Entomopathogenic Nematodes against Insect Pests of Rice

— Rice is one of the important staple crop in the world. Rice pests cause yield reduction as well as value of the crop. A number of insect pests that attack rice plants account for yield losses. In the rice agro ecosystems, many types of entomopathogens such as nematodes, fungi, bacteria, and viruses can reduce pest population. More emphasis should be placed on using an IPM approach where biological control with entomopathogens is also one of the main components. Entomopathogenic nematodes (EPN) as a safe alternative to the use of insecticides against insect pests in IPM of rice.


INTRODUCTION
Rice being a tropical plant is also adaptable to a broad range of climatic, edaphic and cultural conditions. It provides 20% of the per capita energy and 15% of the per capita protein for humans worldwide (Mikkelsen and Datta, 1991). Rice is grown in more than a hundred countries, with a total harvested area of approximately 158 million hectares, producing more than 700 million tons annually. Nearly 640 million tons of rice is grown in Asia representing 90% of global production (Way and Bowling, 1991). Rice production should be increased to supply a rapidly expanding population; however, it has been hindered by a number of diseases and insect pests. Moreover, rapid changes in rice production technologies have created greater frequencies of pest epidemics (Reissig et al., 1986).

II. INSECT PESTS STATUS
A number of insect pests that attack rice plants account for yield losses of 24% worldwide. Insect pest cause at least 20 per cent field losses in rice in India (Pathak et al., 1982). More than 70 species of insect pests are known to feed on rice, and at least 20 of them can seriously affect rice production. A variety of factors can contribute to pest outbreaks, including climatic factors, improper irrigation, high rates of nitrogen fertilizer application, overuse of insecticide. Insect pests attack all parts of the rice plant at all growth stages and some serve as vectors of viruses that adversely affect the plant.
Brown planthopper (Nilaparvata lugens) and white-backed planthopper (Sogatella furcifera) are serious pests of rice. They occur in tropical to temperate areas with high reproductive potential and can cause extensive damage through their feeding activity and transmission of viral diseases like rice grassy stunt and ragged stunt. Nymphs and adults suck sap from the base of plants, just above the waterline. In heavy infestation, these planthoppers can cause hopperburn resulting in browning and wilting of some or all tillers in a hill (Kuno, 1968 them, than the adult weevils feeding on leaves. The yield loss could reach up to70-80% with heavy infestation.

III. MANAGEMENT OF INSECT PESTS
Control of insect pests has primarily depended on the application of chemical insecticides. Chemical insecticides are expensive, besides other disadvantages, including secondary pest resurgence, insecticide resistance, environmental pollution, and impact on nontarget organisms. The utilization of pathogenic microorganisms holds a high possibility for the suppression of rice insect pests (Otake, 1979;Chatterjee et al. 1983;Pathak et al., 1982;Heong, 1983). In recent years more emphasis has been placed on using an IPM approach where biological control is also one of the main components. In the rice agro ecosystems, many types of entomopathogens such as nematodes, fungi, bacteria, and viruses can reduce pest population.
Nematodes have been found associated with most of the insect orders. There are more than 3100 natural associations between insects and nematodes involving 11 orders of nematodes and 23 orders of insects. The association may range from a phoretic relationship to obligate entomoparasitism leading to host death, sterility, reduced fecundity or delayed development. Some that are associated with one host and its special ecology are highly specialized and difficult to propagate on an artificial medium while other less specialized forms have wide host range, can be mass produced on artificial media, and are currently used for control of agricultural pests. Entomopathogenic nematodes (EPN) as a safe alternative to the use of insecticides in IPM of different crops, including rice, have gained worldwide attention (Table 1).

