SRI. Through AWDI= the productivity per unit of water applied becomes greater and a sustained hypo= xic soil condition that prevents root growth under flooded condition is avoided. Intermittent irrigat= ion is also known to alter the microhabitat and the microclimate of the rice pest there by reducing pest build up. In the management of BPH, black bug, case worm and whorl maggot this practice = is helpful. The AWDI practice al= so has potential health benefit in that the intermittent drying of rice field kills immature stages of mosquitoes that transmit diseases such as Malaria and Japanese encephalitis. At this point, location specific pest problem mainly= of termite, cutworm, root grubs and nematodes in drained soils needed to be ke= pt in mind.      = ;

4. F= requent use of mechanical hand weeder: Beginning wi= th 10-15 days after transplanting the use of rotary weede= r increases the number soil pores besides weed control. Weed management using rotary weeder results in better oxidation of root zone durin= g the growth cycle, leading to better development of aerobic microbial environmen= t. Cono weeder operation in = combination with AWDI method of irrigation synergises the activity of beneficial micro organisms like Azospir= illum in the root system of the rice plant, which is able to fix nitrogen from atmospheric sources (Biological nitrogen fixation) to the tune of 50-70 kg/= ha (Ladha et al., 1998).

   &nb= sp;        Among the mineral nutrients, elemental Nitrogen has greater influence on leaffolder incidence. Higher N especially along with inadequate K (Potassium) invariably causes severe pest and disease incidence.&= nbsp; Succulent lush green crop canopy caused by high nitrogen application results in emission of green leaf volatiles (GLV) which are known to attract rice leaffolder adults for oviposition (Ravi, 2006).

Plant protection research opportunities in SRI=

SRI raises a great number of interesting issues and opportunities in the sphere of plant protection. Following are some of the research opportunities in SRI.

Monitoring pe= st and disease scenario in SRI: Limited information is available on insect pest scenario in SRI= at present. As that of insect pest, plant diseases are the major constraint in rice cultivation. Hence a systematic study on characterizing SRI for disease scenario needed to be initiated. Recognizing the significance of the SRI system, under the All In= dia Coordinated Rice Improvement Program (AICRIP) ICAR has initiated a network experiment exclusively on monitoring insect pest scenario in SRI system sin= ce 2006. The experiment is in pr= ogress in seven locations across the country.

Biointensive IPM module to suite SRI: Integrated pest management (IPM) forms the core in rice protection.&n= bsp; The IPM approach envisages utilization of various available control measures effectively, to maintain pest population below economic damage lev= el. Research programme on integrating various components of IPM in conjunction = with SRI principles may help in developing a biointensive IPM module.

Biodiversity: The SRI practices are known to have negative impact on     biodiversity in irrigated rice ecosystem especially on aquatic arthropods which includes so= me of the potential natural enemies of the crop pest. Hence, for a long term perspective, strategies for biodiversity conservation in SRI need to be developed.

Strategies for mosquito vector control: Rice ecosystem has traditio= nally been associated with human diseases like malaria and brain fever because of= the reasons that the vector of the disease, mosquitoes breeds abundantly in the rice fields. Hence, a systematic study in SRI mainly focusing on AWDI and w= ider spacing may contribute for mosquito vector management.

Strengthening human resources at grass root level: SRI being a new technology= , the participating farmers are fully involved in knowledge management. Since SRI= is more feasible for adoption in small farm holdings, the favorable attitude possessed on SRI by such small farmers may be utilized for effective diffus= ion of plant protection know- how and do- how.= This needs to be best explored by the extension specialist who disseminates plant protection.

   &nb= sp;        Rice is a ‘Strategic commodity’ in Asia because it is a most importa= nt constituent in the diet of the poor and main source of income and employmen= t in Asia.&n= bsp; In the coming years, incremental improvements in rice yield are no longer likely to meet the drastically changing global demands urging for complete transformation of production system (Kropff and Bindraban, 2006). SRI may continue to contr= ibute for a higher productivity in rice in the forth coming era of ‘Second green revolution’. In p= lant protection perspective, a more precise and better documented comparison on pests, diseases, nematodes and natural enemies and their biodiversity is ne= eded for better understanding of SRI. These are to be explored in future.