Steinernematids and Heterorhabditids 1331 Ecology and Distribution

After the baiting technique developed by Bedding and Akhurst (1975), random soil surveys were conducted globally in order to find entomopathogenic nematode in temperate, sub-tropical and tropical countries. These nematodes were common in both cultivated and uncultivated soils and their distribution was found to be worldwide (Hominick et al. 1996; Hominick 2002). Steinernematids were much more biologically diversified than Heterorhabditids. The most widely distributed species were S. carpocapsae, which has been isolated from Europe, Australia, New Zealand, India and America followed by S. feltiae from Europe, Australia and New Zealand (Poinar 1990). S. carpocapsae and S. feltiae were widely distributed in the temperate region, whereas H. bacteriophora in the continental Mediterranean climate and H. indica throughout the tropics and sub-tropics (Hominick 2002). Among the most thinly distributed species were S. anomali, which was recovered only from Russia, S. rara from Brazil, S. kushidai from Japan and S. scapterisci from Uruguay. The most prevalent species in the UK was S. feltiae, whereas in Northern Europe it was S. affini (Poinar 1990). The factors affecting the local distribution of entomopathogenic nematodes are soil texture, vegetation and availability of suitable hosts (Griffin et al. 2005). S. affini was found largely in arable lands and grasslands but absent in forests, whereas S. kraussie was common in forests (Hominick 2002). H. megidis and H. indica were extensively found in sandy soils, resulting in a mainly coastal distribution (Griffin et al. 1994, 2000). The distribution of H. indica has also been reported from the soil samples collected from three sites in the date palm growing region in the eastern province of Saudi Arabia (Saleh et al. 2001). Uribe-Lorio et al. (2005) conducted a survey in north Pacific and southeast Caribbean regions of Costa Rica. Out of a total of 41 soil samples, five were positive for entomopathogenic nematodes, with three containing Steinernema and two containing Heterorhabditis isolates. Campos-Herrera et al. (2007) studied the distribution of entomopathogenic nematodes in natural areas and crop field edges in La Rioja, Northern Spain. Five hundred soil samples from 100 sites were assayed for the presence of entomo-pathogenic nematodes. There was no statistical difference in the abundance of entomopathogenic nematodes to environmental and physical-chemical variables, although, there were statistical differences in the altitude, annual mean air temperature and rainfall, potential vegetation series and moisture percentage recovery frequencies. Twenty isolates were identified upto species level and 15 strains were selected of which 11 were S. feltiae, two S. carpocapsae and two S. kraussie. S. kraussie was isolated from humid soils of cool and high altitude habitats and S. carpocapsae was found to occur in heavy soils of dry and temperate habitats. S. feltiae was the most common species with a wide range of altitude, temperature, rainfall, pH and soil moisture, although this species preferred sandy soils.

In course of evolution, entomopathogenic nematode like other terrestrial organisms have adopted unique survival mechanism to resist unfavourable condition and environmental extremes including absence of water, extreme temperature, lack of oxygen and osmotic stress. Survival and pathogenicity of S. carpocapsae has been found greater at lower temperature (5-25°C) than at higher temperature (35°C), whereas survival and pathogenicity of S. glaseri has been found greater at higher temperature (15-35°C) than at the lower temperature (5°C) (Kung and Gaugler 1991). The optimum temperature and moisture requirement for infectivity and survival vary with nematode species as has been reported in case of S. abbasi, S. tami, S. carpocapsae, S. feltiae, S. glaseri and S. thermophilum (Karunakar et al. 1999; Ganguly and Singh 2001; Ganguly and Gavas 2004). Cooler temperature has not been found detrimental to nematode survival (Kaya 1990) but exposure to nema-tode at 35°C or above have proved detrimental to infective juveniles (Schmiege 1963). Hazir et al. (2001) studied the effect of temperature on the infectivity, time of death, development and reproduction of S. feltiae. Five isolates of S. feltiae were used in the experiment: SN from southern France, Rafaela from Argentina, Monterey from California, MG-14 from Hawaii and Sinop from Turkey. The result indicated that all isolates caused 100% mortality of greater wax moth, Galleria mellonella larvae and developed and produced progenies between 8°C and 25°C. At 28°C none of the isolates produced progeny, and the nematodes developed to the first generation adults were unable to proceed to the next generation. In all isolates, penetration efficiency was highest at 15°C and 20°C and emergence time was fastest at 20°C and 25°C. Bhatnagar and Bareth (2003) conducted an experiment to study the survival of H. bacteriophora in sandy loam soil at four moisture levels representing 25%, 50%, 75% and 100% of the field capacity. In saturated soils, 70% of the infective juveniles survived for 75 days. Nematode mortality reached 40% within 15 days in soil with 50% field capacity moisture level and within 5 days in soil with moisture level at 25% field capacity. Jothi and Mehta (2007) investigated the impact of different temperatures on the infectivity and productivity of four entomopathogenic nematodes, viz., H. indica, H. bacteriophoa, H. zealandica and S. glaseri on G. mellonella. All the species of entomopathogenic nematodes caused 100% mortality at a temperature ranging between 30°C and 40°C at 24 h after inoculation. At 48 h after inoculation H. indica and H. bacteriophora caused 100% mortality between 20°C and 27.5°C, whereas H. zealandica was effective at temperature between 22.5°C and 27.5°C. S. glaseri was found to be virulent even at 15°C and continued upto 27.5°C at 48 h after inoculation by causing 100% mortality.

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