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

Authors: SHARIFI-FAR, SHAHRAM - University Of Guelph; Shapiro Ilan, David; BROWNBRIDGE, MICHAEL - Vineland Research And Innovation Centre; HALLETT, REBECCA - University Of Guelph

Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/6/2017
Publication Date: 3/1/2018
Citation: Sharifi-Far, S., Shapiro Ilan, D.I., Brownbridge, M., Hallett, R.H. 2018. The combined approach of strain discovery and the inbred line technique for improving control of Delia radicum with Heterorhabditis bacteriophora. Biological Control. 118:37-43. https://doi.org/10.1016/j.biocontrol.2017.12.004.
DOI: https://doi.org/10.1016/j.biocontrol.2017.12.004

Interpretive Summary: Entomopathogenic (insect-killing) nematodes are small round worms that are used as natural bio-insecticides. These nematodes only kill insects and are not harmful to human or the environment. The efficacy of these nematodes in insect pest control can be reduced due to environmental extremes such as cold temperatures. One method to improve cold tolerance in entomopathogenic nematodes is to discover new strains in naturally cold regions such as Canada. However, beneficial traits (such as cold tolerance) in newly discovered nematode strains can be lost over time due repeated culturing. An approach that can be used to overcome the loss of beneficial traits is to create purebred (“homozygous”) lines of nematodes. In this study, we combined the two methods, i.e., the discovery of a cold-tolerant nematode strain with the creation of pure-bred lines. The result was the creation of stable nematode lines that are superior in killing insect pests in cold temperatures. Our findings improved the ability to use entomopathogenic nematodes in cold climates. Additionally, the combined approach we used may be used as a model to improve entomopathogenic nematodes for other beneficial traits such as virulence (killing power), reproductive capacity, longevity, etc.

Technical Abstract: Entomopathogenic nematodes are potent biocontrol agents but their efficacy can be compromised under unfavorable environmental conditions such as cold temperatures. 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. Trait deterioration can be reduced by creating homozygous inbred lines. In a novel approach to strain improvement, we combined strain discovery with the inbred line technique. The efficacy of ten homozygous inbred lines from a Heterorhabditis bacteriophora strain isolated in Ontario, Canada was determined against pest 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 oC. At 16 oC, 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 oC 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.