The combined approach of strain discovery and the inbred line technique for improving control of Delia radicum with Heterorhabditis bacteriophora

Entomopathogenic nematodes are potent biocontrol agents but their efficacy can be compromised under unfavorable environmental conditions. For example, cold tolerance can be an essential requirement when utilizing entomopathogenic nematodes in cooler regions. Discovery of new nematode species or strains that are adapted to local conditions is one approach that can be used to enhance efficacy. Once a strain is isolated however, serial culturing can lead to loss of beneficial traits (such as temperature tolerance). Trait deterioration can be eliminated or reduced by creating homozygous inbred lines. In a novel approach to strain improvement, we combined strain discovery (to obtain superior traits) with the inbred line technique (to stabilize traits). The efficacy of ten homozygous inbred lines from a Heterorhabditis bacteriophora strain isolated in Ontario, Canada was determined against insect larvae, under low temperature conditions. To assess the impact of serial culturing on nematode performance, two wild-type parent populations (the original ‘non-cultured’ parent population, and a repeatedly ‘sub-cultured’ population) were compared with the ten inbred lines. The first experiment evaluated efficacy against Galleria mellonella under five constant temperature regimes from 8 to 24 °C. At 16 °C, eight inbred lines and the ‘non-cultured’ parents demonstrated efficacy against G. mellonella. At the warmest temperatures, all nematode treatments were effective. In a subsequent experiment, the efficacy of the nematode lines and two commercial H. bacteriophora strains was evaluated at 16 °C against the cabbage maggot, Delia radicum. All treatments except one of the inbred lines caused higher levels of infection than the control. The majority of the inbred lines and the ‘non-cultured’ parental line exhibited superior efficacy compared with the ‘sub-cultured’ population and both commercial strains. This study demonstrates the utility of combining strain discovery with the inbred line approach for improved biological control.