Entomopathogenic nematode thermal tolerance and performance: experimental manipulation for increased efficacy

Thermal physiology of entomopathogenic nematodes (EPN) is a critical aspect of field performance and fitness. To date, however, thermal limits for survival and activity, and the ability of these limits to adjust (i.e. show phenotypic flexibility) depending on recent thermal history, are poorly established especially for non-model nematode species. Through exploring thermal tolerance for important EPN species and attempting to improve and modify these limits, it should be possible to increase field performance in biocontol programmes. Herein we focus on the nematodes Steinernema yirgalemense and Heterorhabditis zealandica, reared on the codling moth, Cydia pomonella.

We initially investigated host dietary effects on cold tolerance for entomopathogenic nematodes through the addition of L-proline and trehalose to codling moth larval diets in an attempt to modify and improve cold tolerance for EPN. We developed a methdology to score upper and lower lethal temperatures for EPN species. Using this we assessed lethal lower temperatures by exposing approximately 1-200 nematodes per temperature to set subzero temperatures for one hour and then scoring survival 24 hours post treatment. We have now established that proline supplementation has limited effect on low temperature tolerance for S. yirgalemense and have therefore continued to assess these effects across a range of 5 other EPN species, including our original focal study species, H. zealandica.

We then employed our developed methodology for assessing lethal lower temperatures and then also examined upper lethal temperatures and determined survival. To see if we could improve the thermal tolerance of these EPN species we then investigated acclimation effects, through holding nematodes at four different temperatures (5, 20, 25, 30ºC) for 24 hours before temperature assays. To see if induced acclimation would translate to ecologically relevant measures, we examined the virulence (infectivity) by inoculating new hosts post acclimation treatments.

Steinernema yirgalemense was found to be a much more heat tolerant species than H. zealandica. Conversely, H. zealandica was more cold tolerant than S. yirgalemense. The EPN species both had strong responses (both negative and positive) to acclimation treatments that then influenced survival. The virulence of these species does not appear to be influenced by thermal acclimation, although more tests are needed to determine how different forms of acclimation may be used for pest-control strategies.

Our results give indication of large differences in thermal tolerance for the EPN species investigated, as well as large responses to acclimation treatments. Given success with our established protocols it would be useful to further test acclimation and virulence, as well as intergenerational artificial selection responses, and relate these findings to pest-control strategies. Entomopathogenic nematodes (EPN) are sensitive to environmental temperatures and their field efficacy is likely linked to thermal tolerance. By exploring thermal tolerance across a range of EPN species and attempting to improve and modify these limits, it should be possible to increase field performance in biocontrol programmes.