Development and population dynamics of Steinernema yirgalemense (Rhabditida: Steinernematidae) and growth characteristics of its associated Xenorhabdus indica symbiont in liquid culture.

Entomopathogenic nematodes have become a valuable addition to the range of biological control agents available for insect control. An endemic nematode, Steinernema yirgalemense, has been found to be effective against a wide range of key insect pests. The next step would be the mass production this nematode for commercial application. This requires the establishment of monoxenic cultures of both the nematode and the symbiotic bacterium Xenorhabdus indica. First-stage juveniles of S. yirgalemense were obtained from eggs, while X. indica was isolated from nematode-infected wax moth larvae. The population density of the various life stages of S. yirgalemense during the developmental phase in liquid culture was determined. The recovery of infective juveniles (IJs) to the third-stage feeding juveniles, was 67 ± 10%, reaching a maximum population density of 75,000 IJs ml- 1 on day 13 after inoculation. Adult density increased after 8 days, with the maximum female density being 4600 ml- 1 on day 15, whereas the maximum male density was 4300 ml- 1 on day 12. Growth curves for X. indica showed that the exponential phase was reached 15 h after inoculation to the liquid medium. The stationary phase was reached after 42 h, with an average of 51 × 107 colony-forming units ml- 1. Virulence tests showed a significant difference in insect mortality between in vitro- and in vivo-produced nematodes. The success obtained with the production of S. yirgalemense in liquid culture can serve as the first step in the optimizing and upscaling of the commercial production of nematodes in fermenters.

Entomopathogenic nematodes for the control of the codling moth (Cydia pomonella L.) in field and laboratory trials.

Three commercially available entomopathogenic nematode (EPN) strains (Steinernema feltiae and Heterorhabditis bacteriophora Hb1 and Hb2) and two local species (S. jeffreyense and S. yirgalemense) were evaluated for the control of the codling moth (Cydia pomonella). In field spray trials, the use of S. jeffreyense resulted in the most effective control (67%), followed by H. bacteriophora (Hb1) (42%) and S. yirgalemense (41%). Laboratory bioassays using spray application in simulated field conditions indicate S. feltiae to be the most virulent (67%), followed by S. yirgalemense (58%). A laboratory comparison of the infection and penetration rate of the different strains showed that, at 14°C, all EPN strains resulted in slower codling moth mortality than they did at 25°C. After 48 h, 98% mortality was recorded for all species involved. However, the washed codling moth larvae, cool-treated (at 14°C) with S. feltiae or S. yirgalemense, resulted in 100% mortality 24 h later at room temperature, whereas codling moth larvae treated with the two H. bacteriophora strains resulted in 68% and 54% control, respectively. At 14°C, S. feltiae had the highest average penetration rate of 20 IJs/larva, followed by S. yirgalemense, with 14 IJs/larva. At 25°C, S. yirgalemense had the highest penetration rate, with 39 IJs/larva, followed by S. feltiae, with 9 IJs/larva. This study highlights the biocontrol potential of S. jeffreyense, as well as confirming that S. feltiae is a cold-active nematode, whereas the other three EPN isolates tested prefer warmer temperatures.