Steinernema yirgalemense inoculum size and timing impact on the population dynamics of in vitro liquid culture shake flask production

Abstract

Summary Optimising the in vitro liquid mass production process for entomopathogenic nematodes (EPN) is a critical step in the development of a cost-effective EPN biopesticide product. Recording the nematode population and growth dynamics in an Erlenmeyer shake flask environment is essential to gaining a better understanding of which factors may influence the final yield. Although infective juvenile (IJ) inoculum concentration has previously been studied, no consensus yet exists as to whether it influences the final yield. This study sought to determine the impact of IJ inoculum concentration and timing on the recovery, growth and yield of the South African EPN isolate of Steinernema yirgalemense in shake flasks. The results indicated that the IJ inoculum concentration positively affects the final IJ yield of 2.88 × 155 IJ ml−1. Moreover, at higher IJ inoculum concentrations, a higher number of reproductive females and males are produced, when recovery percentages were similar. The lowest IJ inoculum concentration treatment was also the only treatment to show signs of an unwanted second generation in the final yield. The IJ inoculation timing trials confirmed that once the bacterium, Xenorhabdus indica, has ended the exponential growth phase and entered the stationary phase, it is ideal to then inoculate the IJ. However, it was found that the S. yirgalemense can be inoculated into the X. indica bacteria solution after 36 h of growth, instead of after 44-48 h. These trials demonstrated the need for further optimisation to produce consistent yields of the highly pathogenic South African EPN isolate of S. yirgalemense.