IV. MODE OF ACTION
Mermithids are a large group of obligate entomopathogenic nematodes that are considered important regulators for some insect populations (Kaiser,1991). Mermithid parasitism results in nutritional depletion, retarded growth, organ disruption, reduced fecundity or sterility and death. The newly hatched second stage mermithid is the infective stage (pre-parasite).Once the mermithid contacts the plant hopper nymph, it uses its stylet to penetrate through the cuticle into the host hemocoel to initiate the parasitic phase. The 3 rd and 4 th stage juveniles occur in the hemocoel.Two to three weeks after parasitization, the 4 th stage juvenile (post parasite) exits its adult host by boring through the thin intersegmental area of the abdominal segment, causing death of the host. After emergence, the postparasite burrows into the soil, molts, and overwinters as an adult (Sutanov et al., 1990;Vandergast and Roderick,2003;Kamminga et al.2012).
Steinernematidae and Heterorhabditidae have attracted most attention as they contain the EPNs Steinernema and Heterorhabditis. The nematodes musualistically associated with insect-pathogenic bacteria. The bacteria Xenorhabdus and Photorhabdus (Family: Enterobacteriaceae) are symbiotically associated with Steinernema spp. and Heterorhabditis spp. respectively. These bacteria are ecologically obligate to EPNs, with specific mechanisms of pathogenicity and their existence in free form in nature is believed to last a very short while due to photo and thermo sensitivity. These nematode bacterium associations meet many criteria for augmentative control of insect pests through inundative releases including: broad host range; ability to kill hosts rapidly : a durable infective stage capable of storage; distribution; and persistence; available mass inexpensive mss production technologies; no evidence of insect immunity; safety to plants and vertebrates; and application with existing spray equipment. The third stage dauer juvenile (DJ) occurs free in the soil and its role is to seek out and infect an insect larva. These free-living, nonfeeding juveniles and developmentally arrested third stage juvenile ranging in length from 0.4mm to 1.1mm. Steinernema gains entry to the insect larva through natural openings (mouth, anus and spiracles). In addition to these modes of entry, Heterorhabditis also gains entry by abrading the intersegmental membranes of the insect. Once in the haemocoel of the insect the DJ releases cells of a symbiont bacterium that it carries in its intestine. The insect haemolymph provides rich medium for the bacterial cells and these begin to grow, release toxins and exoenzymes and kill the insect. The insect dies rapidly, usually within 24-48 h. Generally life-cycle of entomopathogenic nematodes is completed within 12-15 days at room temperature. Depending on the availability of food resource, both heterorhabditis and steinernematids generally complete 2-3 generations within insect cadaver and emerge as infective juveniles to seek new hosts. Imamura (1932) reported that Mermithidae were parasitic on Chilo simplex. Grewal et al., (2006) recorded it in Asia. Pena & Shepared (1985) recorded 50% parasitisation of BHP by Hexamermis sp. in Phillippines. Heong (1983) reported that an entomopathogenic nematode Amphimermis unka caused  Manjunath (1978) and in eastern india by Satpathi (1999 (Rubtsov,1969;1977). Agamermis unka is the most important and common natural enemy in temperate regions. Agamermis species live in the soil and infect hosts from the soil directly or after short migration up to plant stem (Nickle, 1981 (Kuno,1968). As egg production and hatchability of Agamermis are high, inoculative releases into areas where the mermithid population is low or nonexistent appear feasible. To affect plant hopper populations; the mermithid must parasitize a high number of progeny of the migrating population. Both the short-winged (brachypterous form) and the long-winged (macropterous form) adults are susceptible to mermithid parasitism, but the brachypterous form (57%) had higher parasitism than the macropterous form(8%) (Choo et  al.,1989).The brachypterous form is usually found lower on the rice plant where the mermithid is more likely to encounter it. A. unka would be most effective when the migrating adult insects produce few progeny and the parasitic stage of the mermithid occurs in high numbers. About 30% of the natural controls of brown plant hopper in eastern India are due to parasitic nematode (Satpathi et al., 2008). A control strategy would be to reduce the number of progeny produced by migrating adults. This can be accomplished through an integrated manner with chemical or biological insecticides, resistant cultivars, cultural methods, or a combination of these control tactics. Before an integrated pest management system can be incorporated in the field, further studies on the biology of the nematode and its compatibility with current control tactics are needed. Rice water weevil adults can be parasitized by mermithid, as has been reported in native regions (Bunyarat et al., 1977).These parasites may reduce fecundity and cause high mortality in infected adults. Rice blue beetle (Leptispa pygmaea) was reported to be parasitized by Hexamermis (Patel and Shah,1988). This mermithid is already established in the rice fields in Korea, appears to be compatible with some chemical pesticides, and reduces the fecundity of its host (Choo et al., 1998). Cultural practices such as tilling and irrigation can increase the performance of A.unka. We have to enhance the effectiveness of the naturally occurring mermithid into an IPM program to reduce BPH population. Genus identification through molecular technique can help predict and infer mermithid biology, which can ultimately assist in rearing protocols, if mermithids are to be used for future research and incorporation into current management protocols.

PARASITISM / BIOEFFICACY
In India, efforts were made during 1970s to study the effectiveness of exotic EPNs, S.carpocapsae (DD-136)  (Kega et al.,2013;. Steinernema carpocapsae was found to cause mortality against rice water weevil, Lissorhoptrus oryzophilus under laboratory setting but failed to work in the field in Japan (Nagata,1987).In Cuba, there was success using Steinernema spp. against the rice water weevil with up to 80% control in field trials (Carbonell, 1983;Meneses, 1983).In California, research with both S.carpocapsae and Heterorhabditis spp. found that nematodes provided control of rice water weevil larvae when applied to drained soil that was reflooded 8 d later (Grigarick and Oraze,1990).In China, the Otio strain of S. feltiae was found to cause high mortality for larvae (Sun et al.,2006) and mortality rate was affected by time and dose. Efficacy of S.feltiae, S. carpocapsae A24 strain, S.glaseri NC 34 strain, H.bacteriophora and H.zealandica have also been detected in adult weevils (Kisimoto et al.,1987). Mortality of 82.5% and 97.5% was observed in adults of L. oryzophilus treated with S.feltiae and H.bacteriophora, respectively, at 10d after incubation with nematodes (Li et al.,2007). However, the widespread application or adoption of nematodes against rice water weevil in Asia or North America has not been possible for economic reasons (Choo and Rice, 2007).

V. CONCLUSION
Understanding the ecological and behavioral relationships between the nematode and insects could result in proper use of compatible insecticides or other biological control agents in providing an integrated approach to insect management. In vivo production of mermithid has been accomplished with the mermithids from the banded cucumber beetle (Creighton and Fassuliotis, 1982) and from mosquitoes (Peterson, 1984). Similarly, if BPH can be mass produced easily, in vivo production of the mermithid may be used to augment natural population. In addition to production, methods to store the eggs and adults and timing of introduction into BPH populations need to developed. Using the conservation of naturally-occurring population of mermithid, there is a need to implement an effective IPM programme. By understanding the biology and ecology of these